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Turbulence and Moisture
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Why Hot Air Rises; ...Un-named Weather; …Convection Checklist; ...Making Turbulence; ...Turbulence; ...Clear Air Turbulence (CAT); …CAT Occurs; … About Turbulence; ...Reporting Turbulence; ...Moisture; ...Humidity; ...Dew Point; ...Dew Point Statements Reviewed; ...Forms of Moisture; ...Finding Cloud Altitude; ...Ceiling; ...Clouds by Altitude; On Cloud Nine; ...Clouds by Form; ...Special Clouds; ...Fog Types; ...Types of Precipitation;  ..AIRMETS; …Mountains and Winds; …Thunderstorm Theories; …Stormy Weather; …Icing Revisited; …Reading the Wind; …Weather on the Fly; …Precipitation; …Briefing; …Lightning Strikes; …Beyond ETA; …Looking Under the Covers; …Knowing the Difference; …Low Level Significant Prognosis Charts; ...Prog Chart Facts; …A Thunderstorm Has a Life; …Squall Line Thunderstorms; …The Worst of the Worst; …What We Learned in 30-years; …Thunderstorm Classification; …Cloud Classification; …Stability and the Lack of Stability; …U.S. Weather; …Weather Items; …Visible Moisture; …Where's the Turbulence? ..When Is Turbulence? ....Why Moist Air is Less Dense; .. Density Altitude in Brief; ...Summer Flying in Brief;  Learning to Live with Turbulence

Why Hot Air Rises
The pressure altimeter measures the weight of the air above some point. The air weight is measured using the ocean surface as the base. The weight of air is weighed using the weight of mercury as a comparison since a column of mercury one-inch high equals a similar of air one thousand feet high. Atmosphere is standardized using this scale to say that 29.92 inches of mercury is the average weight of the entire atmosphere. This is 1013.25 in millibars.

Weather is produced by the sun's effects on the earth in conjunction with the rotation of the earth and variations in the earth's surface and GRAVITY. The total combination is a heat distribution system based upon ever-changing highs and lows of air pressure. The 750-millibar chart illustrates the 10,000-foot level of air pressure.

Aircraft weigh the air using a pressure altimeter. We correct this weight for the time and place using a dial up number called the Kollsman window. This converts the atmospheric pressure into feet of altitude. Just as we normally set clocks and watches for the area, so does the altimeter do the same for all locally situated aircraft. Only by using the local altimeter setting can aircraft be safely separated in altitude.

Un-named Weather
Weather exists for which we have no terminology. Convective Available Potential energy (CAPE) is a situation where the heat of condensation will create a thunderstorm. When reading the weather look for SBCAPCE OF 2000-3000-j/kg or higher as indicative of conditions creating thunderstorms.

Some storm systems do not move. They are called steady state storms. they continue to exist by drawing warm, msoisst air into itself. TAFs will have existence of CBs.

Convection Checklist:
1. Check air temp and dew point every l00 miles
2. Get specialist to give convective outlook
3. If temp and dew point are below 50 degrees things will be o.k.
4. If temp and dew point are 60 or higher in the morning expect thunderstorms.
5. When enroute check on WSTs (Convective sigments.
6. If temp and dew point are rising…leave the area. Read the outlook
7. Day or night temp dew point over 60 is too dangerous for flying
8. Don't try to out run the problem…get on the ground.

 Making Turbulence
Permanent high pressure exists at the poles. Permanent low pressure exists at the equator. The sun heats the air in the tropics and it settles at the poles. Or it would if it were not for the rotation of the earth. Rotation causes what should be one big circulation cell to divide into three. Where most of the people live and fly, one of these cells have winds that go to the right. The winds at the surface are from the southwest while winds aloft are from the west and northwest. The 30 to 60 degrees of latitude are the battleground of the dry polar winds and the moist winds from the tropics.

If it were not for the sun all airflow would be smooth and orderly. The sun disrupts the flow of air. Turbulence is simply the effort of the air to return to the smooth and orderly flow that would exist were it not for the sun effects.

The student/passenger introduction to turbulence and weather should be, likewise, gradual both as to duration and severity. The better a pilot's knowledge of wind/weather patterns the better able he will be to select desirable conditions. The stress of turbulence is harder on the pilot and passengers than on the aircraft. Turbulence makes flying unpleasant. It is the unpredictability of turbulence that causes this stress. The dislike of turbulence is inherently related to our instinctive fear of falling. Exposure and contemplation directly affect perception of turbulence. Passengers and students detect the demeanor of the pilot/instructor in turbulence and infer the amount of concern required. Gradual exposure is the best way to overcome turbulence anxiety. You don't need to like it to overcome its effects on your flying. Turbulence is harder on people than it is on airplanes. Reduced climb, heading changes, and altitude excursions affect the airplane.

Turbulence is not limited to inside thunderstorms and other vertical clouds; it can be found in areas below and around them. Virga is an indicator of microburst activity. Strong winds over mountainous terrain produces mechanical turbulence. High level turbulence is related to the jet stream. Temperature differences, over 6 degrees Celsius, associated with frontal movements produces turbulence and wind shear. Anything stronger than light turbulence is capable of making aircraft control impossible and causing structural damage. Severe turbulence can tear an aircraft apart.

Turbulence accidents most often occur during takeoff and landing. 40% of turbulence accidents are 40% caused by control problems. If you suspect that there may be a problem the decision not to depart is wise; the corresponding landing choice may not be available. In strong winds minimize the use of flaps and use higher approach speeds. You can even request the use of a taxiway going into the wind. Once on the ground you may be in conditions that do not allow a safe taxi. Get ground help. A friend of mine once found that any attempt to taxi caused him to become airborne.

Va, maneuvering speed is commonly thought of as having to do with control movement. Va is the turbulent air penetration speed. Va is based on weight, the heavier the weight the higher the Va. I find that thinking of it as driving over country railroad tracks. The lighter the car the higher the bounce. Flight into turbulence at below Va makes the plane stall before it breaks. Know your Va. Once way to determine the approximate Va at below gross weights is to change the Va by half the percentage of weight reduction. Find the actual weight reduction below gross as a percentage of gross. Increase the published Va by half of the weight reduction percentage. A 30% reduction of weight would result in a 15% increase in Va. If you are flying light, go slower than the published Va.

There is another speed used in turbulence that is different than Va; it is called structural cruise speed or Vno. Unlike Va this is shown on the airspeed indicator as the meeting point of the orange and green. This is a speed below cruise that is recommended for rough air penetration. Vno does not offer the structural assurances offered by Va. The further below Va you are in turbulence the less likelihood there is of structural damage.

Don't turn in turbulence. Any bank greatly increases the potential to overstress the airframe. The enemy when flying in turbulence is losing control. Fly attitude, stay level, and take whatever altitude nature lets you have. Managed turbulence will not cause structural failure.

Turbulence can cause up/down drafts that exceed the capabilities of a G.A. Aircraft. It is best to maintain level attitude at reduced airspeeds and power settings and not to try to contain altitude. ATC will give you an altitude "range" under such conditions.

Turbulence is caused by rising air. Air rises because of movement across rising terrain or because it is warm. Likewise, air temperature gets colder at higher elevations. Why, then, doesn't the cooler air stop rising? Air pressure decreases as it rises. As air rises into decreased pressure it expands. The expansion causes a decrease in temperature. The lapse rate or change in temperature as at 2 degrees Celsius or about 3.5 degrees Fahrenheit.

Whenever the warmer rising air contains moisture a new factor affects the lapse rate. Warmer air can contain more moisture than cooler air. As warm air, containing moisture, rises, it expands due to decreased pressure and cools. As the air expands and cools it releases heat into the surrounding atmosphere. This process called condensation causes the moisture to become visible at the dew point. The dew point is where air, at a given temperature, contains sufficient moisture for it to become visible. When this process begins to cycle, to repeat itself over and over, we get rising thermals of air and turbulence.

Thermal activity can cause initial warm air to rise, expand, cool, condense, warm, rise, expand etc. at a rate that easily exceeds the climb rate of an aircraft. We often see this as initial scattered puffy clouds, which, as the sun rises, become cumulus clouds. Any one or all of the cumulus can become a towering cumulus thunderstorm. A given thunderstorm can have winds and turbulence exceeding the structural capacity of any aircraft. For this reason, any flight planned into areas subject to this type of weather should be on the ground by mid morning.

A special form of thermal is called a dust devil. Dust devils do not occur nearly as often as thermals and they are often quite visible but not if in a paved area. Dust devils are spinning thermals that move with the prevailing winds. Fly upwind of dust devils. Do not takeoff or land in the vicinity of a dust devil.

All thermals can be avoided by flying at altitudes above their cap level. This means that flying above the clouds will be relatively smooth. The hotter it is the higher you must fly. Maintaining level flight in thermal conditions requires that the pitch attitude of the aircraft be varied to counter the up and down drafts of the thermal area. Slow down if it gets rough.

Clear Air Turbulence (CAT)
CAT occurs where there are high altitude winds above 15,000 feet. There are no clouds or other visual warnings of CAT. CAT is most frequent on the northern side of the CAT in winter.

Only three deaths from CAT have occurred since 1981. CAT is turbulence that occurs without convective clouds. There are three jet streams that are common sources of CAT; jet streams called the polar front, the subtropical, and the polar night. The polar front divides the cold polar and warm tropical air masses from 25° north to 42° north in the summer. Core wind is usually at 30,000'. The subtropical is between 20 and 30 degrees north latitude. It occurs in three waves around the world mostly in the winter at 35 to 45 thousand feet. The Polar night is near the Arctic circle in the winter and has little effect on the U.S.

Cat is usually near the tropopause just between the troposphere and stratosphere. At temperatures between -55 to -65 C it is above the highest clouds. CAT is just upwind of a trough and just downwind of a temperature avection. CAT is hard to forecast because it is erratic as to dimension and time. CAT occurs in conditions of above 110-knot jet streams.

CAT Occurs:
--Near jet streams faster than 110 knots.
--In vicinity of mountains crossed by jet stream.
--In vicinity of mountain waves.
--On the 300-millibar chart a 20-knot isotach within 1250 miles of another will cause horizontal shear CAT.
--Curving jet streams are likely to have turbulent edges.
--Wind shift areas of troughs and ridges are sources of CAT.

Advective turbulence is due to strong winds over rising surfaces. Cloud shapes and patterns are indicative of these winds. Look for lenticular clouds as indicators of mechanical turbulence. The wind velocity, terrain, and the lack of stability in the air mass determine the turbulence. Sigmets for low altitude turbulence occur in such conditions. Partial advective turbulence avoidance requires altitudes 50% greater than that of the terrain

Convective turbulence is due to uneven heating of the earth's surface due to texture or surface covering. Solar heating or cold air moving over heated surfaces causes instability. Instability of the air is turbulence. Without the heat there is stability and no turbulence. From late morning to late afternoon the solar rays will cause thermal turbulence. The lighter the wind the higher the thermals and the greater the turbulence. In summer convective thermals can reach to 14 thousand. In winter five thousand is typical. Haze reduces the strength of thermals. A thunderstorm is a convective thermal of at least 10,000 feet.

Close to the surface there is a layer of air called super adiabatic. After a night of keeping the night air warmer this layer is stable. The rising sun heats the earth below this layer by passing heat energy right through it. The ground heats the air touching the earth. This air rises through the super adiabatic layer.

The initial early morning warming efforts of the sun will only affect a few hundred feet above the surface but as the day progresses the more direct rays of the sun heat the earth sufficient to cause rising thermals of many thousands of feet. Only a stable layer of air powerful enough to cap the rising thermal can stop it.

Rising thermals of air are like ever thinner columns which may blend until forming a mushroom top similar to an atomic explosion. A black surface area will cause the first and most rapidly rising thermal. The strongest rising air is at the microfront or upwind side of the thermal. Widely varying velocities occur along the thermal microfront. Where you see cumulus clouds you are seeing the tops of thermals.

About Turbulence
--Obstruction causes lee side, downwind turbulence

--Vertical currents under clouds and over hot surfaces

--Wind Shear
--Weather front, T-storms, microburst wind direction changes

--Clear Air Turbulence (CAT)
--Change in OAT at altitude near jet stream

--Things rock back and forth in place

--Things will slide

--Things will jump

--Loss of control, possible damage

--Less than 1/3 of time

--From 1/2 to 2/3 of time

--2/3 or more of the time

--Auto-Pilot off
--Know all the ways there are to turn off the autopilot and electric trim
--Efforts of autopilot may break aircraft or autopilot
--Hand-fly the plane
--Trim slightly nose-up and hold thumb on yoke
--Reduce to Va

--Straight and level flight
--Ignore altitude changes, avoid banks or turns

--Rich mixture
--Avoid brief loss of power due to fuel flow

--Study wind flow
--Avoid over flight of terrain that might cause turbulence
--Get PIREPs
--Fly over water

--Watch outside air temperature
--Inversions have smooth air above
--Airliners anticipate CAT by OAT first

Reporting Turbulence:

--Occasional- less than 1/3 of time
--Intermittent - 1/3 to 2/3 of time
--Continuous - more than 3/2 of time
Categories Subjective and dependent on size and weight of aircraft
--Light - slight and rapid no change in altitude or attitude
--Report as light chop or light turbulence
--Moderate - Greater intensity than light - seat belts strain felt -objects move.
--Severe - Large abrupt changes in altitude and attitude - variations in airspeed - objects tossed around.
--Extreme - Aircraft control in doubt -

The presence of moisture in the air is the determining factor as to whether a given body of air will become stable or unstable. Colder air is heavier than warm air. Cold air will not rise unless it is surrounded by even colder air. It is the relative temperatures of neighboring air bodies that determine whether air will rise or fall.

When the air contains water it is conditionally unstable and will remain in an unsaturated state until it reaches near the dew point. At or near the dew point it will become visible moisture (cloud). Any uplifting of this conditionally unstable air will eventually cause it to condense, become saturated (visible) and unstable. Any lifting of unstable air will cause an acceleration of the lifting action.

The moist air does not cool as much as dry air. This means that any uplifting is more likely to increase the temperature difference between the warmer moist air and the surrounding air. The rising acceleration of a moist air column increases as the temperature difference increases. This increasing acceleration is a characteristic of unstable air.

A dry air column cools at a standard environmental rate of 5.5 degrees Fahrenheit for every thousand feet rise in elevation. Without moisture it will cool and tend to descend or level off at the same level as the surrounding air. Stability or instability of any air column is determined by its relative temperature lapse rate to that of its surrounding air.

Atmosphere can be either stable or unstable. Any uplift of stable air will result in its return to its original level. Any uplift of unstable (warmer) air will cause it to expand and cool but at a lower rate than the surrounding air. This air mass will continue and accelerate if the air is either relatively warm, moist or both. The earth's surface and its immediate air is heated unevenly. Turbulence is caused when warmer air moves up through cooler air.

The standard environmental lapse rate of 3.5 F or 2 C per 1000 feet is not realistic. We can determine approximate lapse rates by taking thermometer reading every thousand feet. The likelihood of any flight having this standard is slim. We can determine the approximate freezing level above a known elevation by deducting 2-degrees Celsius from the initial Celsius altitude/temperature for every thousand feet, until reaching zero.

Humidity relates to the water vapor in the air. Humidity is usually spoken of as relative humidity at a particular percentage. The amount of water vapor in the air divided by the amount capable of being there at a given temperature multiplied by 100 give this percentage.

When water is heated it evaporates into the air. Air becomes saturated when it can no longer evaporate any more moisture. At this point the moisture becomes visible and it has reached its dewpoint. One way air meets its dewpoint is by cooling. When temperature and dewpoint are given in a METAR as being close together, the pilot should expect that any cooling will result in visible moisture.

Air will not become any colder than the dewpoint.. As air cools to its dewpoint, water vapor condenses, releases heat and warms the air. Evaporation takes up heat, which is why we cool off with water.

Dew Point
Dew - Formed when warmer air releases moisture to a colder surface. Frozen dew is hard and ice-like. It is not caused by sublimation.

A physcrometer is one way, using a wet sensor and a dry sensor, and usually thermocouples to measure the temperature and a way to move air through an insulated enclosure. Then you compute humidity or dew point or some other quantity. A direct measurement can be done by electronically cooling a mirror surface and determining by reflected light when a fog forms on the mirror, and measuring the mirror surface temperature. Some automatic dew point sensors work that way.

The primary weather enemy of the VFR pilot is visibility. Visible moisture reduces visibility. Moisture becomes visible at the dew point. This is the point at which moisture in the atmosphere condenses and becomes visible. This is very important near bodies of water. Once formed only warming or frontal passage can change temperature/dew point spread.

When the spread between the dew point and temperature gets within ten degrees you should pay close attention to its direction and speed of change. This change can be quite rapid. Within four degrees visible moisture can appear because particles in the air give the moisture something to cling to. A fast cold front, even if weak, can lower the temperature to near the dew point and eliminate the VFR sky. This can happen quite rapidly on cool summer evenings in California but is most certainly to happen in the early morning.

The temperature determines the amount of moisture required to saturate the air. Warm air will hold more moisture than cold air. Saturation occurs at a given temperature when the same amounts of moisture are condensing as are evaporating. This is the dew point. It is recorded as a temperature only as a convenience.

The dew point formula can be used as follows.
Fahrenheit temperature minus dew point divided by 4.4.
Change result to thousands.

The moisture that air can hold in vapor form is a function of air temperature. Cool air will contain less moisture than warm air. At some point the temperature can be reached at which the air/moisture combination will become saturated and visible. This is 100% relative humidity and the dew point.

The dew point spread is used in weather reports to advise pilots of the possibility and probability of visible moisture occurring. The spread is the difference between an existing temperature and the current dew point temperature. When the spread difference is within four degrees dust nuclei in the air can make the moisture become visible very quickly

You can estimate the altitude of clouds when knowing the dew point spread at the surface. For every two degrees of spread there will be a thousand-foot altitude gain for the formation of visible moisture otherwise known as a cloud. If the situation is one of instability where the surrounding air is cooling at a rate faster than the adiabatic rate cumulus clouds will form with a flat bottom and development at the top. Stable conditions cause the air mass to slowly cool with the formation of stratus clouds. At nightfall, the temperature will drop; you can expect the formation of fog when the dew point spread gets down around four. It is unlikely that fog will form if the spread stays above four.

Dew Point Statements Reviewed
Opinion after initial statements.

1. "Warm air can hold more water vapor than cooler air so cooler air will reach its dew point ...sooner"
Whether the air is warm or cool is not the issue. The issue is the spread between the temperature and the dew point. Warm air with a small dew point spread is much more dangerous than cooler air with the same spread.

*Because* warm air (say: above 20 or 25 deg C) can hold more moisture (and therefore has more moisture to condense), the visibility will start to lower with dew point spreads as much as 3 or 4 degrees. When the temperature is down around minus 10 degrees C or less, visibility rarely deteriorates until the dew point spread is less than 1 or 2 degrees C.

2. "Humid air rises relative to dry air because of the lightness of water vapor as a gas."
That may be technically true, but air-masses tend to have more-or-less the same humidity characteristics throughout the entire air mass. So there is no "relative to dry air" for the supposed convection to take place.
Humid air tends to exhibit instability more than dry air in practice... because humid air, in rising, will reach the condensation point sooner. Thereafter it will not cool as rapidly, and is more likely to remain warmer than the rest of the environmental air, and hence more buoyant. Dry air if often the result of a high pressure system, which produces a general subsidence in the area, and is not at all conducive to rising anything.

3. "A unit of air rising will cool 5.5 degrees F per 1,000 feet of altitude in relation to the larger mass of air surrounding it, until it reaches temperature equilibrium. If this rising air does so at a rate less than the
DALR (dry adiabatic Lapse Rate), the air is stable. Air is unstable when rising at a rate greater than the DALR."
I thought air rose *AT* the DALR, not "less than" or "greater than". I am sure the statement was *supposed* to say something like.... "...after rising, if the resultant temperature of the risen air is less than the ambient temperature of the environment at that level, then the air is stable. Air is unstable if the resultant temperature of the risen air is greater than the ambient temperature of the environment at that level."

4. In a later paragraph: "Air rising at or slower than the MALR (moist adiabatic lapse rate) is more stable than air rising at or faster than DALR".

WHAT? I have no idea what she is trying to say. The MALR and DALR have nothing to do with speed. And if we are talking about the speed of temperature change, the MALR is ALWAYS "slower" than the DALR, and in any case, the air is either cooling *AT* the MALR or *AT* the DALR.

5. There is the description of the famous formula for determining cloud height from dew point spread, along with the statement: "It indicates changes in base heights taken over time; that is, are the clouds lowering?"

This formula is useful primarily for the determination of the bases of cumulus* clouds in fair weather, and possibly stratocumulus following a cold front. Be *very, very* careful trying to apply it to any other meteorological situation. In fact... DON'T!

6. In a sidebar on Density Altitude is the statement: "Even though you may think that humid air is more dense than dry air, the fact is that the molecules of water suspended in the air force other gas molecules farther apart to make room for them. So, even though humid air may weigh more per unit volume, there is actually less air for your wing and propeller to use."

In humid air, the molecules of water replace the molecules of air. Since air is mostly nitrogen and oxygen, whose atomic masses are 28 and 32 respectively, and water has an atomic mass of 18... every replaced molecule results in a lighter and lighter air mass. Humid air does never, Never, NEVER "weigh more per unit volume" than dry air. Your wing and propeller uses molecules of water vapor just the same as the molecules of any other gas. It is just that these molecules happen to be lighter, thus are easier to displace, thus provide less "reaction", and therefore less lift.

Forms of moisture
Water in its various forms, vapor, liquid, and ice, is the energy source for making weather. The heat energy transferred as water changes form is called latent heat. This means it is lying in wait, ready to use the heat difference in the surrounding atmosphere to make a form change. Vapor changing to condensate gives up heat to the atmosphere. The hotter atmosphere rises making way for new air to be heated. The process feeds on itself to make thunderstorms.

Heavy rain can cause a greater loss in aircraft performance than can wind shear. Laminar airfoils (Piper, P-51) are most affected by surface roughness caused by beads of water, corrosion, frost, etc. Avoid landing during heavy rain. Avoid slow flight situations such as a go-around. If unable to avoid, fly a higher than normal speeds.

Finding cloud altitude
If you know the temperature-dew point spread in degrees, 4.5 F can divide it into the degrees. The dividend in thousands is the AGL base level of the clouds and the end of turbulence if there are no clouds.

If the temperature/dew point difference is 44 degrees clouds are likely to form at 10,000 feet. When dew point temperatures reach 50-degrees; you have reached conditionally instability where the suspended water contains enough heat potential to cause thunderstorms. When the dew point temperatures reach 30-degrees; you have sufficient water to cause clouds.

---Unsaturated air cools at the dry adiabatic lapse rate of about 3 degrees C per thousand feet of altitude.
---Dew point decreases at about 0.5 degrees C per thousand feet.
---Take the temperature and the dew point, subtract to get the difference
---Divide by 2.5 to get how many thousand feet AGL the clouds will form.

Figured from ground level so MSL heights will vary with altitude of airport. Ceiling report requires qualified observer. A ceiling exists when cover is reported as broken or overcast. Any thin or partial qualification will not give a ceiling. A weather service ceiling is the height above the surface of the base of the lowest layer of clouds or obscuring phenomena that hide more than half of the sky. Reported as broken or overcast. ATC must turn on the beacon, daytime, when the ceiling is below 1000' or the visibility is less than three miles.

A ceiling is the vertical visibility into the surface based obscuration that hides all of the sky. Any ceiling reported by a ground observer is above ground level. Ceilometers are useable below 12,000'. Cloud height can be estimated by ATC being able to note when Mode-C altitude reading is correlated with aircraft disappearing into the clouds.

VFR flying: If reported ceilings are not 1000' above the highest obstruction within five miles of your track, don't fly. Don't fly if weather is not improving. Low visibility is aviation's greatest weather killer. Moisture is the greatest reducer of visibility. The best indicator of low visibility is the dew point/temperature spread. Evening is more critical than morning because the spread is more likely to be narrowing.

Clouds by Altitude
(Classified by World Meteorological Organization)
By Altitude: (the top of a given cloud is determined by where the air stops rising. Super cool drops are at the top of clouds)

On Cloud Nine
There are nine classifications of cloud levels. Cumulonimbus is the ninth and highest.
As air rises and cools clouds form. For every 1000' unsaturated air cools 5.33-degrees F for every 1000' of rise. This is known as the dry adiabatic lapse rate. When the dew point is reached, a cloud forms. Condensing releases heat. When this happens, the cooling occurs at the moist adiabatic rate of 3.3-degrees F.

The trigger for rising air can be from surface convective heating (sun), orographic lifting as with rising terrain, air mass movements, or nearby moist air. Moist air to the extreme will only hold 6% vapor by weight. This makes this air 2% lighter than dry air. Relative to the surrounding air, cold air will sink, just as warm air will rise.

As a pilot you look at forecasts of thunderstorms, high winds, and turbulence as accompanying cold fronts. Along with warm fronts we look at low clouds, haze, continuous rain, or snow. The weather exists along the cold front and along and ahead of the warm front. Turbulence results from the strong winds associated with the thunderstorms.

The weather term for fronts originated during WWI in Norway where the movement of the weather was likened to trench warfare. This so called Norwegian cyclone model has a left-turning spinning of low pressure with cold air north and warm air to the south in conflict. The rotation of the earth imparts the left spin of air. The center rises, is sucked away by high altitude winds and replaced by air from below. A storm exists. The differences in temperature between the air masses is the fuel that keeps it changing.

Clouds by form
There are numerous combinations with prefixes and suffixes to identify clouds by form:
--Cirrus - feathery or wispy ice crystals above 20,000
--Alto - Mid-range at 6,500 to 20,000
--Cumulus - Heaped up, puffy cotton balls
--Cumulonimbus (CBs)

Is the mature thunderstorm with heavy rain, turbulence, lightning and hail. The gust front of a CB is the strongest and most dangerous when it descends to the ground and precedes the arrival of the thunderstorm by several miles.
--Cumulonimbus mammatus
Huge bumps below a thunderstorm. Area of extreme turbulence exists below.
--Towering Cumulus
--Stratus - flat, layered
--Stratus types (rising air less than one inch per second)
--Cirrostratus...form at high altitudes as warm front arriving
--Altostratus ...mid altitude as warm front nears
--Low altitude stratus to 6500
--Stratocumulus cumulogenitus...after cold front, patchy smooth above over wide area
...form from lifted moisture, overcast, rain, snow, and ice. These are CB wantabes. If sufficient moisture and uplift are available they become CBs. First appear in forenoon and may continue to grow if conditions allow. Not likely to mature if first appear in late P.M. You can't outclimb a growing cumulus even at night. Don't go VFR unless sure of conditions.
--Stratus fractus...warm and stationary fronts, scud
--Nimbo stratus...rain producer
Rain clouds of gray and dark gray. Rain will reduce visibility and rain from higher clouds will cause lower clouds to form. It takes 4000' of clouds to give rain.
Known as the thunderstorm cloud, its symbol or abbreviation is Cb. Rising air may reach 14 inches per second.
In mountainous regions this almond or lens-shaped cloud indicates the possibility of severe turbulence on the leeward downwind) side of the mountain.
--Embedded Thunderstorms
Cumulonimbus clouds obscured by massive cloud layers. Embedded thunderstorms are associated with warm and stationary fronts but may occur in a slow-moving, shallow cold front.
This horizontal cloud shape points in the direction of storm movement.
-- An airplane in a charged cloud is an invitation for a lightning strike.

Special clouds
Dew Point Clouds
Water vapor that condenses may form dew on a colder surface but in nature it causes clouds and fog. Any partially saturated air that is cooled will condense to form an initial haze if there are nuclei particles available in the atmosphere to trigger the process.

Cap cloud-

The cap cloud is at the top of the ridge making the wave and may extend several thousands of feet upward. It contains significant moisture and will cause a rapid accumulation of ice. They have been known as Foehnwall clouds or a mountain cap. The further down the lee side of the mountain the stronger the wind.

Rotor cloud
The rotor cloud is located downwind from the ridge but at ridge height. The rotor cloud can usually be seen. The rotor cloud is a sideways tornado forming upwind and dying downwind.

Lenticular cloud
Distinctive laminar shapes that form at the apex of wave forms. Do not attempt to hold altitude, go for the ride.

Fog types
Fog is a cloud on the ground. A low-down cloud. Fog forms when the air can't hold the moisture it contains and lets it condense. The temperature at which the moisture condenses is called the dew point. The more moisture the higher will be the dew point or the required temperature to make that moisture visible.

Prediction of fog is difficult both by the forecaster and the pilot. Recognizing the fog potential is important for planning departure and arrivals. As temperature and dew point come closer the air becomes saturated and will hold no more water vapor. A decrease in temperature of and increase in water vapor causes condensation and fog. Dense fog occurs when the spread is zero. Condensation nuclei can cause fog to form when the spread is several degrees but the fog will not be considered dense. More than one process may be in effect and can cause very rapid development of fog conditions.

In Northern California it is not unusual to have clear weather in the twilight before dawn with a fog forming at sunrise. The fog will become thicker as the sun evaporates the surface moisture. As the sun rises it will repeatedly cut through and thin the fog only to again evaporate the surface moisture and make more fog. The cycle may continue all day with a possibility of weather remaining below VFR minimums. As the sun declines in the afternoon there is a chance for slight improvement due to a decrease in the evaporative power of the sun.

---Three types of West Coast fog
---Worst and rarest is stratus between 1,000 and 1,500 feet, but occasionally on the ground
---Less rare is radiation fog, a shallow fog from radiation cooling of the earth on calm, cloudless nights. -------The earth cools the humid air it, causing condensation.
---Usually it is advection fog forming off the Pacific coast when moist air passes over the cold Ocean.
--- This is the fog and low clouds that run the eight day summer cycle air conditioning of the Bay Area.
---Elsewhere it is the upslope fog is common as moist air masses are blown and lifted by mountains
---Precipitation fog exists where warm rain falls on cooler air, raising the humidity giving fog
---Precip fog derives where warm fronts exist but can occur with other types of fronts when moving slowly.
---Ice fog is a type of radiation fog in cold conditions where the fog is formed as ice crystals.
---Clouds and fog are the same except for the way they form
---Cloud formation involves turbulence, while fog does not.

Radiation fog
(Ground fog-tule fog-valley fog, etc.)
Radiation or ground fog is formed near the cool ground when a surface inversion exists. The nuclei attract water and grow into fog. When the air has condensation nuclei that are hygroscopic, water will be attracted at less than 00% humidity. Salt is such a nuclei. You are better off not to use your landing light since it will blind you to the runway. Without the light you can use the edge lights and you may not even notice the fog.

Radiation fog becomes more prelevant just after sunrise because the sunshine reaches the earth and evaporates any surface moisture. This raises the dew point in the air above the surface and causes the formation of fog. Radiation cooling occurs when temperature is lowered on clear nights when earth's heat and moisture meets colder air with cooling down to dew point. The required heat transfer occurs when the warmer earth radiates heat into the cooler air. Clouds act as a warm blanket that absorbs heat only to reflect it back towards earth. This prevents fog from forming. Radiation fog is most common during the winter high-pressure periods when the nights are long. Fog will not form if the temperatures are below 28-degrees Fahrenheit. At -20F ice fog will form. Below 0-degrees Fahrenheit radiation fog can form over ice or snow cover.

Fog starts at the ground and can exist from a few feet (ground fog) where it is possible to walk or drive without seeing the ground. Radiation fog can rise to several hundred feet in the air. If there is no wind, it can persist for several days. It may be possible to fly over radiation fog and see vertically to an airport and still not have the slant visual range needed for airport operations. In the central valley it is called valley fog. Surface heat and no wind. The fog will be thicker in the valleys because the cold, heavier air flows downward. If winds exist the radiation fog is less likely to form due to mixing. The fog will finally dissipate when the earth and air are both warmed by the sun. This process proceeds from the outer edges to the center of thickness.

Frontal Fog
Or Precipitation Fog occurs when a rapidly falling pressure may cause fog to form to the center of the low-pressure center. Warm rain on colder air may cause evaporation in multiple layers of stratus and fog. This fog forms when rain falls into colder air. It is typical of certain warm front conditions when the warm front overrides colder air.

Advection fog
(marine layer fog)
Cold water or land overridden by warm moist air blowing along the Pacific Coast during summer. The horizontal movement of the air makes the required heat transfer. Air is cooled by contact with cold arctic water from Alaska. Additional cooling occurs when the air rises. Moderate on-shore winds move the air that will form a fog layer one to 3000 feet above the surface. Fog will move inland and back out to sea, as the interior land valleys become hotter and cooler. Fog layer moves progressively inland over several nights while retreating partially during the day. An eight day cycle, four days with increasingly dense fog followed by four days of retreat, is typical for the Bay Area but the cycle may be changed by weather fronts.

Upslope fog
Warm moist air blown up sides of mountains. Air cools as it rises. Humid air as it rises will cool to dew point and form upslope fog. Often occurs in conjunction with avection fog before it acquires depth needed to cross a mountain. Down-slope winds prevent the formation of fog due to warming of the air.

Ice fog
This fog is composed of ice crystals. Common in Arctic or very cold regions. Forms below -20 degrees Fahrenheit. Forms in populated areas using hydrocarbon nuclei from heat sources. Will form rapidly at very low altitudes only a few feet thick.

Steam fog
(sea smoke)
Similar to hot kettle of water. Warm water causing rising vapor into colder air. Makes Grand Banks off New England very foggy near the water. Early winter conditions in the Arctic or Great Lakes.

Types of Precipitation

Droplet size separates rain from drizzle. Drizzle has closely spaced droplet size of less than two-hundredths of an inch. (200 to 500 micrometer diameter.) Individual droplets seem to float and will not disturb a smooth water surface. Rain drops are greater, four hundredths of an inch, than drizzle droplets and will cause ripples and splashes in still water. Smaller liquid drops are called cloud droplets.

Freezing rain
Difference is droplet size. Rain is more than 500 microns. Freezing rain is a hazard where it is too warm for snow but cold enough at least part of the time for droplets to freeze on contact with a cold surface. When rain falls through colder air below it may become super-cooled. It is still rain but on contact with a freezing surface will instantly freeze. Ice pellets, U.S. commonly called sleet, were once freezing rain. Warmer air will be at a higher altitude. Freezing rain differs from freezing drizzle only in that the 500-micron droplet size separates the two. Descending through snow into freezing temperature means you can expect to enter freezing rain.

White out conditions can occur without warning. Visually similar to ground (radiation) fog. The snow being blown from the surface rises a few feet above the surface and cuts off visual reference. Wet snow indicates that colder temperatures exist above.

The amount of moisture in snow is the major variable as a cause of aircraft icing. Do not rely on visibility as an indicator since a fluffy snowflake will reduce visibility up to ten times the reduction caused by a small dense flake while they both can contain the same amount of water and icing potential. Good visibility in snow conditions cannot be used as an icing indicator. Don't fly into a snow shower that you cannot see through. Clouds the produce snow usually contain ice. Ice will form on any airplane at near freezing temperatures plus moisture. Carburetor ice can form at temperatures between fifty and 100 degrees F.


Usually occurs on stationary objects as thin granular on upper surfaces. Due to sublimination of water vapor directly to frost in below freezing weather. Frequently at night when temperature and dew points are below freezing. This condition provides the moisture required to crystallize on aircraft. Can occur in flight when aircraft skin is cooled below freezing prior to descent into moist air or even drizzle. Forms much as due but air is below freezing. Frost is white and opaque. It is formed by sublimation since it goes directly from vapor to ice without going through the liquid stage.

Frost causes a rough surface and inhibits the smooth flow of boundary-layer air. This slower air separates from the airfoil sooner and decreases lift. Frost will cause a 10% increase in stall speed.

AIRMETs tend to be overly conservative in weather analysis. Forecasters in Kansas City issue Airmets. They are issued every six hours making four per day. If an AIRMET needs to be changed the text is preceded by AMD while those that follow are headed UPDT for update.

All AIRMETs are about moderate conditions that affect smaller aircraft. An AIRMET warning of icing is titled Zulu. An AIRMET about turbulence is titled Tango. IFR conditions over more than 50% of an area has AIRMETs titled Sierra which also includes winds over 30 knots and mountain obscuration. The worst conditions appear first.

More common AIRMET abbreviations are:
ISOLD isolated
WDLY SCT widely scattered
SCT scattered
NMRS numerous

Mountains and Winds
By themselves mountains are not dangerous; nor are winds. Even hills are not dangerous. With the right amount of wind a hill can be just as dangerous as a mountain in winds. In one respect hills are more dangerous. Hills can be deceptively dangerous. On a hot day, a hill, even a small hill, with the right altitude, a wind velocity from the right direction and velocity can have sufficient density altitude and turbulence to bring down most any aircraft. Let me illustrate.

A high time engine can easily lose up15% of its designed horsepower. With a 3% loss per thousand feet of density altitude we have still more loss of power. Density altitude also affects propeller and wing efficiency. The little hill just described becomes a monster.

Turbulence increases with wind velocity. My personal limits are not to fly the Sierras when forecast winds approach 20 knots. If I do not have two thousand extra feet of altitude when approaching a ridgeline I will cross at an angle regardless of the performance capability of the aircraft. Valley observations do not apply to the mountaintops. Winds and density altitudes are not visible.

Make a performance chart for your aircraft relating to percentage of gross weight. Make a trial flight at altitude to confirm your performance with the current engine and load. Plan for only 90% of your best performance as a margin of safety. I am a sea-level pilot most of the time. When I fly in the hills and mountains of California I try to chose my conditions carefully. I start watching the weather several days before a mountain flight. I watch the departing fronts as well as the arriving fronts.

I file a flight plan and insure it by maintaining radio contact with all available radar facilities. On occasion I will climb to an altitude using portable oxygen just to get the insurance I need. I have found that PIREPS are the best weather information but it may be too localized for my route.

Thunderstorm Theories
We know what a thunderstorm does but we don't know just why. The classification of thunderstorm stages as cumulus, mature and dissipating took place over 50 years ago. Twenty years ago the downdraft became wind shear and microburst. Lightning is still a group of theories looking for a solid explanation. We have plenty to study with 15,000 worldwide strikes an hour. U.S. and China both have about 10,000 thunderstorms a year but the we have about a thousand tornadoes. They have hardly any. Tornadoes occur on the south or southwest side of a thunderstorm since that is toward the moisture source of the Gulf of Mexico.

Weather is electromagnetic with electrons trading off from earth to atmosphere and back again Lighting is the visible process of balance restoration for the electromagnetic forces. To get the lighting we need a lifting force (sun's heat) and some moisture for conductivity.

The physics of a storm requires free convection of water vapor containing heat. Condensation releases the heat when temperature and dew point meet. We now have water heavy enough to fall. How it falls determines the kinds of thunderstorm. Water that falls into the updraft effectively kills the storm quickly. If the wind blows the water away from the updraft we will have a storm for a longer time.

The worst of thunderstorm weather tends to be in the vicinity rather than in the storm itself. The weather in the storm tends to be in one direction, while the neighboring areas have air that is making rapid and erratic changes of direction. Changes of 500 feet per minute are about six miles per hour but are considered severe wind shear. Without a wind shear alert system these conditions may be unnoticed.

A downdraft is usually the most powerful part of a thunderstorm. Downdraft wind speeds have been recorded as high as 150 mph with 120 mph not uncommon. There are two types of micro-bursts. The wet ones occur east of the Rockies, the dry ones west of the Rockies are associated with virga. (rain that does not reach the ground)  Microbursts rarely last longer than five minutes. The safest way to deal with a sudden change in altitude or speed is to execute a full power go around and come back again if necessary five minutes later.

Flying in wind shear conditions requires the pilot to:
1. Prevent altitude loss by flying at Vx as necessary but at Vy if stalling potential exists.
2. Fly close to Vref for your weight (make a chart) to avoid stalling
3. Climb when you can since altitude is insurance.
4. Carry speed and power on the landing approach

Stormy Weather
… is more than just a 1930's song
--Convective growth of cumulonimbus requires moisture and unstable air
--Instability can be caused by heat, temperature differences, or terrain.
--Midwest and mountains are where most thunderstorms occur.
--High winds aloft cause thunderstorms to be more severe and capable of lasting an hour.
--You must know prevailing winds to avoid thunderstorms.
--Avoid the downwind side of thunderstorms
--A downdraft below a thunderstorm can destroy any aircraft.
--Upwind and far away is the place to be around thunderstorms
--Few accidents are directly related to lightning.
--Many lightning discharges are horizontal
--Worst storms are MCCs known as mesoscale convective complexes can cover several states.
--MCC tornadoes last longer than the usual 20 minutes.
--Virga is a warning that downdrafts and turbulence exist.
--In heavy rain lean the mixture, use alternate air for best power
--Ice exists around thunderstorms, clear ice is the worst that can happen near the top and downwind.
--Avoidance is the best weather alternative, land and refuel to give time for storm to leave or die..

Icing Revisited
--When in visible precipitation turn on all anti-icing devices.
--Ice can stop the engine.
--Ice has weight but greatest hazard is airflow disruption.
--It takes very little ice to bring down an airplane.
--Frost forms when moist air freezes to a cold surface.
--Rime ice looks like cake frosting. It is in stratus clouds between -10 to 20 degrees Centigrade.
--Clear ice is glass-like and comes from freezing rain near the tops of cumulus clouds.
--Mixed ice has the worst features of rime and clear ice. It can bring an aircraft down very quickly.
--The worst way to avoid ice is to fly along a front looking for a place to cross.
--The pitot tube is a likely place for icing to occur if it is not heated before the fact.
--Losing the static air is a sure way to get inaccurate airspeed and altimeter readings.
--Carburetor heat, windshield defroster and alternate air should be used in icing conditions.
--A propeller will lose 20-percent of it efficiency in icing conditions.
--Cycling a propeller at different speeds can throw off ice on occasions.

Reading the wind
Winds blowing toward the mountains are going uphill and will cause any moisture to precipitate.
--Southerly winds are likely to cause poor visibility and a chance of thunderstorms in high humidity.
--Compare actual winds with the forecasts. Unexpected winds make forecasts inaccurate.

Weather on the Fly
--Being unable to anticipate weather changes is a major cause of weather related accidents.
--Record temperatures each 1000 feet on climb-out to figure lapse rate and compare with forecasts.
--You can see weather enroute. Check With FSS/Flight Watch (122.0) to see if weather is as forecast.
--Monitor 122.0 while enroute
--Give and ask FSS for PIREPS, radar and satellite picture descriptions.
--If temperatures are lower than forecast you can expect weather problems.

--Rain reduces visibility and is usually accompanied by turbulence.
--Embedded thunderstorms can be hidden by rain.
--Rain is conducive to carburetor ice.
--Rain at near freezing temperatures will form clear ice on an aircraft.
--Showers and thunderstorms have winds that make both flying and landing difficult.
--Crosswinds and wet runways reduce the landing capability of both aircraft and pilot.
--Be aware of terrain elevations when trying to circumvent rain showers.
--Many radar facilities are limited in ability to help you around or out of weather.
--Avoid rain forecast areas when flying at night.

FAR 91.103 requires pilot to acquire all available weather
--Briefings can be obtained by phone, DUATS or personal visit.
--Briefing can be outlook, standard, or abbreviated.
--Standard briefing gives adverse weather warning, flight recommendation, synopsis of system movement, current conditions, METARs, PIREPS, NOTAMS, and forecasts. .
--Always ask for NOTAMS, known delays, GPS status, and anything else you can think of.
--Abbreviated briefing requires that you be specific about information you want.
--Monitor 122.0 (Flight Watch) and listen in on nearby AT?IS, AWOS and ASOS to keep current.
--Current conditions when compared to forecasts allow you to adjust your plans to minimize risk.

Lightning Strikes
--March through May have 42 percent of events
--Lightning avoidance is more common than a few years ago.
--No lightning accidents in past six years.
--Since 1983, only five accidents caused by lightning damage.
--Flash blindness caused two additional accidents.
--Cold fronts cause 25 percent of accidents.
--Most likely strike altitudes are between five and seven thousand feet.
--Engines can be seriously damaged by lightning strike.
--Lighting can cause shock wave and overpressure sufficient to damage an aircraft.
--Sferics devices record past strikes not of what is to come.
--Half of strikes occur near thunderstorms.
--Strikes occur outside of heavy rain for as far as 10 miles away.
--Aircraft seem to cause strikes.
--Outside air temperature for 30 to 34 Fahrenheit is region of most strikes.
--Go down and slow down is good escape strategy

Beyond ETA
--Look for changes along the route
--Look for weather to each side by checking AWOS and ASOS
--Compare forecast and enroute altimeter settings in the chance that pressures are up for improving weather.
Check altitude temperatures and note changes from forecasts as related to time

Looking under the Covers
--Flight safety is based not on when to fly but on when not to fly.
--Consider what could go wrong with the forecasts and locate your options.
--The most likely error will be in the timing of a weather event.
--Check for any change in stability
--Note any change in Z-level from that forecast.
--Be able to interpret the words you hear into a set of viable options.
--Any flight close to the ground requires special cautions.
--Showing another pilot a new skill is a hazardous situation.
--Know your airplane's performance limitations.
--Maintain minimum safe altitudes and turn away from rising terrain.
--Don't fly into a situation where you have not planned your options.

Knowing the Difference
--A forecast is an estimate of conditions in UTC time at locations delineated by VOR locations
--A forecast is a prognosis (prog) with a valid time (VT) window by time and day.
--The closer between a progs preparation and valid time the greater the accuracy.
--Observations are reports of real-time conditions.
--Airmets (WAs), sigmets (Ws), convective sigmets (WSTs), urgent pilot reports (UUAs, are observations.
--A chart made of observations is an analysis made at a point in time.
--The four-panel Composite Moisture Stability chart is an example of an analysis..
--PIREPs, airport surface reports, Radar Summaries, Weather Depiction Charts, and Pressure Charts are others.

Low-Level Significant Prognosis (forecasting) Charts
First digit front type: (Note even numbers at surface, odd numbers above surface)
0. Surface quasi-stationary
1. Above surface quasi-stationary
2. Surface warm
3. Above surface warm
4. Surface cold
5. Above surface cold
6. Occluded (wrapped around)
--Occluded fronts are hard to forecast and occur unexpectedly.
7. Squall line
8. Intertropical
9 Converge line

Second digit intensity (Note grouping of threes)
0. Squall line none
1. Weak and weakening
2. Weak no change
3. Weak and strengthening
4. Moderate and weakening
5. Moderate no change
6. Moderate and strengthening
7. Strong and weakening
8. Strong no change
9. Strong and strengthening

Third digit characteristics
0. None
1. Decreasing activity
2. No change
3. Increasing activity
4. Intertropical
5. Forming or existence expected
6. Quasi-stationary
7. Waves (may cause front to separate into two low-pressure areas)
8. Diffuse
9. Position doubtful

Prog Chart Facts
--Surface to 24,000 feet
--Four charts are made every six hours.
--Top charts show freezing levels forecast including ground level
--Bottom charts show surface conditions, fronts, pressure areas and precipitation forecast
--Charts made seven hour before valid times of 12 hour forecasts.
--Need to be updated prior to flight for real time conditions. (Analysis is what happened at a given time.)
--Helps show how the fronts will be moving and changing.
--You need 'code chart' available as above.
--Four digits are millibar pressures. (Less than 1000 means bad weather)
--Solid blue line is surface freezing level
--Dashed green lines are above surface freezing levels every 4000 feet.
--Dashed yellow areas are turbulence areas.
--Add two zeros to numbers associated with symbols to get altitudes predicted.
--Trofs are areas of clouds and precipitation
--Blue scalloped areas are marginal VFR (MVFR).

A Thunderstorm has a Life
--A region of dry air lies ahead of the thunderstorm path.
--An arcus cloud (wave shaped) forms an arc before the moving thunderstorm.
--A gust front is the strong wind burst preceding the moving thunderstorm.
--Cold air in front of the thunderstorm is sucked up into the thunderstorm.
--The cold air vaporizes and can become supercooled water that crystallizes above Z-level into ice.
--This ice forms cirrus clouds ahead of the anvil as the storm dissipates.
--Mamatus clouds form behind the anvil directly above the storm.
--At maturity rain falls through the core of the storm and sucks dry air from behind the storm as it falls.
--The falling rain and air creates severe turbulence.
--The energy of a thunderstorm comes from the latent heat releases as water vapor condenses. .
--A dry line is a boundary between desert air moving eastward and humid air from the Gulf. It is the major
source of Plains thunderstorms.
--Most thunderstorms occur at night.

Squall Line Thunderstorms
--Avoid flying near or into squall lines.
--Squall lines are common east of the Rockies in the sprint and summer.
--Pre-frontal squall lines can precede cold fronts by 100 miles.

The Worst of the Worst
The MCC (mesoscale convective complexes) are massive convective weather systems of 250 miles in diameter.
The MCC is initiated by an inversion containing a cluster of thunderstorm with a rain area to the north. Winds
below the inversion are light but above the inversion the winds increase in speeds, reaching over sixty knots only a few hundred feet above ground level. This is a low-level jet from the south, usually. Flying up or down in this region will pass through wind shear turbulence.

Nighttime is best from MCC cluster thunderstorms with lightning shows and tops above forty-thousand feet. Nighttime storms die out in the morning but as the MCC moves eastward it may reform as an MCC the following night. Summer rain in the plains comes from MCCs. During this rain an aircraft could enter severe turbulence with no warning.

Acquired Thunderstorm Knowledge since 1975
Downbursts are now detectable by Terminal Doppler Weather Radar. The downburst is air falling with rain from a thunderstorm to the ground and then spreading horizontally. A microburst is a downburst that is less than 2.5 miles in diameter. A microburst is a concentrated intense downburst. A dry microburst is a western event falling with virga. Virga is rain that does not reach the ground. When a dry microburst hits the ground it arrives as a circular dust storm expanding rapidly. A wet microburst is hidden by surrounding rain. An aircraft flying into a microburst will be driven into the ground.

The mature stage of a thunderstorm is indicated by falling rain. The mature stage is a turbulent destructive combination of rising updrafts, falling rain and microburst. The downdraft flows in the direction of storm movement in a frontal gust that may reach 40 knots. Cold air falling with the rain will eventually cut off the thunderstorm engine. Any takeoff or landing into a frontal gust is in trouble. A pilot near a thunderstorm must
expect strong shifting winds.

An upper air weather disturbance is a supply of cold air ready to be fed down the middle of a thunderstorm.
This air over a warm moist air is a thunderstorm waiting to happen.

Thunderstorm Classification

Most thunderstorm illustrations show the rare single cell storms. Multi-cell storms or clusters are more common because the cells tend to 'breed' additional cells. Airmass storms form in the humid afternoons without the need of a weather from or a low pressure system. These storms are easily avoided by pilots because they are highly visible and easily avoided.

Squall line storms often extend for hundreds of miles preceding a frontal line and followed by areas of stratus with poor visibility and low ceilings. These storms disrupt cross-country flights.

The largest of all cells are the super cell thunderstorms that spawn tornadoes, micro bursts, hail lighting and heavy rain. The super cell storms form at the southwestern end of a squall line or can exist individually. I once saw one that covered the entire western end of Kansas.

Derechos (day-RAY-chos) are storms consisting of a series of downbursts with winds over 50 knots. As it moves along a distance over two-hundred miles it spread destruction along the way.

Cloud Classification
--vertical development

Ragged clouds indicate turbulence while cauliflower like cumulus clouds can be seen growing up. You can never judge by appearance what may happen in a soft looking cloud. The up and down turbulence may still be going on inside or on the other side. If a cloud looks dangerous, it probably is.

Billowing clouds are made by wind shear conditions and are indicative of severe turbulence. Roll clouds occur
in the cold gust front that precedes a thunderstorm near the ground. Lenticular clouds are lens-shaped that occur often in a series as strong winds blow across mountains. Severe turbulence is to be expected.

Stability or the Lack of Stability
--A pilot needs to know weather because so much depends on this knowledge.
--The U.S. usually has only four weather-makers at one time across the country.
--Cold fronts run north and south and have greatest thunderstorm potential.
--Warm fronts with curves that extend southward are more difficult to evaluate.
Unstable air will continue to rise after the lifting force stops. This condition causes thunderstorms.
--Any rising air means that elsewhere air will be falling. Weather is determined by the rise and fall of air.
--Stable air, if lifted, will return to its original altitude when the lifting force ceases. Weather will be calm.
--Air that goes up cools at a regular rate regardless of the surrounding air. If the surrounding air is cold then the rising air will continue to rise. If the surrounding air is relatively warm in relationship to the rising air, the air will stop rising.
--The presence of cold air aloft in either a closed low or an upper air disturbance will cause an unstable
condition to exist. Warm air below will rise and a high humidity will increase the heat exchange and increase instability.
--By flying early in the day before the sun heat the earth we can avoid these conditions.
--Weather forecasts are based on expectations of clouds and rain as a basis for air instability. It is advisable that pilots keep track of the outside air temperature. By comparing it to the winds aloft temperatures you can come up with your own analysis of the stability. As you climb every 1000 feet check the temperatures to compare the standard rate of change of two-degrees per thousand to the actual change. If it stays at two or even gets warmer the region is stable. Any cooling faster that two is unstable and if it reached 3.5 degrees it is very unstable.
--Air that is warmer that the air below makes a stable condition called an inversion. Inversions usually happen on clear nights as the earth cools into the sky. Inversions can occur in high pressure areas because of sinking air. All inversions are stable and cause poor visibility conditions. The possibility of embedded thunderstorms exist. Inversions can have a wind shear zone of severe turbulence.

U.S. Weather
Low pressure areas are formed from the surface and spiral upwards with counterclockwise winds. The rising air cools and condenses into clouds and usually precipitation. This is bad weather for flying. High pressure areas have descending air with clockwise winds that flow outward at the surface. The descending air warms and causes clouds to disappear giving clear skies and good flying conditions but not always. The warm air can cause an inversion with reduced visibility. Winter storms and low-pressure areas have very low pressures.

Troughs (TROFs) extend from low-pressure areas and move across the U.S. much as does a sine wave moves across an oscilloscope screen. While fronts form in trofs not all trofs have fronts. Trofs without fronts are depicted with dashed lines on the map. Trof air is coming together from several directions and rising. Expect clouds and rain. A trof can become circular and form a low-pressure area called a cut-off low with very erratic winds and conditions that may vary widely from expected conditions. Much of trof conditions depends on the upper air conditions.

The weather systems of the U.S. flow west to east with any north south movements being caused by weather from Canada and the Gulf respectively. At trof that goes from south to north is an upper atmosphere trof. A trof that goes from south to north are called upper air ridges. Bad weather accompanies to cold air of an upper level trof. A ridge is a good flying weather signal with warm air.

Weather Items
1. What is a dry line when depicted on a surface analysis chart?
According to
Dry Line (Meteorology)
A boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains....

2. What is the J/KG value on the convective outlook (AC) Pg 4-41/2 in FAA Av Wx Services?
Enthalpy joule per kilogram J/kg m2/s2
ELHE (J/kg) = latent heat of vaporization at 0 C = 2500000
ELHS (J/kg) = latent heat of sublimation at 0 C = 2834000

3. Definition of Mist and Fog
--Mist requires surface visibility of 5/8 mile
--Fog requires surface visibility of less than 5/8 mile.

PATWAS stands for Pilot's Automatic Weather Telephone Answering Service. When you dial 1-800-WX BRIEF and choose to listen to the recorded observations and forecasts, PATWAS is what you are listening to.

Visible Moisture
Be it fog, haze, or cloud there must be a precise blending of ingredients for moisture in the air to become visible.  Every visible droplet will have a small hygroscopic particle inside. These particles are attractive to moisture and  are always present in air by the thousands per cubic inch. Relative humidity reaches 100 when the moisture-
laden particles become visible. A light wind and warm conditions will lower the relative humidity and make the air 'invisible'.

Radiation fog has many names such as ground fog, valley fog, tule fog, etc. This fog requires still air with a warm surface heating cooler air above. We have a surface inversion with saturated air (visible) below dry air above. In northern California this condition can make driving and flying especially dangerous when you can see well at windshield height but nothing of the road or runway.

It is not unusual to have good visibility just before dawn and find that the higher the sun gets the deeper becomes the fog. Such a condition can keep an airport zero-zero all day. Aircraft over the airport can see the runways but the slant visual range is below the mile required for SVFR. There is no burn-off of radiation fog. It evaporates when the surface warms sufficient to lower the relative humidity to 50 percent.

Advection fog is moved by wind from offshore over into land areas. It is a layer with a well-defined low ceiling and top. You can have a ceiling of 300' and ten-mile visibility. This fog will burn off. It is a sea fog or sea smoke that can cover vast areas of California. An approaching warm front or passing cold front will leave moisture looking for hygroscopic particles. When the dew point/temperature spr ead is within five you can expect haze and possibly fog. Haze has over 5/8ths mile visibility; fog has less than 5/8th visibility.

Where's the Turbulence?
More severe turbulence is experienced where the jet stream is centered about am even faster jet core. Jet cores are west of squall lines. The worst turbulence is on the west (cold) side of the jet core.


--Obstruction causes lee side, downwind turbulence

--Vertical currents under clouds and over hot surfaces

--Wind Shear
--Weather front, T-storms, microburst wind direction changes

--Clear Air Turbulence (CAT)
--Change in OAT at altitude near jet stream

--Things rock back and forth in place

--Things will slide

--Things will jump

--Loss of control, possible damage

--Less than 1/3 of time

--From 1/2 to 2/3 of time

--2/3 or more of the time

Controlling the Aircraft
--Auto-Pilot off
--Efforts of autopilot may break aircraft or autopilot
--Hand-fly the plane
--Trim slightly nose-up and hold thumb on yoke
--Reduce to Va

--Straight and level flight
--Ignore altitude changes, avoid banks or turns

--Rich mixture
--Avoid brief loss of power due to fuel flow

--Study wind flow
--Avoid Over-flights of terrain that might cause turbulence
--Get PIREPs
--Fly over water

--Watch outside air temperature
--Inversions have smooth air above
--Airliners anticipate CAT by OAT first

When is Turbulence?
(David B. Cole) wrote
Approximately 30 miles NW of the airport we suddenly descended to 2500. It was from there on out that I experienced the worst turbulence I had ever experienced in a small plane. My head hit the ceiling at least four times. I kept the airspeed below Va as the plane essentially did what it wanted. We rolled 25-30 degrees un-commanded on several occasions and once the sudden movement of the yoke nearly took my wrist with it.

This was the first time I could say that I was somewhat nervous in turbulence. The bigger boys going into Newark were reporting moderate turbulence and before switching to the tower my instructor reported
that we had severe turbulence. Yet he remained calm and somehow I still managed to keep altitude to within 100 feet, with the exception of a few excursions down 150 feet.

Just FYI - if you were managing to hold altitude within 100 ft most of the time, and the worst deviation was 150 ft, this was not severe turbulence. Severe turbulence is when unsecured objects are flying about the cabin; the airplane is often uncontrollable, etc. What you experienced was moderate turbulence. As a rough rule of thumb, if you are hitting your head on the ceiling, that's moderate turbulence. Anything less is light. When you can't keep your chart on your lap or the airplane on course and altitude, that's severe.

The Problem Continues
We were vectored for LOC Rwy 22 approach at CDW. I tuned and ID'ed the localizer, but was so whipped that I forgot to set the OBS head from GPS to Nav. When I finally figured this out, thanks to my instructor, the needle was centered and I still had a 30-degree intercept. To make a long story even longer, I eventually got on
course, although we were still getting smacked around fairly hard. The surface winds were from 260 at 12 kts, so we circled to Rwy 27, although I made the mistake of descending to the straight-in minimums.

The major hazard of moderate turbulence is fatigue, and fatigue is a recipe for pilot error. You made several, but of course that's what training is all about. I'm glad to see that your instructor pointed this out for you, but I'm going to offer you a slightly different perspective.

More Advice
It's all very well to say that when you recognize fatigue you should land, but the reality is you are eventually going to fly IFR fatigued, just like you have driven in bad weather fatigued. You will sit in some pilot lounge for several hours, being safety conscious and waiting for the really bad weather to pass. You will then decide that the weather isn't that bad anymore, and you're not that tired, and it will all be OK. Then you will find yourself being beaten up, tired, and needing to shoot an approach to get home - possibly with some equipment failure. Of course you could always decide to only fly for fun, and never fly when there's any pressure to be anywhere at any particular time - but then what's the point of the instrument rating? So assuming you are actually going to use the rating and stay current, you need to be prepared for the day you will have to use it when you are not at your best - without making the mistakes you did on that last approach. How do you get to that point? Practice, man, practice.

Why Moist Air is Less Dense 
---Water is a unique substance which we routinely encounter in all three states - gas, liquid and solid. 
---Each one has effect on how and when we fly. 
---The point of transformation of water into one form and back again provide pilots with the greatest challenge. 
---As a gas water affects air density, visibility and therefore, aircraft and human performance. 
---Adding water vapor to the air reduces its density. 
---78% of our atmosphere is Nitrogen (with an atomic weight of about 28). ---Another 21% is Oxygen (with a weight of 32). 
---Other gases amount to one percent. Carbon Dioxide (weight 44). 
---Water varies none to about 5%, and its weight is 18, 
---Water reduces the average weight of air.. 
---The lighter the air is, the faster a wing or propeller must move to get the same lift or thrust.. 
---As water vapor cools and forms clouds and fog, 
---As air warms, it can hold more water vapor. cooling the air raises the relative humidity 
---This cooling can come from a contact, radiation, a colder air, or as the result of the entire air mass rising.

Density Altitude in Brief
 ---Only a comparison of density altitude and aircraft performance makes takeoff safe. 
---Lean the mixture for density altitude takeoffs.(see POH procedure) ---Safer to make two half loads than one heavy load at high density altitude airports. 
---Density altitude conditions for multi-engine aircraft is even more critical.

Summer Flying in Brief 
---The number one accident factor is aviation is situational awareness ---Summer haze can hide storms and obstructions. 
---Over the desert morning flights are less turbulent than those in the afternoon. 
---Approach all ridges at a 45 and make shallow banks near aircraft service ceiling 
---Fill tanks full in the summer to reduce moisture forming in the fuel at night ---The higher you fly the more you should drink water

Learning to Live with Turbulence:
---You will NEVER learn to enjoy turbulence. I have in the near vicinity of my home airport an area where all the winds coming through the Golden Gate make their way to the great Central Valley.
I use this area between Benicia and Cordelia along a freeway to introduce and build a tolerance for turbulence.

While you will never ENJOY turbulence, you will find that a tight grip on the yoke will give your two jolts for every one caused by nature. Keep a 'feather' touch on the yoke and lift wings with the rudder. As I do, find a place where you can turn your turbulence on and off so as to build your tolerance.

I flew across the Sierra Mountains three times with different instructors before I ventured across by my self. Each time I insisted on going a different way to extend my field of options. I enjoy craps so my wife and I fly to Reno several times a year. In my lifetime I could have bought a casino. I now have flown to Las Vegas six or seven different ways. Every flight is has just enough excitement to make the flight memorable.

Your training flights usually have gone in a triangle in one direction. You will learn a great deal flying them backwards. Just this week I took an inexperienced pilot on successive night flights to six airports in the SFBay Area. The second flight was in the reverse direction but done with more student confidence. Last night we flew to six uncontrolled airports in the Central Valley. Next flight would be in the Sierra foothills. The three hours of FAA minimum time is not enough.

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Continued on 5.538 Avoiding Icing and Thunderstorm