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Weather and Wind
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National Weather Service; Item on Weather; Getting the Weather; Becoming Weather Wise; Static Electricity: ...Making Weather; ...Weather Stability; ...Mesoscale Weather; ...California Weather; ...Air Mass Weather; ...Fronts; Warm Front; Cold Front; Occluded Front; Stationary Front; ...Weather Demonstration; ...Inversions; ...Pressure; ...Moisture and Weather; Wind Indicators; ...Natural Wind Indicators; Wind Shear; ...LLWAS; ...Mountain waves; ...Jet Stream; ...California Surface Winds; ...Headwinds; Relative Wind; The Weather She is A'changing; Weather Advice; Opinion; Weather Aloft; Weather Notes; ...Reading Weather Winds; Whether to Fly in the Weather; The Smog Layer; ...PIREP Wind; ...Deteriorating Weather Lesson; ...Weather Web Sites; ...

National Weather Service
1. Convective Outlooks (AC) Use connect-the-dots method to define area.
2. Severe Weather Outlook Charts (graphic of AC) also called Convective Outlook charts use slgt, mdt, high to describe area weather. The charts connect the dots.
3. Radar Summary charts (snapshots of past weather and rain clouds can extend much farther.)
4. SIGMETs (WS) Severe turbulence, icing, wind shear when first detected and then 55 minutes after every hour. Uses connect-the-dots method to define area.
5. Convective SIGMETs (WSTs) Significant meteorological information of severe, embedded, and lines or 40% coverage of Level 4
6. Center Weather Advisories (CWAs) ATC frequency will give advance warning of severe weather.
7. Radar Reports (Rareps)locates weather by direction and distance 060/075; NC =no change; MT=maximum tops; +/+= strong storms increasing intensity; LN=line; A=area; C=cell;
8. Terminal Aerodrome Forecasts (TAFs) Airport forecasts every 8-hours mention adverse weather and thunderstorms
9. Area forecasts (FAs)
10. Hourly Surface Observations (METARs) from airports with weather observers who mention storm activity.
11. Thunderstorm Forecasts
12. Lifted Index is measure of temperature at altitude; K index is measure of moisture and saturation. As a fraction the lower the fraction the greater likelihood of severe weather.
13. Composite Moisture and Stability Chart
14. Severe Weather Watch Bulletins (WWs)
15. PIREPs

Item on Weather
New weather site identifier codes are now in effect. Many remain the same.

Checking the AWC Web site now counts as a legal weather briefing according to FAA

39 U.S. Towers are are getting what it takes to predict local weather

Getting the Weather
As a pilot you want first of all to get an overview of the big picture. Begin with the upper-air forecast charts such as the 500-millibar chart, which is about halfway up the inch-square air column or 18,000 feet. This chart shows the various pressure centers, troughs, ridges and winds. By becoming familiar with these charts and the accompanying weather the pilot can begin to anticipate and interpret weather.

Once you see the relationship of the 500-millibar chart and surface weather you are better able to understand and orient yourself to weather hazards. Additional information comes available through the infrared and water vapor. These visual images show the details of visible weather systems. Infrared works at night as well as day. Interpretation requires practice. Only by knowing where a storm is expected to go can you even make preliminary flight plans. Doppler systems (Nextrad) is giving us a forward step in this weather process.

Becoming Weather Wise
For a beginning a pilot should make a practice of watching the evening TV weather and the early morning weather channel. Next, by using DUAT or calling a Flight Service Station (FSS) available nation wide at 1-800-WX-BRIEF you can obtain a briefing. These can help you meet the FAR 91.l03 preflight requirement of having all available information as part of the preflight. When below freezing level temperatures are encountered an important part of the preflight is to assure that the oil breather tube is not ice-clogged. Additionally, when cold weather arrives, the grease in the throttle wire cable can become so congealed that movement is restricted. Do not takeoff in an aircraft where throttle movement is restricted in this manner. Climbing into still colder weather may prevent any movement.

As a minimum a pilot needs to know, for local flights, airport conditions, fog prediction, freezing level, turbulence, local notams, and winds at 3000. Weather briefings are rapidly becoming less available except through computerized access by the pilot.

Radar returns are based only on water. Wet is not always convective. Water is not a thunderstorm until it includes vertical development and Vertically developed clouds can contain severe turbulence and icing. Low clouds with flat tops usually do not have turbulence or icing. Virga is indicative of evaporation cooling of the air, which accelerates downward as microbursts. Virga is most dangerous when surface temperature is warm, winds are light, and dew point spread exceeds 35 degrees.

Static Electricity
Static electricity is a common phenomenon. It is not dangerous unless it builds enough to produce a spark. It is invisible and silent medium that causes many aircraft fires. Susceptibility to static electricity exists when fuel is being
poured between plane and any ungrounded to plane fueling source. Plastic fuel cans and plastic funnels are the most common sources. Even carrying plastic fuel cans in plastic truck beds is a potential five hazard. Do not drain fuel into plastic buckets.

Making Weather
Atmosphere has three levels. Troposphere extends from sea level up to 35,000 feet around the poles and to 65,000 feet at the equator. The standard temperature lapse rate of the troposphere is 2-degrees C per 1000 feet of altitude. On top of the troposphere is a thin layer called the tropopause, which acts as a moisture barrier. The top of the troposphere is the bottom of the atmosphere. The 850-millibar air pressure level will be lower in true altitude than warmer air. All air pressure is the measure of the weight of air above. The stratosphere is next extending to 22 miles high and because of the tropopause barrier has no moisture and a constant temperature of -55-degrees C. 50% of the world's atmospheric weight is below 18,000 feet.

Consider a rotating spherical envelope. A mixture of gases-occasionally murky and always somewhat viscous. Place it around an astronomical object nearly 8000 miles in diameter. Tilt the whole system back and forth with respect to its source of heat and light. Freeze it at the poles of its axis of rotation and intensely heat it in the middle. Cover most of the surface of the sphere with a liquid that continually feeds moisture into the atmosphere. Subject the whole to tidal forces induced by the sun and a captive satellite. Then try to predict the conditions of one small portion of that atmosphere for a period of one to several days in advance.

A pilot must learn to be skeptical about what the weather is for now, what it is forecast to be, and what he hopes it to become. The entire process is so capricious that even in 1997 the gigabite computers can't keep up.

All of the factors in the foregoing paragraph are factors in making weather but only three main factors, heat, water, and wind are of major importance. There are three kinds of heat transfer in the atmosphere:
--Convection is that which causes thunderstorms.
--Radiation is that which causes ground fog
--Conduction is that which causes you to drop a nail held by the fingers when you are heating the other end.

Weather Stability:
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 are 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.

Mesoscale Weather
Small area weather conditions. These are the weather that occurs in the valleys around the bay area. Each small area will have weather distinct from its neighbors. This is very typical of California coastal regions

California Weather
Ocean water temperatures determine the weather world wide but most dramatically in California. Warm water in the Gulf Stream (El Nino) causes precipitation; cold water in the stream causes stratus and fog. California fog requires cold water and warmer air. This creates an inversion where the colder air is below the warmer. The temperature increases (instead of usual decrease) with altitude.

The cold Alaska Current follows the western coast of North America and increases the moisture content of the air in the Pacific High while decreasing its temperature. FOG. This California fog would stay along the coast were it not for the warm interior valleys and desert of California. The heat from the deserts causes low pressures from rising air, which is very attractive to the air in the Pacific High. The result is a strong surface pressure gradient creating winds that carry the fog ashore.

Bay area fog, were it not for occasional fronts, follows a four day cycle of the fog coming on shore, into the bay, into the east bay and into the Central Valley. Mesoscale weather will exist in the various areas both during the four-day fog arrival and during the four-day retreat back to sea.

An air mass extends for over 1,000 miles. The air mass acquires its properties from the surface below. North America has three air mass areas, Arctic, Polar and Tropic. Thus air mass types are polar and tropical with sub-classes of maritime or continental. The formation of air masses is seasonal. Air masses change as they move. Where two air masses meet we have a frontal zone where the colder mass moves below the warmer. The air mass may be far ahead or far behind the surface front. Worst weather is when fronts move fastest.

Cold fronts are usually in the winter and the passage is shown by clearing, wind shifts, a rise in pressure and colder temperature. The extent of clouds depends on the stability of the air.

Warm Front
A warm air mass shows several days ahead of the front with a slow progression (less than 20-kph) of very high clouds as it overrides relatively cooler air below. First comes cirrus, lowering to cirrostratus, becoming still lower altostratus which changes to alto cumulus. Finally comes nimbo stratus with continuous light rain or snow. Imbedded thunderstorms. Visibility goes below minimums. Freezing rain occurs in winter. Frontal passage gives good flying for a few days. Warm and stationary worst weather.

There is a variety of warm and cold fronts. The main difference being stability. Stable air masses can have weather dependent on the moisture available. Stable warm air masses can have clouds to 25,000' with icing.

Cold Front:
A cold air mass appears quickly at over 20-kph. Any cirrus becomes altostratus and altocumulus changing quickly into stratocumulus. When the front hits strong gusty shifting winds accompanied by nimbostratus or cumulus. Produces summer thunderstorms and squall lines in front. Heavy bursts of localized rain can cause flooding. Passes quickly and leaves two days of cumulus clouds for bumpy flying below. Cold fronts change when they cross mountain ranges. Pacific cold fronts are moist before the Sierras, dry afterwards. Across the Rockies it picks up moisture from the Gulf and will intensify. Dry cold fronts will be bumpy but VFR.

Cold fronts are areas of lowest pressure and strongest winds. Wind coming over water carries moisture. Wind direction and velocity can vary dew point spread. If you stand with your back to wind point to the area of low pressure. This tells you where the fronts are located. Winds over water or blowing up slope are likely to worsen weather conditions. When forecast winds are in error then the entire weather forecast is in error. Westerly winds bring better weather. Southerly winds bring worse weather. Surface winds 100 miles to the west are likely to arrive at your location in two hours. The pilot who makes note of frequent changes in altimeter settings is in an area of strong winds.

The cold air of the world descends to the surface near the poles. Moving southward it deflects to the right. This polar front and its associated low, the Aleutian low cause much of the U. S. weather. The Aleutian Current is a north to south cold current that causes the weather along the Pacific coast.

Three elements mix to form thunderstorms, excess moisture, a lifting force, and unstable air. Thunderstorms are forecast in the area forecast (FA) and terminal forecasts (TAFs). Any time the lifting index has a negative number you can expect thunderstorms. Any thunderstorms upwind of your flight with the three elements present indicates a no-fly zone. Fly to the bright spot and blue. What your eye sees is just as valid as radar.

Thunderstorms are either air mass which are isolated and easy to avoid or severe. Severe thunderstorms have hail, 50-knot winds with possible tornadoes. Avoid flying on the upwind side and the north side of storm lines if given a choice.

Turbulence beyond the pilot's and aircraft's capability is to be expected in or near a thunderstorm. PIREPs are a pilot's best friend in turbulence since it is real time information. Avoid any reported moderate or higher level of turbulence. Proper interpretation of PIREPS, those real time and position reports by flying people, is critical to safe weather flying.

Occluded Front:

Occurs when faster cold front catches and winds around a warm front. Worst of both systems. Stratocumulus and cumulonimbus clouds lower slowly but clear quickly. If a low-pressure area is gaining strength rapidly, it will slow down. The fronts will rotate around the low. The cold front will overtake the warm front and result in an occlusion.

Stationary Fronts:
Sometimes spring fronts blend together without any clearing and bring days or a week of weather having nothing but low levels of fog, haze and smog. Following cold fronts are needed to clear out stationary fronts.

In the winter where cold air is under the warm layer and the temperature difference is over 10 C, forecasts of improving weather are inaccurate. The conditions are too stable to make a change likely.

Weather Demonstration
Place pencil horizontally between praying hands out in front of you. Left hand is northern cold air and the right is southern warm air. The low exists between your hands. Move left palm down to roll the pencil. The pencil rolls cyclonically to the left.

The cyclone draws warm air down from the south and sends it above the sinking cold air. The cold front moves to the southwest and a warm front forms east. The bigger the cyclone low the stronger the weather.

Any time the air temperature is warmer above than the air below is called an inversion. An inversion may be surface based or an inversion aloft. Inversions can be the cause of poor visibility and turbulence. Inversions are most common in late spring and early fall.

Surface inversions begin at sunset when the earth's surface temperature cools rapidly on a cloudless clear night. This allows the air close to the surface to cool while the air only a few feet higher remains warm. This condition can last till mid-morning. The condition is most likely to exist when winds are very light and unable to cause a mixing of the air layers. The heat from a rising sun will increase the mixing of the layers and eliminate the inversions. In regions like Alaska such inversions can be dangerous to aircraft due to frost forming on the aircraft.

The inversion aloft is caused when air masses flow in such a way as to cause the warm air to pass over the cold air. If the upper layer of air is also descending its increased pressure will cause further warming. This is a common summer condition on the west coast of the U. S. This is also called a subsidence inversion. This condition causes the stratus (layered) clouds of avection fog, smog layers, and reduced visibility.

Inversions exist by providing warmer air aloft than exists in air at lower altitudes. The temperature lapse rate is upside down. An inversion prevents vertical air motion. Large, slow-moving high-pressure systems are a common source of inversions. The top of an inversion layer is flat.

There are four pilot clues to the existence of an inversion:
1. Cloud formations can occur with as much as a 6 degree dew point spread.
2. An illusion in inversion visibility conditions makes things seem further away.
3. There is an increased possibility of low level wind shear.
4. The air will be very stable and give smooth flight above the inversion of either type.

High pressure has a clockwise down spiraling cold air mass giving good flying weather. This is generally true about highs moving in from the Pacific Ocean but not true if they come down from Canada. Where it comes from is important. The Bermuda High brings days of poor visibility to the Eastern U. S.

Low-pressure areas are counterclockwise upward spiraling systems that bring bad weather. The Low draws in moist air and forms spiraling cloud formations of low cumulo or stratocumulus rain clouds. The cold front formed by a Low is {,} comma shaped.

Flying from warm air into cold air is effectively flying from high to low pressure. The effect in a standard lapse rate is 35' for every degree Celsius. Every three degrees colder with a constant altimeter setting and altitude effectively puts you 100 feet lower.

As altitude increases above sea level the pressure of the air decreases at a rate of two pounds per square inch for every 2000' below 18,000'. This pressure altitude in used to determine oxygen requirements for humans. When the temperature change (lapse rate) is greater than standard the density altitude will be higher than the pressure altitude. This affects aircraft operation by decreasing power, lift, and thrust.

The barometer is a lineal scale used for weighing the air on a square inch of the earth surface. The aneroid barometer uses a closed bellows and a circular scale to do the same thing. The barometer that uses the weight of air to measure altitude above the earth is called an altimeter. The standard setter for all forms of barometers is the mercury barometer, which reads in Hg on a scale in which one inch of mercury movement in a tube is equivalent weight of one thousand feet of air if in the same tube. This change is weight is known as the lapse rate.

At sea level the weight of the air column is 14.7 pounds per square inch or the air pressure required to raise the mercury column to 29.92 inches or 1013.2 millibars. For aeronautical purposes measures are varied through the use of a kollsman window which computes weight from mean sea level (msl) without regard to terrain. As daily air pressures vary the altimeter is re-set to read changes due to the adjusted lapse rate.

By FAR you must have your altimeter set from a figure given within 100 n.m. in a worse case situation. Fact is, settings should be obtained from the closest source available. All radar facilities are required to give this setting at least once during any continuous contact with an aircraft. This setting can be confirmed + 300 feet just be having the pilot read back his altitude.

From a weather viewpoint changes in air pressure are always significant. The pressure trend that is to lower pressure means that air is being drawn in along with moisture that at will at some point condense into visible moisture. Rising air pressure reverses this process. These pressure changes in the U.S. mean that the rise and fall will cause fronts to move across the country. On average, one front crosses a given point once a week in summer and two or three such passages in the winter. Warm or cold in the passage of the fronts is always relative to the front preceding. Cold fronts move faster than warm fronts and tend to wrap-up on them when they touch. The wrapped area is known as an occluded front, which usually has the worst aspects of both the warm and cold fronts. It would not be nature's way to have the best qualities survive as offspring.

Moisture and Weather
The movement of moisture is reminiscent of a multiple juggling act where the three forms of ice take turns going through six different stages of transformation. Vapor can condense into liquid and sublimate into ice while liquid can evaporate back into vapor and the ice can sublimate into vapor. The ice that sublimated from vapor can melt into liquid and be created by freezing liquid. This entire process can be diagramed as the three forms in a line with a large circle connecting vapor and ice on the outside while liquid in the center is connected by two smaller circles one to vapor and the other to ice. The required words along the arcs of the circles are only sublimation twice hot and cold from vapor to ice and ice to vapor. Liquid and vapor are connected by the hot condensation arc and the cold evaporation arc. Liquid and ice are connected by the hot freezing and the cold melting. With a little effort the diagram can be made.

In the diagram we have the three phases of moisture gas, liquid and solid. All are forms of water. The forms are determined by the speeds at which the molecules move. The molecular speeds are determined by temperatures. Air is saturated when for every molecule evaporated another is condenses. The relative humidity is 100%. When dew point rises above 60 degrees we have a condition of high humidity. The dewpoint is the temperature where dew forms. When the air temperature and dew point are close together contaminants in the air facilitate the formation of liquid molecules which form visual obscuration.

The form changes of water while being caused by temperature changes, in turn cause temperature changes.
Each form change there is an exchange of heat. Every storm is a heat engine during which the change in water form either releases or collects heat.

Wind Indicators
Wind often is very stressful to the pilot. Takeoffs and landing techniques require greater skill under the influence of wind. Even the light and variable wind adds complexities that are difficult to plan for. Having specific wind information can either add to or subtract from a pilot's stress.

Runway selection is primarily based on wind direction except where noise abatement rules prevail. With experience a pilot learns to read winds from water, dust, DME, GPS, the windsock and weather forecasts. Regarding the latter, one thing you can be certain of is that the winds will NOT be as forecast. It is not a good idea to read the wind from a tetrahedron. Most tetrahedrons have locks that allow them to be positioned for the preferred runway without regard to wind direction. The windsock is the best wind indicator at an airport and should be noted on downwind and especially on final. What you see on downwind will enable you to make a wind adjusted pattern. A disproportionate number of landing accidents are caused by a pilot's failure to adjust the pattern for wind direction and velocity.

The FAA wind cone (sock) can be of several colors but usually orange and either 8 or 12 feet long. Windsocks are fully extended in 15-knot winds. I make a practice of having student's practice reading windsocks by comparing ATIS or tower wind directions and velocities with their guesses from the windsock. At one time Concord, CA had five windsocks. It was not unusual to have them all showing different direction and velocity.

A weather related accident is most likely to be blamed on the wind. Wind does have a lot to do with your flying. The unequal heating of this planet's mixed surfaces of land and water causes wind. The time of the year and the weather is related to wind direction and velocity. The world is not heated evenly as it moves around the sun, changes distance, leans, and turns. The uneven heating causes the movement of air masses seeking equilibrium. This uneven heating also causes pressure differences and pressure gradient forces. This causes wind. The greater the pressure differences in you altimeter setting the stronger will be the winds. The warmth of the earth causes vertical movement of air.

Local small-scale winds are caused by the flow of air to replace the air that is rising. Water instead of absorbing heat reflects most of it. Land absorbs instead of reflecting. The hotter land causes rising air thermals. When the sun goes down the land gives back to the atmosphere much of the heat acquired during the day. Land makes this change at five times the efficiency of water. At night the water is warmer than the land and the flow of the wind reverses.

Where the pressure isobars bend the effect of centrifugal force is greatest. This is the force the wind has upon itself due to differing pressure gradients. The rotation of the earth also creates a force against the air surrounding the earth. This coriollis effect causes the winds in our hemisphere to be deflected right. This coriollis effect deflects anything projected into space. The winds of the southern hemisphere are deflected left. This effect is weakest at the equator, which explains the absence of extreme highs and lows there.

Gustave Coriollis mathematically described the effect of the earth's rotation on moving objects in the atmosphere. The effect begins at the equator a zero and increases toward the poles. The effect also increases as the wind speed increases.

Winds should flow parallel to isobars but frictional force affects the flow of air below a couple thousand feet. Wind strength increases above this level. This frictional force makes the wind turn across the isobars into the low at a 30-degree angle. It is only above the frictional layer that the winds parallel the isobars.

When conditions indicate strong wind conditions take an early look at the Va for your aircraft and its present weight. The higher winds are the slower you should go. This is usually done beginning with the Va for gross weight and find the percentage you are under that weight. Easy to do on the E6B or on paper by making the following equation.. (One way to do it.) Make a fraction with your 'actual weight as the numerator over the gross weight as the denominator equals N over 100. Multiply the diagonal numbers actual weight by 100 and divide by gross weight. This is the percentage of actual weight to the 100% of gross weight. Subtract the two percentages to find the difference. Divide this difference by three and using the answer as knots use it as the amount to reduce your Va speed. Example: If you find that you are 10% below gross then you would lower your Va at gross speed by three knots. Not at complicated as it seems once you do it several times.

A mountain wave can be dry and cloudless with as much turbulence as if a visible cloud exists. Winds of 25-knots perpendicular to mountain ridges with increasing velocity at higher altitudes will contain such waves. Wide areas of weather instability are the best souses of mountain waves. The same wave system can last for a couple of days. Waves can occur over low hills if the winds are strong enough.

A Foehn wind (Chinook) are warm dry winds descending off mountains that can melt two feet of snow in a day. They are as dangerous as mountain waves. Any encounter with a downdraft demands that the pilot be prepared to do a 180 or have sufficient excess altitude to allow him to 'dive out' of the situation. This can be considered mountain drainage winds, which have thunderstorm like gust fronts.

These winds can be microscale weather involving less than one hour and a very few miles. Pilots flying in such conditions need to know the warning signs of markedly different barometric readings of neighboring mountain airports. Rotor clouds and unsteady lenticulars in a line serve as a warning of such conditions. Virga usually indicates extreme turbulence. These conditions may be of very short duration about mid-morning. The cloud type tells the kind of turbulence to expect.

Flying in turbulence is best done with a very light yoke and lots of rudder. Bring up a dropped wing with rudder not yoke. Rudder use otherwise must be coordinated and in choppy conditions do a rudder dance. If turbulence causes pitching of the nose, make major correction with power changes. I have found it better to put in the initial power change I feel necessary and then take half of the change off immediately the correction begins to occur. Use elevator to adjust for altitude excursions. When the aircraft acts like a bronco or a canoe counter primarily with rudder input and gentle yoke pressures.

NTSB reports that up to 40% of all weather related light aircraft accidents occur where the pilot cannot cope with the winds near the surface. The best way of avoiding adverse winds is just to stay on the ground. If airborne, barring engine failure, landings are optional. If the winds contribute to a bad approach make the easy choice of throwing in your hand and going around. A given airplane accident is the final result of decisions made by pilots. When an accident occurs that can be attributed to poor decision-making it is difficult to find why an otherwise cautious and competent pilot would do such a thing.

Natural Wind Indicators
Calm 1 mph vertical smoke, water like mirror
Light air 1-3 drifting smoke, ripples
Light breeze 4-7 wind felt, leaves rustle, vane moves wavelets, crests do not break
Gentle breeze 8-12 leaves always move, light flags extend large wavelets, crests break, foam,
scattered caps
Moderate breeze 13-18 branches move, dust and paper raised, small longer waves, frequent caps
Fresh breeze 19-24 trees sway, crested wavelets on lakes, moderate waves, whitecaps, spray
Strong breeze 25-31 large branches move, wires whistle, large waves, spray white foam crests
Moderate gale 32-38 whole trees move, heaping waves seas, white waves breaks in windblown streaks

Wind Shear
Low level wind shear alert system (LLWAS) is a predictive system when wind changes of 20 knots are measured. Will give a 90-second warning of a microburst ahead. By carrying extra speed into such conditions you can trade speed for altitude. Do not bank and do not change configuration

Invisible atmospheric phenomenon can be present on all sides of the thunderstorm, in the downdraft under the cell, and in a gust front which can precede a thunderstorm by 15 miles or more. Wind shear is a local variation, at a given time, of wind velocity with distance. It is measured by dividing two-velocity variation by the distance between them. A wind speed difference of 50 knots over one mile would be a shear of 50 knots per mile. Wind shear is caused by thermal eddies near the ground or by strong winds crossing rough terrain.

A rapid agitation and whirl of air which changes or varies wind velocity by 15 knots, or a change of vertical velocity of 500 fpm or a change in direction. Wind shear includes lateral shear, rotor winds, dust devils, macro bursts and micro bursts. Vertical velocities of over 4000 fpm and wind speed changes over 90 knots have been recorded. Between 1% and 2% of all thunderstorms create severe wind shear.

Wind  shear can be a problem for all aircraft and becomes a problem for light aircraft especially when encountered when operating at full power with no reserve performance available as for climb or takeoff. Unlike jets, the propeller aircraft gets a rapid response from power applications so where reserve performance is available the effects of wind shear can be corrected. You can expect wind velocity to decrease as you get closer to the ground. The wind direction will shift clockwise at the same time.

Majority of wind shear accidents occur in Colorado and California. A thunderstorm need not be close by. Even distance storms or virga can cause potential danger. Moisture is not a necessary factor. The best technique is avoidance. Be aware that wind shear is likely in the vicinity of virga, convective activity in formation of thunderstorms and frontal passage. About ten "wind shear accidents" occur every year with 9 of these light aircraft. The area of greatest danger for wind sheer is when landing or taking off.

The winds will vary in both direction and speed with altitude. The landing pilot must be prepared to have higher velocities and different directions even from pattern altitude to touchdown. ATIS winds are taken about twelve feet above the surface by a multi-cup wind turbine called an anemometer. This means that all winds above and below this height will be of a different velocity and direction. The closer to the ground the more to the right will be the wind direction and of less velocity due to earth friction. Thus running out of rudder authority at 500 feet above the airport means that some of that authority will be regained at ground level.

With the advent of the Low Level wind shear Alert System (LLWAS) and Terminal Doppler Weather Radar (TDWR) you can expect at larger busy airports to receive wind shear alert notices on the ATIS. Along with the previous aircraft's PIREP you have been properly advised.

You know it is there, but not where or how long or strong. Wind shear conditions involve a low altimeter setting, rain, cool weather, and gusty 20 to 40-knot winds and variable directions. Any pilot would be ready for an instant abort. Any rapid change in decent, speed is sufficient reason to initiate the go around. Light aircraft should be in a clean configuration to begin with and you should not fixate on any single instrument until the VSI has a clear constant indication of a climb. While in the tailwind wind shear, power will not increase speed nor stop descent. Should you first experience the increased performance headwind side of the wind shear be aware that the tailwind effect is yet to come. Get all the altitude you can because you will quickly use it up.

Wind shear is change of wind speed or direction in a short time. When shear exceeds ability of plane to

accelerate or decelerate its affects performance.

Low level wind shear alert system has some defects.
Inversions can affect the operation of the anemometers.
Radios may be so busy that pilots don't understand warnings
Instruments fail to give warning
Placement of anemometers allow errors to occur
Not enough anemometers on approach corridor

Mountain Waves
To have a mountain wave, you need a mountain obstruction to the airflow of 25 knots or more at an angle within 30 degrees perpendicular to the obstruction. The wind velocity should increase with height and exist in a stable air mass. The smooth updraft on the windward side gives severe turbulence on the lee side. Turbulence can occur below lenticular clouds. Always cross ridges at 2000' AGL and remember at 10000' you have lost 30% of your engine power. To get out, terrain permitting turn downwind. In wave conditions, a mile one way or the other can make a big difference. Actually there are two different mountain waves, one that is smoothly flowing and another that is breaking much as would an ocean wave be smooth before breaking.

Three types of clouds come with the mountain wave which comes with strong winds over jagged terrain. The basic cause is large differences in ressure in a stable air mass across a mountain range. The mountains force the air by orographic lifting to higher altitudes. The stable air will go lower once past the mountain and may be forced into a series of cycles much like a wave. The wave can extend for hundreds of miles with the top of each cycle indicated by a lenticular cloud.

During January and February pay particular attention to weather reports of rapidly falling pressure, temperature changes across mountains, winds over thirty knots and strong surface winds. Oddly, a difference of a couple of miles can make significant difference in the mountain wave effect. The problem is, which couple of miles.

Strong winds perpendicular to mountain ridges generate altocumulus standing lenticular clouds (ACSL) which are lens shaped and stationary on the lee side of the ridge. Below the ridge on the lee side can be rotor clouds but the roll may or may not be visible. A rotor is capable of crashing an airliner. An unusual cloud called the Kelvin-Helmholtz can be a part of the mountain wave sequence.. Kelvin-Helmholtz clouds look like multiple breaking waves. Flight in the vicinity of Kelvin-Helmholtz clouds will cause multiple excursions in altitude and airspeed. Advise ATC when this is happening and request a block altitude that will allow these deviations.

The jet stream is a belt of high altitude winds just at the tropopause level. The easterly wind speeds increase in winter and shifts from the poles toward the mid-latitudes. Wind shear and turbulence is common on the northern side of the stream. The speed is caused by temperature differences. The presence of a jet stream is best detected by temperature. Wind must be over 50 knots and can be up to 200 knots.

The jet stream was actually discovered and became a factor in flying in WWII. B-29s on high altitude bombing missions over Japan found that the bombsights were often unable to cope with both the slow and fast ground speeds being encountered by bombers flying in the jet stream. The great inaccuracies that were occurring prompted the low level fire bombing that resulted in the collapse of Japan.

California Surface Winds
42% of accidents, mostly while landing, are caused by wind factors. Most non-winter California winds are near calm until the sun factor kicks in about 10 a.m. The winds continue until near sunset as which time as the sun factor decreases so do the winds. These diurnal (daily) conditions are most constant in the coastal areas.

California valley winds are like other diurnal winds. They are controlled by the cycles of heating and cooling via the sun. Winds go up the valley during heating and down the valley during cooling. Knowing this you can make an early decision as to probable runways. In most accidents, poor judgment, not strong winds, seems to be the biggest problem. A majority of wind shear accidents occur in Colorado and California. Thunder storm need not be close by. Even distant storms or virga can cause potential danger.

For all the years of my flying my wife has contended that I always fly into headwinds. Once again a greater wisdom proves her right. A headwind is determined by the difference between your true airspeed and your ground speed regardless of direction. The airplane, once airborne, does not 'know' there is a wind.

There will always be more headwinds than tailwinds because a portion of the near direct crosswinds will require the aircraft to crab into those winds to track the desired course. Thus, the fact that there is a crosswind the reduction in ground speed to obtain the desired course results from a headwind component. The faster you fly into this crosswind the better but there will always be that residual headwind component.

Relative wind

---Any wind created by motion will act opposite to the direction of motion.
The relative wind is not always where the nose points as in slip or skid
---The stall is caused by angle of attack not lack of relative wind

The Weather She Is A'changing
Weather forecasting is making huge advances and disseminating the information to pilots is not keeping up. Pilots need to know where the moisture and instability is. The weathermen locate the moisture and find where movement toward instability makes weather of interest to pilots. The critical element is the timing involved.

Weather cannot be forecast in spaces less than fifty miles across and airplanes take time to cover that distance. Before, during, and after weather can be quite different while the pilot passes through this much space. The timing of the forecast and the pilots passage coincide only incidentally.

The best sources of moisture and instability come from the some 700 balloons that are sent aloft every day from widely scattered places in the U.S. It is the paucity of soundings that places pilots at risk. The voids can only be filled by PIREPS, ASOS> AWOS and satellites. Now mathematics and statistical theory are available to help the weathermen more accurately predict the meeting times of moisture and instability. This system is called the Rapid Update Cycle model and allows three-hour updates through the use of sounding simulations.

The charts and graphs from these simulations will soon become an important part of a pilots preflight planning. When making a trip you can go to:
use the directions and click on Skew-T diagrams and go to your local or enroute AWOS or
ASOS sites and get forecast soundings for your time enroute. By comparing the dewpoint of the clouds, winds and temperatures you can determine their predictive accuracy with what occurs as you fly. 

Weather Advice
(Roger Halstead)
When it comes to inadvertent VFR into VMC Too many pilots think it's pretty much "If I'm careful it won't happen, no it can't happen to me!" Unless a pilot makes it a practice to stay on the ground unless it's very low humidity, calm, and clear there is a very high chance that pilot will some day have some hair-raising stories to tell.

It happens along the coasts, it happens in the Midwest, it happens in the great lakes and it happens in the mountains. And there is a *lot* more to staying out of the clouds than just getting a good briefing and watching the clouds ahead with the idea that you can always turn around.

I've had weather close in from ahead, from behind, from above, and from below. The weather can be sneaky, or it can be quicker than any thing most of us around here fly. There are a number of things to remember about the weather if you wish to stay out of trouble. The instrument rating is a great place to start, but it comes with no guarantees and is no substitute for good judgment, nor is it a cure for bad judgment.

First, remember that the weather is always changing whether for better or worse is often up for debate.

Second, remember that although the science of weather prediction has come a long ways, it has a long ways to go.

Third, Remember that the most current conditions come from others flying around out there too. OH! 3.B: Automated and computerized weather stations will be quite happy to lie to you.

Fourth, the weather as based on those Pilot Reports (PIREPS) is a whole lot more accurate than the briefing you may have had an hour or two...or more back. So it does pay to learn a few things about how the weather behaves where you are flying.

There are just two kinds of weather patterns. Normal and not normal!

We are just heading into a season where, particularly East of the Mississippi, a front can move through, stop, back up and take a whack at the stuff still standing that it missed the first two times through. Nor is this unique to the country East of the Mississippi. Just because a front moved through an hour or two ago, does not mean that it will keep going. Particularly in Spring and again in Fall, but it's not unheard of for them to put it in reverse most any time of the year. Now, for those who thing they will always be able to turn around I can site one specific case.

Fifth: Remember storms can form in place! Sometimes a small place and sometimes they cover thousands of square miles. A couple of years back, I was checking the weather on a beautiful clear, warm summer day. Not a cloud in the sky and nothing on RADAR. It was humid, but there was a wide dewpoint/temperature spread so I grabbed my DUATS briefing and headed for the car.

I was getting ready to start the car, but something seemed strange. It looked like close to 30% cloud cover, yet less than 5 minutes prior it was clear. I decided to go check the RADAR again. There were now clouds over a wide area and what looked like a tiny thunderstorm clear over by Grand Rapids...(90 miles SW). I decided to wait for one more update on the RADAR.

Low and behold in less than 2 minutes that storm was over 30 miles across. five minutes later I was operating a weather net on the local 2-meter repeater and that storm covered an area about 150 miles SW through NE
and about 60-80 miles from north to south. It then sat there and beat the crap out of us for a good hour or more with a whole bunch of level 4s and 5s. Remember that entire line formed in about 5 minutes. That's a long way to go with a 150, 172, or Cherokee. *Any one* near the center of that line in a small plane would have been in deep doggie do as it formed over an area too fast for some one to fly out. To top it off when they form like that you really don't know if you are flying out or in.

Sixth: *Always*, and I emphasize that always keep one eye over your shoulder. Some years back my wife and I headed for home from Zypher Hills Florida. It was a tad hazy, but the ceiling was over 2000. So we headed north and hit rain by Dade City. That's a whole ten miles. So it was back the ZH for an hour and then another try.

This time we headed over toward Tampa Bay Exec. Not too bad. It looked a little stinky right in the Tampa bay area, but exec which is NE looked pretty good . So...we started up the coast. After all it was only about 30 miles to sunshine. Unfortunately we were down to counting gaiters before we made the sunshine and although I'd been keeping an eye over my shoulder, it was a lot thicker back there than I'd thought. I headed back for Tampa bay Exec just as fast as I could go. We got out of there around 1:00 PM the next afternoon.

Seventh: Keep aware of your surroundings If you ever get caught up at twilight and the temperature/dew point spread is small you may get to experience one of those adrenaline rushes first hand. The entire landscape can be hidden in clouds faster than you can say "Look!". The fog formation can follow the terminator if conditions are just right. That is as fast as you can point to the eastern horizon and swing your arm to point at the western Horizon. It's like waving a wand to watch the fog form. Remember that for fog to form it has to give off some heat. *Sometimes* that heat is just enough to cause the fog to clear for five or ten minutes. Just don't bet your life on it.

Eighth: Be flexible! Don't be afraid to wait to take off, or to set down and wait awhile if you are already up there. It's a whole lot cheaper to stay over night, or have someone come and get you than it is for the insurance company to take care of the survivors.

Then there is the wind! It shows up when un-forecast and sometimes isn't there when you expected to get another hundred miles out of that tank of gas. And...sometimes it's there as a head wind instead, when you really, really did need that extra hundred miles.

You may find convective activity, or even fog when FSS is still reporting clear. Particularly in the great plains, or western areas it's a long, long ways between RADAR and reporting stations. There are many areas (including
valleys out east) where they can not see even as low as 5000 AGL. It usually does blow in the right direction, sooner-or-later, but just how close to the forecast time is still a crapshoot. Although....A front only has to move a tad, north, or south, or it can be early or late and have the wind 180 degrees to what was expected.

Some times the only way to keep from getting the snot beat out of you and the passengers is to climb higher, only to find that you've been watching that same town for the last hour. And....isn't that the second train you've seen? And then again sometimes you climb higher only to find that today is the day the jet stream decided to come down and give us mere mortals who fly airplanes with propellers a look at what see normally rides high above us. Now, I'll guarantee that none of your passengers will get bored in that case.

Ninth: Always have a way out!

Finally, There is no real secret about staying out of the weather. You can't with 100% certainty, but you can keep those odds acceptably low.

Get a good briefing, listen for PIREPs, keep looking outside, look over your shoulder once in a while and talk with FSS. Notice if the weather is getting better, or worse. Is it better, or worse than forecast? Don't be too proud to call time and spend a night or two in a four corners truck stop. If things are going really good, don't forget to pay attention as that is natures way of lulling you into a false sense of security just before striking.

Above all! If the trip is one of appreciable distance where you are likely to be put under pressure to end up some where on a schedule...Go commercial, or take a train.

Yes, I know that is why most say they got the license, but with others along, you may be the only one having any fun...And you would like to have company next time? But that's another story.

There's so much detailed but vital information to learn about aviation weather, from whether hail's ahead of the storm or behind it, to why it's to important to reset the altimeter on a trip, that a beginner concentrating on the mechanics of clear-air flying can seriously underrate how much data has to be pasted into the brain. Not just memorized, but made second nature. You can't put everything on checklists. A lot of it has to be properly filed
in the brain.

Weather Aloft
--Chart temperatures in Celsius and winds with arrows in 5/10knot increments
--Text figures local with first two digits wind direction with added zero and two digit wind speed.
--Performance charts use temperature
--Area forecast predicts VFR clouds and weather
--AIRMET Sierra gives IFR conditions and mountain obscuration
--AIRMET Tango gives turbulence
--AIRMET Zulu gives icing conditions
--Constant pressure charts AC00-45E,
--Low pressure areas should be avoided
--Isobars close together indicate strong winds
--Upper winds flow parallel to lines
--A three-digit number by lines indicate meters of altitude by adding a zero
--Station models give data of temperature, dew point, wind, wind speed, meters of altitude (above) and change
--700 millibar is for about 10,000 feet and 500 millibar for 18,000 feet.
--500 millibar lines east and west portray calm winds
--North/south lines mean winds will mix warm and cold temperatures, expect storms.
--Forecasting of turbulence is right only half the time.
--A SIGMET requires a forecast of severe turbulence. (rare)

Weather Notes

--When winds aloft are not as forecast, neither will the weather.
--Stronger southerly winds mean worse weather
--The arrival of fog is more accurately forecast than its dissipation.
--Never fly into a known fog situation with the expectation that it will lift.
--The closer you get to bad weather the higher you can expect the cloud tops to be.
--Airframe icing requires lifting and moisture. New buildups are more likely to have ice than old clouds.
--Expect severe turbulence where occluded fronts exist.
--Between two areas having separate strong southwesterly winds, expect an occlusion.
--A closed low aloft makes a complete circle is called a cut-off low are difficult to forecast. May move west.
--500 millibar is 18,000' and determines significant weather since upper levels support what happens below.
--When multiple lows exist, one will become dominant.
--A cold front that passes and has behind it northerly winds is slowing and may stop.
--Stationary fronts breed new fronts.
--The strength of a cold front is directly related to lthe extend of the temperature drop.
--A narrow frontal zone with heavy rain is bad news.
--Area with widely scattered thunderstorms can be flown through, carefully.
--Area with scattered presents more problems.
--Broken or clusters of thunderstorms are best watched from the ground.
--Severe turbulence should be expected from any thunderstorm
--When it looks bad it probably is bad; the meaner it looks the meaner it is.
--Thunderstorms are weakest in midmorning.
--Lower is better in area of thunderstorms.
-- The visual illusion when flying in precipitation is that you will be higher than your actual path.
--Mentally fly every day when 'weather' exists to see what you might be doing.

Reading Weather Winds
3000 9900 - Wind less than 5 knots and variable
4000 2507 - Wind 250 @ 7
6000 2521+11 - Wind 250 @ 21 Temp +11 degrees C
9000 2432+06
12000 2538-01
18000 2465-15
24000 2591-24
30000 750239 - Wind 250 @ 102 knots temp -39 degrees C
34000 750149
39000 760861

On the last three you subtract 50 from the first two digits and put a 1 in front of the second two digits. Then the last two digits are the temp in Celsius (below 0). So 750239 gives you:
75 - 50 = 25: 250 degrees
02 plus the one = 102 knots
39 is -39 degrees below zero.
-Richard, PP-ASEL wannabe

Whether to Fly in Weather
--Preflight sources are your eyes, TV weather, newspapers, FSS weather, DUATs, internet and PIREPS
--Best webs are and
--Best VFR charts are significant weather prognostic, constant pressure analysis and the radar summary
--Best IFR charts are the VFR charts plus icing forecassts and 12-hour low level significant progs
--Route weather can be obtained by phoning an FSS and using the TIBS recordings
--Present weather is best from PIREPS through the 122.0 flight Watch frequency
--Local airport weather is becoming more available through AWOS and ASOS with radio and phones.
--In flight icing is number one cause of inflight G.A weather accidents
--Thunderstorms are the main cause of flight delays
--Weather forecast improvements are on their way.
--Greatest error is not 'if'. It is 'when" the weather will happen that is now the greatest variable.

The Smog Layer
Most of my flying is within 3000' AGL because I like it there. I once flew from California and back from Oshkosh with an average AGL altitude below 1000'. This means that I had no forecasts of winds because local climate is dominant. It means that I see plenty of smog when in densely populated areas. I will be warm in the summer and cold in the winter and freezing rain most common. I know the cooler it is the more shallow will be the weather condition. Weather conditions here will be mostly IFR between the smog and the surface. I will find most of the water, hot winds and cumulo-granite. Up and downdrafts and whirling winds will be likely. All of this still unable to be forecast but well publicized as it occurs..

It is here that every flight begins and ceases. Local charts are more likely to warn me of obstacles and experience has taught me to fly low in the good times to set my limits when conditions are less than ideal. I know that I do better flying in my home area by knowing the micro-climate of very valley and community. Still I will find the unexpected but I will keep always one last option available.

The Problem
 For purposes of filing an in-flight PIREP, how do you calculate or estimate wind velocities and directions you are experiencing? I have been told this is easy if you have a GPS.

 Solution #1 1) Start with your indicated airspeed, OAT, and altitude, and compute TAS (most ASI's have an IAS/TAS function built right in). Read your heading off the DG (or compass). 2) Get your GS (ground speed) and track from the GPS. 3) Plug these two vectors into an E6B (whiz wheel or electronic) to compute wind speed and direction. 

Roy Smith Solution #2 A lot of GPS units will calculate it, you have to put in some information, like your heading and your indicated airspeed. Also, if the winds are close to 90 degrees to your direction of travel, you need VERY accurate heading information. 

An easier way, and very accurate, is to turn into the wind until you have your slowest GS. Then take the difference between it and your TAS. That's your wind. Doug

Deteriorating Weather Lesson
Thanks for the 'star wars' bit. I had a similar experience only today but not nearly as touching. I am re-treading a pilot 20 years out of the cockpit who purchase a two year old Grumman AA5-B with all the goodies. However today we are going flying intentionally head on into a weather front. My pilot friend learned to fly in Hawaii and is weak in dealing with weather problems.

Before we started I told him we were going to contact FSS and get a route specific briefing. We were sitting near the hangar door and during the preflight briefing and weather discussion were able to watch the weather changes as the front approached some 40-50 miles away.

The weather was making fairly rapid changes in ceiling, wind velocity and temperature. We departed directly toward the front, knowing that the leading edge to our left would be hooking around behind us and having the potential to make our return more difficult. 25-miles out we contacted Center for flight following. Shortly thereafter Center told us that the weather at our destination was rapidly deteriorating. and asked of our intentions.

My student gave a sequence of altitude, visibility and general weather condition reports to Center as we proceeded toward the Front. We had to climb because of terrain and our visibility soon dropped to less than ten miles with very light rain. As we approached 4000' the digital temperature gauge dropped over one degree a minute soon to reach zero Celsius. We put on pitot heat and watched the rain quickly turn to heavy. We initiated our reversal of flight direction immediately while reporting the sudden change in conditions. Soon things were much improved and my student wanted to take another look to the south, as though we might be able to fly around the front. This direction would soon require a change in frequency from Center to Norcal. Even though we had previously given our aircraft information the controller asked again what we were flying. A Grumman Tiger", said my student adding that his instructor was giving him an introductory flight into rapidly deteriorating weather conditions.

At once the controller began to initiate casual conversation regarding his Tiger and the places he had taken his Tiger to various aircraft-type gatherings in the past few months. All of which my student had attended. He was very interested and complementary about the purpose of our flight. Granted the weather was such that he was not at all busy but it made the flight all the more enjoyable that my student while under stress from the MVFR weather found a kindred spirit on the other end of the radio.

We only flew an hour but my student came away with much more confidence that he could now study the weather books knowing what was important. He could get a weather briefing and have ready-made questions that need to be asked.

Weather Web Sites
Here are some links to get you started.
These websites provide METAR decoding information.
This website provides decoding information for the GFA (graphic area
This website provides information regarding the FD (winds and temperatures
This website has information on interpreting weather-satellite images
These websites provide more complete information on aviation weather

Jet stream

Satellite vis+ir
Satellite GEOS west large vis+ir
Satellite GEOS west medium animated vis+ir

NavCan local area aviation weather manuals (these are excellent and while
they provide good general information for everyone they should be required
reading for anyone planning to fly in Canada. The first chapters are
generic and the rest is location specific)\Publications\LAK\default.xml
BC local area aviation weather manual (NavCan)

Mountain Weather Education (for climbers)

We both are Amatuer Astronomers, and that is actually how I met him, and
there is this site/tool we use called the "Clear Sky Clock", and even though
it is done by the Canadian Government, there are Clear Sky Clocks for many
places in the U.S, he has one for the observatory (converted garden shed) in
his back yard :-). So, after looking at the weather forecast and going,
"hmmm...", he was like "Hey... maybe saturday looks ok... Clear Sky clock
says it will be clear Friday and since these guys don't forecast more than a
few hours out, maybe we will get lucky... :-)"

If you are curious, his Clear Sky Clock is for the "Highlands Astro-shack"
in Renton, Washington, and you can find all the clear sky clocks at at

He also has a reciever in his basement that can pick up the signals from the
weather satelites as they go over and processes the signals and posts the
pictures on a web page... sometimes they are better than the ones from NOAA,
and sometimes his computer is on the fritz or they are noisy.

Wade Hasbrouck

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