a http-equiv="Content-Type" content="text/html; charset=iso-8859-1">Prevailing Winds

Flight Environment



Since the atmosphere is addressed to the earthby gravity and rotates with the earth, there would certainly be no circulation if some pressure did notupset the atmosphere"s equilibrium. The heating of the earth"s surface by the sunlight isthe force responsible for producing the circulation the does exist.

You are watching: Fast-moving currents of air that occur above the friction layer are called _____.

Because of the curvature of the earth,the most straight rays of the sun strike the planet in the vicinity of the equator resultingin the best concentration the heat, the largest feasible amount that radiation, and themaximum heating of the atmosphere in this area of the earth. In ~ the same time, thesun"s rays strike the earth at the poles at a very oblique angle, bring about a muchlower concentration that heat and much less radiation so that there is, in fact, an extremely littleheating that the atmosphere over the poles and also consequently really cold temperatures.

Cold air, being an ext dense, sinks andhot air, being much less dense, rises. Consequently, the rising warmth air at the equatorbecomes even less thick as that rises and also its press decreases. An area that lowpressure, therefore, exists over the equator.


Warm air rises until it will acertain height at which that starts to spill over right into surrounding areas. At thepoles, the cold dense air sinks. Waiting from the upper levels that the atmosphere flowsin on height of it increasing the weight and also creating an area of high pressure at the poles.

The air the rises at the equator does notflow straight to the poles. Because of the rotation of the earth, over there is a develop up of airat around 30° phibìc latitude. (The exact same phenomenon wake up in the southern Hemisphere). Several of the wait sinks, causing a belt the high-pressure at this latitude.


The sinking wait reaches the surfaceand operation north and also south. The air the flows southern completes one cabinet of theearth"s circulation pattern. The air that flows north becomes part of another cellof circulation in between 30° and 60° phibìc latitude. At the exact same time, the sinkingair at the phibìc pole operation south and collides with the air relocating north native the 30°high press area. The colliding wait is required upward and an area of low pressureis created near 60° north. The third cell circulation sample is produced betweenthe north pole and also 60° north.

Because that the rotation the the earthand the coriolis force, air is deflected to the right in the northern Hemisphere. Asa result, the motion of wait in the polar cabinet circulation produce the polar easterlies. In the circulation cell the exists in between 60° and 30° north, the activity ofair produces the prevailing westerlies. In the tropical circulation cell, thenortheast profession winds room produced. These are the so-called permanent wind systemsof the each.

Since the planet rotates, the axis is tilted, and also there is more land mass in the north hemisphere than in the southern hemisphere, the actual worldwide pattern is much much more complicated. Instead of one large circulation between the poles and also the equator, there are three circulations...Hadley cell
- low latitude air activity toward the equator that with heating, rises vertically, through poleward activity in the upper atmosphere. This creates a convection cell the dominates tropical and also sub-tropical climates.Ferrel cell - A mid-latitude median atmospheric circulation cell for tasiilaq.net called by Ferrel in the 19th century. In this cabinet the air operation poleward and also eastward close to the surface and equatorward and also westward at greater levels.Polar cell - wait rises, diverges, and travels toward the poles. Once over the poles, the wait sinks, developing the polar highs. At the surface ar air diverges external from the polar highs. Surface winds in the polar cell space easterly (polar easterlies).


There space two main pressures which affect the movement of wait in the upper levels. The pressure gradient causes the waiting to move horizontally, forcing the air directly from a region of high press to a an ar of short pressure. The Coriolis force, however, deflects the direction the the circulation of the air (to the best in the northern Hemisphere) and causes the air to circulation parallel to the isobars.

Winds in the top levels will certainly blow clockwise roughly areas that high pressure and counterclockwise approximately areas of low pressure.

The rate of the wind is determined by the push gradient. The winds room strongest in regions where the isobars room close together.


Surface friction plays an essential role in the speed and direction of surface ar winds. Together a result of the slowing under of the air together it moves end the ground, wind speed are much less than would certainly be meant from the pressure gradient top top the tasiilaq.net map and also the direction is readjusted so the the wind blows throughout the isobars right into a center of low pressure and out of a center of high pressure.


The impact of friction usually does not extend an ext than a couple of thousand feet right into the air. At 3000 feet over the ground, the wind blows parallel come the isobars v a speed proportional to the press gradient.

Even permitting for the impacts of surface ar friction, the winds, locally, execute not always show the speed and direction that would certainly be expected from the isobars ~ above the surface ar tasiilaq.net map. This variations space usually because of geographical attributes such as hills, mountains and huge bodies of water. Except in such as mountain regions, the impact of terrain attributes that reason local sports in wind extends usually no greater than around 2000 feet above the ground.


Land and also sea breezes are led to by the distinctions in temperature end land and also water. The sea breeze occurs during the day as soon as the floor area heats more rapidly than the water surface. This outcomes in the press over the soil being lower than the over the water. The press gradient is often solid enough for a wind to blow from the water come the land.


The floor breeze blows at night as soon as the land i do not care cooler. Climate the wind blows in the direction of the warm, low-pressure area over the water.

Land and also sea breezes are really local and influence only a small area along the coast.


Hills and valleys significantly distort the airflow linked with the prevailing pressure system and also the press gradient. Strong up and also down drafts and eddies construct as the air flows up end hills and down into valleys. Wind direction transforms as the wait flows roughly hills. Periodically lines that hills and mountain varieties will act together a barrier, holding back the wind and deflecting it so that it flows parallel come the range. If over there is a happen in the mountain range, the wind will certainly rush through this happen as v a tunnel with substantial speed. The airflow have the right to be supposed to stay turbulent and erratic for some distance together it flows out of the hilly area and into the flatter countryside.


Daytime heating and also nighttime cooling the the hilly slopes result in day to night sports in the airflow. At night, the sides of the hills cool by radiation. The waiting in contact with them i do not care cooler and therefore denser and also it blows down the slope into the valley. This is a katabatic wind (sometimes also called a hill breeze). If the slopes space covered through ice and also snow, the katabatic wind will blow, not only at night, but also during the day, carrying the cold thick air right into the warmer valleys. The slopes that hills not covered by snow will certainly be warmed during the day. The waiting in contact with them i do not care warmer and less dense and, therefore, operation up the slope. This is one anabatic wind (or sink breeze).

In mountainous areas, neighborhood distortion the the air flow is even much more severe. Rocky surfaces, high ridges, slim cliffs, steep valleys, all integrate to produce unpredictable circulation patterns and turbulence.



Air flowing across a mountain selection usually rises relatively smoothly up the slope of the range, but, as soon as over the top, the pours under the various other side with considerable force, bouncing up and also down, producing eddies and also turbulence and likewise creating powerful vertical waves that may prolong for great distances downwind of the hill range. This phenomenon is known as a hill wave. Note the up and also down drafts and also the rotating eddies created downstream.

If the waiting mass has actually a high humidity content, clouds of really distinctive appearance will develop.

Cap Cloud. Orographic lift reasons a cloud to type along the optimal of the ridge. The wind tote this cloud down along the leeward slope where it dissipates through adiabatic heating. The basic of this cloud lies near or below the peaks of the ridge; the top may reach a few thousand feet over the peaks.

Lenticular (Lens Shaped) Clouds form in the tide crests aloft and also lie in bands the may prolong to well over 40,000 feet.

Rotor Clouds form in the roll eddies downstream. They resemble a lengthy line the stratocumulus clouds, the bases of i m sorry lie below the hill peaks and also the top of which might reach to a significant height over the peaks. Sometimes these clouds build into thunderstorms.


The clouds, being very distinctive, deserve to be viewed from a good distance and provide a visible warning of the hill wave condition. Unfortunately, periodically they are installed in various other cloud systems and are hidden from sight. Periodically the air mass is very dry and also the clouds carry out not develop.

The severity that the mountain wave and the elevation to which the disturbance that the waiting is influenced is dependence on the strength of the wind, its edge to the variety and the stability or instability the the air. The many severe hill wave conditions are developed in solid airflows that are blowing at right angles come the selection and in secure air. A jet present blowing practically perpendicular come the mountain selection increases the severity the the tide condition.

The mountain wave phenomenon is not restricted only come high hill ranges, such together the Rockies, however is likewise present come a lesser degree in smaller hill systems and even in currently of little hills.

Mountain waves current problems come pilots for number of reasons:

Vertical Currents. Downdrafts that 2000 feet every minute space common and downdrafts as good as 5000 feet per minute have actually been reported. They take place along the downward slope and are most severe at a height equal to the of the summit. An airplane, caught in a downdraft, might be forced to the ground.

Turbulence is usually very severe in the waiting layer in between the ground and the top of the rotor clouds.

Wind Shear. The wind speed varies dramatically between the crests and also troughs of the waves. The is usually most severe in the wave nearest the mountain range.

Altimeter Error. The boost in wind speed results in one accompanying to decrease in pressure, which subsequently affects the accuracy of the pressure altimeter.

Icing. The freezing level varies considerably from crest come trough. Major icing can occur due to the fact that of the large supercooled droplets sustained in the strong vertical currents.

When flying end a hill ridge wherein wave problems exist: (1) stop ragged and also irregular shame clouds—the irregular shape indicates turbulence. (2) approach the hill at a 45-degree angle. The you need to suddenly decide to rotate back, a rapid turn deserve to be made away from the high ground. (3) protect against flying in cloud ~ above the mountain crest (cap cloud) because of strong downdrafts and also turbulence. (4) enable sufficient height to clean the greatest ridges through altitude to preventive to stop the downdrafts and eddies ~ above the downwind slopes. (5) always remember the your altimeter deserve to read end 3000 ft. In error ~ above the high next in hill wave conditions.


A gust is a rapid and irregular fluctuation of differing intensity in the upward and also downward motion of air currents. It may be linked with a rapid adjust in wind direction. Gusts are resulted in by mechanical disturbance that results from friction between the air and also the ground and by the unequal heating of the earth"s surface, specifically on hot summer afternoons.


A squall is a sudden increase in the toughness of the wind of much longer duration than a gust and may be led to by the i of a quick moving cold prior or thunderstorm. Prefer a gust, it may be accompanied by a rapid change of wind direction.


Diurnal (daily) sports of wind is led to by strong surface heating during the day, which causes turbulence in the reduced levels. The an outcome of this disturbance is the the direction and also speed of the wind at the higher levels (e.g., 3000 feet) has tendency to be moved to the surface. Due to the fact that the wind direction at the higher level is parallel come the isobars and its speed is higher than the surface wind, this transfer reasons the surface ar wind to veer and also increase in speed.

At night, over there is no surface heating and also therefore much less turbulence and the surface ar wind has tendency to resume its normal direction and speed. The backs and decreases. Check out VEERING and BACKING section below for much more info.


Friction between the relocating air mass and surface functions of the planet (hills, mountains, valleys, trees, buildings, etc.) is responsible for the swirling vortices that air typically called eddies. Lock vary substantially in size and intensity relying on the size and also roughness of the surface ar obstruction, the rate of the wind and the degree of stability of the air. They deserve to spin in either a horizontal or upright plane. Stormy air and strong winds produce an ext vigorous eddies. In secure air, eddies have tendency to quickly dissipate. Eddies created in mountainous locations are particularly powerful.


The bumpy or choppy up and down activity that signifies the presence of eddies provides it an overwhelming to keep an plane in level flight.


Dust devils are phenomena that occur quite typically on the warm dry plains of mid-western phibìc America. They can be of adequate force to current a danger to pilots of light airplanes paris at short speeds.

They are tiny heat short that form on clear warm days. Given a steep slide out rate brought about by cool waiting aloft over a hot surface, small horizontal wait movement, couple of or no clouds, and also the noonday sunlight heating level arid soil surfaces come high temperatures, the air in contact with the ground becomes super-heated and also highly unstable. This surface layer of waiting builds till something triggers an upward movement. Once started, the hot air rises in a column and draws an ext hot air right into the base of the column. Circulation begins roughly this heat low and also increases in velocity till a little vigorous whirlwind is created. Dust devils room usually of short duration and are so named since they room made clearly shows by the dust, sand and debris the they pick up indigenous the ground.

Dust devils posture the biggest hazard near the ground wherein they are many violent. Pilots proposing come land on superheated runways in locations of the mid-west wherein this phenomenon is common should scan the airport because that dust swirls or grass spirals that would indicate the presence of this hazard.


Tornadoes room violent, circular whirlpools that air associated with serious thunderstorms and are, in fact, an extremely deep, focused low-pressure areas. They space shaped favor a tunnel hanging out of the cumulonimbus cloud and also are dark in appearance because of the dust and also debris sucked into their whirlpools. They selection in diameter from around 100 feet to one half mile and move over the floor at speeds of 25 come 50 knots. Their route over the floor is usually just a few miles long although tornadoes have been report to cut destructive swaths as long as 100 miles. The good destructiveness the tornadoes is led to by the really low push in your centers and also the high wind speeds, i beg your pardon are deemed to be as an excellent as 300 knots.


Wind speeds because that aviation purposes are to express in knots (nautical miles every hour). In the tasiilaq.net reports on united state public radio and also television, however, wind speeds are given in miles every hour if in Canada speeds are provided in kilometers per hour.

In a conversation of wind direction, the compass point from which the wind is blowing is taken into consideration to it is in its direction. Therefore, a north wind is one the is blowing native the north towards the south. In aviation tasiilaq.net reports, area and aerodrome forecasts, the wind is always reported in degrees true. In ATIS broadcasts and in the information provided by the tower because that landing and also take-off, the wind is report in degrees magnetic.


The wind veers as soon as it alters direction clockwise. Example: The surface wind is blowing from 270°. In ~ 2000 feet that is blowing native 280°. The has readjusted in a right-hand, or clockwise, direction.

The wind backs when it changes direction anti-clockwise. Example: The wind direction at 2000 feet is 090° and at 3000 feet is 085°. It is transforming in a left-hand, or anti-clockwise, direction.

In a descent from several thousand feet over the ground come ground level, the wind will usually be uncovered to earlier and likewise decrease in velocity, as the result of surface friction i do not care apparent. In a climb from the surface to several thousand feet AGL, the wind will certainly veer and also increase.

At night, surface cooling reduce the eddy movement of the air. Surface winds will ago and decrease. Conversely, during the day, surface ar heating increases the eddy motion of the air. Surface ar winds will veer and increase as more powerful winds aloft mix to the surface. See DIURNAL VARIATIONS section over for an ext info.


Wind shear is the suddenly tearing or shearing result encountered along the leaf of a region in which over there is a violent readjust in wind speed or direction. It deserve to exist in a horizontal or upright direction and produces churning motions and consequently turbulence. Under part conditions, wind direction alters of as much as 180 degrees and speed transforms of as lot as 80 knots have actually been measured.


The effect on plane performance that encountering wind shear derives native the reality that the wind can adjust much much faster than the aircraft mass can be increased or decelerated. Significant wind shears deserve to impose penalties on one airplane"s performance that are past its capabilities come compensate, especially throughout the an essential landing and take-off phase of flight.

In setravel Flight

In seafaring flight, wind shear will likely be encountered in the transition zone in between the press gradient wind and the distorted neighborhood winds at the lower levels. The will additionally be encountered once climbing or descending with a temperature inversion and when passing with a frontal surface. Wind shear is likewise associated through the jet stream. Plane encountering wind shear may experience a succession of updrafts and also downdrafts, to reduce or profit in headwind, or windshifts that disrupt the developed flight path. That is not normally a major problem since altitude and airspeed margins will be enough to against the shear"s disadvantage effects. On occasion, however, the wind shear may be severe enough to cause an abrupt increase in fill factor, which could stall the aircraft or inflict structural damage.


Near the Ground

Wind shear, encountered near the ground, is much more serious and potentially really dangerous. There space four usual sources of short level wind shear: thunderstorms, frontal activity, temperature inversions and strong surface winds passing roughly natural or manmade obstacles.

Frontal Wind Shear. Wind shear is generally a trouble only in fronts with steep wind gradients. If the temperature difference across the front at the surface is 5°C or an ext and if the former is relocating at a rate of around 30 knots or more, wind shear is most likely to be present. Frontal wind shear is a phenomenon connected with rapid moving cold fronts but can be present in warm fronts as well.


Thunderstorms. Wind shear, connected with thunderstorms, occurs as the result of two phenomena, the gust front and also downbursts. As the thunderstorm matures, strong downdrafts develop, win the ground and spread the end horizontally follow me the surface well in breakthrough of the thunderstorm itself. This is the gust front. Winds can change direction through as much as 180° and also reach speeds as an excellent as 100 knots as far as 10 mile ahead that the storm. The downburst is really intense localized downdraft flowing out of a thunderstorm. The power of the downburst can exceed plane climb capabilities. The downburst (there space two species of downbursts: macrobursts and also microbursts) generally is much closer to the thunderstorm 보다 the gust front. Dust clouds, roll clouds, intense rainfall or virga (rain the evaporates before it will the ground) are due to the opportunity of downburst activity but over there is no method to accurately guess its occurrence.


Temperature Inversions. Overnight cooling create a temperature inversion a few hundred feet above the ground that deserve to produce far-reaching wind shear, especially if the reverse is coupled through the low-level jet stream.

As a nocturnal inversion develops, the wind shear near the height of the turning back increases. It normally reaches its maximum speed quickly after midnight and also decreases in the morning together daytime heater dissipates the inversion. This phenomenon is known as the low-level nocturnal jet stream. The low level jet present is a paper of strong winds, thousands of miles long, thousands of miles wide and thousands of feet special that creates over flat terrain such together the prairies. Wind speed of 40 knots space common, however greater speeds have been measured. Short level jet streams are responsible because that hazardous short level shear.


As the reverse dissipates in the morning, the shear airplane and gusty winds relocate closer to the ground, causing windshifts and increases in wind speed close to the surface.

Surface Obstructions. The irregular and turbulent flow of air around mountains and hills and through mountain passes reasons serious wind shear difficulties for aircraft approaching come land in ~ airports near mountain ridges. Wind shear is a phenomenon linked with the hill wave. Together shear is almost totally unpredictable however should be meant whenever surface winds room strong.

Wind shear is likewise associated v hangars and huge buildings at airports. As the wait flows approximately such big structures, wind direction changes and wind speed rises causing shear.

Wind shear occurs both horizontally and vertically. Upright shear is most common near the ground and can attitude a serious risk to airplanes throughout take-off and also landing. The aircraft is paris at lower speeds and in a relatively high drag configuration. There is little altitude obtainable for recovering and also stall and maneuver margins space at their lowest. An airplane encountering the wind shear phenomenon might experience a large loss the airspeed because of the sudden readjust in the loved one airflow together the aircraft flies into a new, moving air mass. The abrupt autumn in airspeed may an outcome in a stall, developing a dangerous case when the airplane is only a couple of hundred feet turn off the ground and an extremely vulnerable.


Narrow bands of exceedingly high rate winds are well-known to exist in the greater levels of the setting at altitudes ranging from 20,000 to 40,000 feet or more. Castle are known as jet streams. As numerous as three major jet streams might traverse the north American continent at any kind of given time. One lies throughout Northern Canada and one across the U.S. A third jet stream might be as much south together the north tropics yet it is somewhat rare. A jet currently in the mid latitudes is usually the strongest.


The jet stream appears to be closely associated with the tropopause and with the polar front. It commonly forms in the break in between the polar and the dry tropopause wherein the temperature gradients room intensified. The average position that the jet stream shears southern in winter and north in summer with the seasonal migrate of the polar front. Since the troposphere is depths in summer 보다 in winter, the tropopause and the jets will certainly nominally be at higher altitudes in the summer.

Long, strong jet streams room usually also associated v well-developed surface ar lows in ~ deep upper troughs and lows. A low emerging in the wave along the frontal surface ar lies southern of the jet. Together it deepens, the low moves close to the jet. As it occludes, the short moves phibìc of the jet, which the cross the frontal system, close to the point of occlusion. The jet flows roughly parallel come the front. The subtropical jet present is not associated with fronts but forms because of strong solar heater in the equatorial regions. The ascending air turns poleward at really high levels but is deflected through the Coriolis force into a strong westerly jet. The subtropical jet predominates in winter.


The jet streams flow from west to east and may encircle the entire hemisphere. More often, due to the fact that they space stronger in some areas than in others, they rest up right into segments part 1000 to 3000 nautical mile long. They room usually about 300 nautical miles large and may be 3000 come 7000 feet thick. This jet currently segments relocate in an easterly direction adhering to the activity of press ridges and troughs in the top atmosphere.

Winds in the main core of the jet stream are the strongest and also may reach speeds as good as 250 knots, back they room generally between 100 and also 150 knots. Wind speed decrease toward the outer edges of the jet stream and also may it is in blowing at only 25 knots there. The rate of decrease of wind rate is considerably greater on the north edge than on the southerly edge. Wind speeds in the jet currently are, top top average, significantly stronger in winter than in summer.

Clear air Turbulence. The many probable place to expect Clear Air disturbance (CAT) is just above the main core that the jet stream close to the polar tropopause and just below the core. Clear air turbulence does not happen in the core. CAT is encountered more frequently in winter when the jet present winds space strongest. Nevertheless, CAT is not constantly present in the jet currently and, due to the fact that it is random and also transient in nature, the is almost impossible come forecast.

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Clear air turbulence might be connected with various other tasiilaq.net patterns, particularly in wind shear linked with the sharply curved contours of strong lows, troughs and also ridges aloft, in ~ or below the tropopause, and also in areas of strong cold or warmth air advection. Mountain waves produce severe CAT the may expand from the mountain crests to as high as 5000 feet over the tropopause. Because severe CAT does pose a danger to airplanes, pilots should try to prevent or minimization encounters with it. This rules of thumb may assist avoid jet streams with strong winds (150 knots) in ~ the core. Strong wind shears room likely above and below the core. CAT within the jet currently is much more intense over and come the lee of hill ranges. If the 20-knot isotachs (lines joining areas of same wind speeds) are closer 보다 60 nautical mile on the charts reflecting the locations of the jet stream, wind shear and also CAT room possible.

Curving jet streams are most likely to have turbulent edges, especially those that curve around a deep press trough. As soon as moderate or major CAT has actually been reported or is forecast, change speed to unstable air speed immediately on encountering the an initial bumpiness or even before encountering the to prevent structural damages to the airplane.

The locations of CAT are usually shallow and narrow and elongated v the wind. If jet stream turbulence is encountered with a tail wind or head wind, a turn to the right will uncover smoother wait and much more favorable winds. If the CAT is encountered in a crosswind, that is not so essential to change course together the stormy area will be narrow.