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

Flight Environment



Since the atmosphere is fixed to the earthby gravity and also rotates with the earth, there would 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 creating the circulation the does exist.

You are watching: A steep pressure gradient ________.

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

Cold air, being more dense, sink andhot air, being much less dense, rises. Consequently, the rising heat air in ~ the equatorbecomes even less thick as the rises and its press decreases. An area the lowpressure, therefore, exists end the equator.


Warm wait rises till it reaches acertain elevation at which the starts to pour out over right into surrounding areas. At thepoles, the cold dense air sinks. Wait from the top levels of the environment flowsin on height of it boosting the weight and creating one area that high press at the poles.

The air that rises in ~ the equator walk notflow straight to the poles. Due to the rotation the the earth, over there is a construct up the airat around 30° north latitude. (The very same phenomenon wake up in the southern Hemisphere). Several of the wait sinks, leading to a belt the high-pressure in ~ this latitude.


The sinking waiting reaches the surfaceand flows north and also south. The air the flows southern completes one cell of theearth"s circulation pattern. The air that flows phibìc becomes part of one more cellof circulation between 30° and 60° north latitude. In ~ the same time, the sinkingair at the north pole flows south and also collides through the air relocating north native the 30°high press area. The colliding wait is forced upward and an area of low pressureis developed near 60° north. The 3rd cell circulation pattern is developed betweenthe north pole and also 60° north.

Because of the rotation that the earthand the coriolis force, waiting is deflected come the right in the north Hemisphere. Asa result, the activity of wait in the polar cell circulation to produce the polar easterlies. In the circulation cell that exists in between 60° and also 30° north, the activity ofair produce the prevailing westerlies. In the tropic circulation cell, thenortheast profession winds are produced. These space the so-called long-term wind systemsof the each.

Since the planet rotates, the axis is tilted, and also there is more land fixed in the northern hemisphere than in the southerly hemisphere, the actual global pattern is much much more complicated. Instead of one large circulation between the poles and also the equator, there room three circulations...Hadley cell
- short latitude air movement toward the equator that through heating, rises vertically, with poleward movement in the upper atmosphere. This creates a convection cell that dominates tropical and sub-tropical climates.Ferrel cell - A mid-latitude mean atmospheric circulation cell for derekwadsworth.com named by Ferrel in the 19th century. In this cell the air flows poleward and also eastward near the surface and also equatorward and westward at greater levels.Polar cell - waiting rises, diverges, and travels toward the poles. When over the poles, the air sinks, creating the polar highs. At the surface air diverges outward from the polar highs. Surface ar winds in the polar cell space easterly (polar easterlies).


There are two main forces which affect the movement of air in the top levels. The push gradient reasons the wait to relocate horizontally, forcing the air directly from a an ar of high push to a an ar of short pressure. The Coriolis force, however, deflects the direction of the circulation of the waiting (to the appropriate in the north Hemisphere) and also causes the air to flow parallel to the isobars.

Winds in the upper levels will certainly blow clockwise around areas the high pressure and counterclockwise roughly areas of low pressure.

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


Surface friction plays vital role in the speed and also direction of surface winds. As a an outcome of the slowing down of the air as it moves over the ground, wind speeds are less than would certainly be intended from the push gradient top top the derekwadsworth.com map and also the direction is changed so the the wind blows across the isobars right into a facility of low pressure and out the a facility of high pressure.


The result of friction generally does not extend much more than a pair of thousand feet into the air. In ~ 3000 feet above the ground, the wind blows parallel come the isobars with a speed proportional to the pressure gradient.

Even enabling for the results of surface ar friction, the winds, locally, execute not always show the speed and also direction that would be meant from the isobars ~ above the surface ar derekwadsworth.com map. This variations space usually because of geographical functions such as hills, hills and huge bodies of water. Other than in such as mountain regions, the impact of terrain features that cause local sports in wind extends generally no higher than around 2000 feet over the ground.


Land and also sea breezes are brought about by the distinctions in temperature end land and also water. The sea breeze occurs during the day once the land area heats an ext rapidly than the water surface. This results in the push over the land 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 land breeze blows in ~ night once 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 very local and affect only a small area along the coast.


Hills and valleys considerably distort the airflow associated with the prevailing pressure system and the press gradient. Strong up and down drafts and also eddies construct as the air flows up over hills and also down right into valleys. Wind direction changes as the air flows approximately hills. Periodically lines the hills and also mountain varieties will act as a barrier, holding back the wind and deflecting it so the it operation parallel come the range. If over there is a pass in the mountain range, the wind will rush with this happen as through a tunnel with considerable speed. The airflow deserve to be intended to remain turbulent and also erratic for some distance together it flows the end of the hilly area and into the flatter countryside.


Daytime heating and nighttime cooling the the hilly slopes lead to day to night sports in the airflow. In ~ night, the political parties of the hills cool by radiation. The air in call with them becomes cooler and therefore denser and also it blows under the slope right into the valley. This is a katabatic wind (sometimes additionally called a hill breeze). If the slopes room covered with ice and also snow, the katabatic wind will certainly blow, not just at night, but also during the day, transporting the cold dense air into the warmer valleys. The slopes that hills not spanned by snow will be warmed throughout the day. The air in call with them becomes warmer and less thick and, therefore, operation up the slope. This is one anabatic wind (or valley breeze).

In like the mountain areas, regional distortion that the air flow is even much more severe. Rocky surfaces, high ridges, slim cliffs, steep valleys, all integrate to create unpredictable flow patterns and turbulence.



Air flowing throughout a mountain selection usually rises reasonably smoothly increase the slope of the range, but, when over the top, that pours under the various other side with significant force, bouncing up and also down, developing eddies and turbulence and additionally creating powerful vertical waves the may expand for an excellent distances downwind of the mountain range. This phenomenon is well-known as a hill wave. Keep in mind the up and down drafts and also the rotating eddies formed downstream.

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

Cap Cloud. Orographic lift reasons a cloud to type along the peak of the ridge. The wind dead this cloud under along the leeward slope where it dissipates v adiabatic heating. The base 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 wave crests aloft and also lie in bands that may prolong to well above 40,000 feet.

Rotor Clouds form in the rojo eddies downstream. Castle resemble a long line the stratocumulus clouds, the bases of i m sorry lie below the mountain peaks and the top of which might reach to a considerable height above the peaks. Occasionally these clouds construct into thunderstorms.


The clouds, being really distinctive, have the right to be seen from a good distance and provide a clearly shows warning that the mountain wave condition. Unfortunately, sometimes they are embedded in various other cloud systems and also are hidden from sight. Occasionally the waiting mass is an extremely dry and the clouds carry out not develop.

The severity of the mountain wave and also the height to i m sorry the disturbance of the waiting is influenced is dependent on the strength of the wind, its angle to the selection and the stability or instability of the air. The many severe mountain wave conditions are created in strong airflows that room blowing at ideal angles come the selection and in stable air. A jet stream blowing virtually perpendicular come the mountain selection increases the severity the the wave condition.

The mountain wave phenomenon is not restricted only to high hill ranges, such together the Rockies, but is also present to a lesser degree in smaller mountain systems and even in currently of tiny hills.

Mountain waves existing problems to pilots for numerous reasons:

Vertical Currents. Downdrafts that 2000 feet every minute are 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 in ~ a height equal to that of the summit. One airplane, caught in a downdraft, might be compelled to the ground.

Turbulence is usually incredibly 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 troughs the the waves. It is usually many severe in the wave nearest the hill range.

Altimeter Error. The boost in wind speed outcomes in one accompanying diminish in pressure, which consequently affects the accuracy that the press altimeter.

Icing. The freeze level varies substantially from crest come trough. Serious icing deserve to occur due to the fact that of the huge supercooled droplets continual in the solid vertical currents.

When flying over a hill ridge whereby wave conditions exist: (1) protect against ragged and also irregular shaped clouds—the rarely often rare shape shows turbulence. (2) method the mountain at a 45-degree angle. The you need to suddenly decide to rotate back, a rapid turn can be made far from the high ground. (3) protect against flying in cloud top top the hill crest (cap cloud) since of strong downdrafts and turbulence. (4) enable sufficient elevation to clean the highest possible ridges with altitude to preventive to prevent the downdrafts and also eddies on the downwind slopes. (5) constantly remember the your altimeter deserve to read end 3000 ft. In error on the high side in hill wave conditions.


A gust is a rapid and also irregular fluctuation of differing intensity in the upward and downward movement of waiting currents. It might be linked with a rapid change in wind direction. Gusts are caused by mechanical turbulence that results from friction between the air and also the ground and also by the unequal heating of the earth"s surface, an especially on hot summer afternoons.


A squall is a sudden rise in the strength of the wind of much longer duration 보다 a gust and also may be brought about by the passage of a quick moving cold prior or thunderstorm. Prefer a gust, it might be accompanied by a rapid change of wind direction.


Diurnal (daily) variation of wind is caused by strong surface heating during the day, which causes turbulence in the reduced levels. The result of this turbulence is that the direction and also speed the the wind in ~ the greater levels (e.g., 3000 feet) has tendency to be transferred to the surface. Due to the fact that the wind direction at the greater level is parallel come the isobars and its rate 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 ar heating and also therefore much less turbulence and also the surface ar wind has tendency to resume its normal direction and also speed. That backs and decreases. Check out VEERING and also BACKING section below for more info.


Friction between the relocating air mass and surface attributes of the planet (hills, mountains, valleys, trees, buildings, etc.) is responsible because that the swirling vortices the air generally called eddies. Castle vary substantially in size and intensity depending upon the size and also roughness the the surface obstruction, the speed of the wind and the degree of security of the air. They have the right to spin in one of two people a horizontal or upright plane. Stormy air and solid winds produce much more vigorous eddies. In secure air, eddies tend to quickly dissipate. Eddies produced in mountainous locations are specifically powerful.


The bumpy or choppy up and also down movement that signifies the presence of eddies provides it daunting to store an plane in level flight.


Dust devils room phenomena that occur quite frequently on the hot dry levels of mid-western north America. They deserve to be of sufficient force to present a danger to pilots of irradiate airplanes paris at low speeds.

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

Dust devils posture the best hazard close to the ground wherein they are many violent. Pilots proposing to land ~ above superheated runways in areas of the mid-west whereby this phenomenon is common should scan the airport for dust swirls or grass spirals the would suggest the existence of this hazard.


Tornadoes space violent, circular whirlpools the air connected with major thunderstorms and are, in fact, an extremely deep, concentrated low-pressure areas. They room shaped favor a tunnel hanging out of the cumulonimbus cloud and also are dark in appearance due to the dust and also debris sucked into their whirlpools. They selection in diameter from about 100 feet come one fifty percent mile and also move end the floor at speed of 25 come 50 knots. Their course over the soil is usually just a couple of miles lengthy although tornadoes have been report to cut destructive swaths as lengthy as 100 miles. The great destructiveness of tornadoes is brought about by the really low press in your centers and also the high wind speeds, i beg your pardon are understood to be as great as 300 knots.


Wind speeds for aviation objectives are express in knots (nautical miles per hour). In the derekwadsworth.com reports on us public radio and television, however, wind speed are given in miles every hour when in Canada speeds are given in kilometers per hour.

In a discussion of wind direction, the compass allude from i m sorry the wind is punch is considered to it is in its direction. Therefore, a phibìc wind is one that is blowing native the north towards the south. In aviation derekwadsworth.com reports, area and also aerodrome forecasts, the wind is always reported in levels true. In ATIS broadcasts and in the information offered by the tower because that landing and also take-off, the wind is report in degrees magnetic.


The wind veers when it alters direction clockwise. Example: The surface ar wind is blowing from 270°. In ~ 2000 feet that is blowing indigenous 280°. It has changed in a right-hand, or clockwise, direction.

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

In a descent from several thousand feet above the ground to ground level, the wind will usually be discovered to back and also decrease in velocity, as the result of surface ar friction becomes apparent. In a climb from the surface ar to several thousand feet AGL, the wind will certainly veer and also increase.

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


Wind shear is the sudden tearing or shearing effect encountered follow me the sheet of a region in which there is a violent readjust in wind rate or direction. It have the right to exist in a horizontal or upright direction and also produces churning motions and consequently turbulence. Under some conditions, wind direction transforms of as lot as 180 degrees and speed alters of as much as 80 knots have actually been measured.


The effect on airplane performance the encountering wind shear derives from the fact that the wind can change much much faster than the plane mass deserve to be accelerated or decelerated. Severe wind shears deserve to impose penalties on an airplane"s performance the are past its capabilities to compensate, especially during the an essential landing and also take-off phase of flight.

In seafaring Flight

In cruising flight, wind shear will likely be encountered in the change zone between the pressure gradient wind and the distorted regional winds in ~ the lower levels. It will likewise be encountered when climbing or descending with a temperature inversion and also when passing with a frontal surface. Wind shear is also associated with the jet stream. Plane encountering wind shear may experience a succession of updrafts and downdrafts, reductions or benefit in headwind, or windshifts that disrupt the developed flight path. It is not generally a significant problem since altitude and airspeed margins will certainly be enough to against the shear"s disadvantage effects. On occasion, however, the wind shear may be severe sufficient to cause an abrupt rise in fill factor, which might stall the airplane or inflict structural damage.


Near the Ground

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

Frontal Wind Shear. Wind shear is typically a problem only in fronts with steep wind gradients. If the temperature difference across the front in ~ the surface ar is 5°C or more and if the front is moving at a rate of around 30 knots or more, wind shear is likely to be present. Frontal wind shear is a phenomenon connected with rapid moving cold fronts however can be existing in heat fronts together well.


Thunderstorms. Wind shear, connected with thunderstorms, occurs as the an outcome of two phenomena, the gust front and downbursts. As the thunderstorm matures, strong downdrafts develop, strike the ground and spread out horizontally follow me the surface well in advancement of the thunderstorm itself. This is the gust front. Winds can adjust direction by as lot as 180° and reach speeds as good as 100 knots as far as 10 mile ahead of the storm. The downburst is an extremely intense localized downdraft flowing out of a thunderstorm. The strength of the downburst can exceed plane climb capabilities. The downburst (there are two species of downbursts: macrobursts and microbursts) commonly is lot closer come the thunderstorm 보다 the gust front. Dust clouds, roll clouds, intense rainfall or virga (rain the evaporates prior to it get the ground) are due to the possibility of downburst activity but there is no method to accurately suspect its occurrence.


Temperature Inversions. Overnight cooling creates a temperature inversion a couple of hundred feet above the soil that have the right to produce far-reaching wind shear, particularly if the reverse is coupled v the low-level jet stream.

As a nocturnal reverse develops, the wind shear close to the optimal of the reverse increases. It usually reaches its best speed soon after midnight and decreases in the morning as daytime heater dissipates the inversion. This phenomenon is known as the low-level nocturnal jet stream. The low level jet stream is a sheet of solid winds, thousands of miles long, thousands of miles large and thousands of feet special that forms over flat terrain such together the prairies. Wind speeds of 40 knots space common, yet greater speeds have actually been measured. Short level jet streams room responsible for hazardous low level shear.


As the inversion dissipates in the morning, the shear aircraft and gusty winds move closer to the ground, resulting in windshifts and increases in wind speed close to the surface.

Surface Obstructions. The irregular and turbulent flow of air around mountains and also hills and also through hill passes causes serious wind shear difficulties for plane approaching to land in ~ airports near hill ridges. Wind shear is a phenomenon associated with the hill wave. Such shear is virtually totally unpredictable however should be supposed whenever surface winds are strong.

Wind shear is likewise associated with hangars and large buildings in ~ airports. Together the wait flows roughly such large structures, wind direction changes and wind speed increases causing shear.

Wind shear wake up both horizontally and vertically. Upright shear is most typical near the ground and can pose a serious peril to airplanes during take-off and landing. The plane is paris at lower speeds and also in a reasonably high drag configuration. There is tiny altitude accessible for recovering and also stall and also maneuver margins room at their lowest. An plane encountering the wind shear phenomenon may experience a large loss that airspeed due to the fact that of the sudden readjust in the family member airflow as the aircraft flies into a new, moving air mass. The abrupt autumn in airspeed may result in a stall, developing a dangerous situation when the plane is only a few hundred feet off the floor and very vulnerable.


Narrow bands the exceedingly high speed winds are known to exist in the higher levels that the environment at altitudes varying from 20,000 to 40,000 feet or more. Castle are known as jet streams. As many as three significant jet streams may traverse the phibìc American continent at any kind of given time. One lies across Northern Canada and also one across the U.S. A 3rd jet stream might be as far south together the northern tropics however it is rather rare. A jet present in the mid latitudes is usually the strongest.


The jet stream shows up to it is in closely connected with the tropopause and with the polar front. It frequently forms in the break in between the polar and the tropical tropopause where the temperature gradients are intensified. The mean position the the jet present shears southern in winter and also north in summer through the seasonal migrate of the polar front. Since the troposphere is depth in summer than in winter, the tropopause and the jets will nominally be at greater altitudes in the summer.

Long, strong jet streams room usually likewise associated v well-developed surface ar lows in ~ deep upper troughs and also lows. A low emerging in the tide along the frontal surface lies south of the jet. Together it deepens, the low moves near the jet. As it occludes, the short moves north of the jet, which the cross the frontal system, close to the allude of occlusion. The jet flows about parallel come the front. The subtropical jet stream is not associated with fronts but forms due to the fact that of strong solar heater in the equatorial regions. The ascending air transforms poleward at very high levels yet is deflected by the Coriolis force into a solid westerly jet. The subtropical jet predominates in winter.


The jet streams circulation from west come east and may encircle the whole hemisphere. An ext often, because they are stronger in some locations than in others, they break up right into segments part 1000 come 3000 nautical mile long. They are usually around 300 nautical miles vast and may be 3000 come 7000 feet thick. This jet stream segments relocate in one easterly direction adhering to the motion of push ridges and also troughs in the top atmosphere.

Winds in the central core that the jet stream space the strongest and may reach speeds as good as 250 knots, return they room generally between 100 and also 150 knots. Wind speeds decrease toward the outer edges that the jet stream and also may be blowing at only 25 knots there. The price of diminish of wind rate is significantly greater top top the northern edge than on the southerly edge. Wind speeds in the jet stream are, on average, significantly stronger in winter than in summer.

Clear air Turbulence. The most probable place to mean Clear Air turbulence (CAT) is just over the central core the the jet stream close to the polar tropopause and also just below the core. Clean air turbulence does not occur in the core. CAT is encountered more frequently in winter as soon as the jet present winds space strongest. Nevertheless, CAT is not always present in the jet present and, due to the fact that it is random and transient in nature, it is virtually impossible come forecast.

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Clear air turbulence may be connected with other derekwadsworth.com patterns, specifically in wind shear linked with the sharply bent contours of solid lows, troughs and ridges aloft, at or below the tropopause, and in locations of solid cold or heat air advection. Hill waves create severe CAT the may prolong from the mountain crests to as high as 5000 feet over the tropopause. Since severe CAT does pose a risk to airplanes, pilots should try to protect against or minimize encounters with it. These rules of thumb may assist avoid jet streams with solid winds (150 knots) at the core. Solid wind shears are likely over and below the core. CAT within the jet present is an ext intense above and to the lee of hill ranges. If the 20-knot isotachs (lines joining areas of same wind speeds) are closer than 60 nautical miles on the charts showing the places of the jet stream, wind shear and CAT space possible.

Curving jet streams are likely to have turbulent edges, specifically those the curve around a deep pressure trough. When moderate or serious CAT has actually been reported or is forecast, change speed to stormy air speed instantly on encountering the first bumpiness or even prior to encountering that to protect against structural damage to the airplane.

The locations of CAT room usually shallow and also narrow and also elongated with the wind. If jet stream turbulence is encountered with a tail wind or head wind, a turn to the appropriate will uncover smoother waiting and much more favorable winds. If the CAT is encountered in a crosswind, the is no so essential to change course together the rough area will certainly be narrow.