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

Flight Environment



Since the environment is resolved to the earthby gravity and also rotates through 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.

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

Cold air, being much more dense, sinks andhot air, being much less dense, rises. Consequently, the rising warmth air in ~ the equatorbecomes also less thick as the rises and its pressure decreases. One area of lowpressure, therefore, exists over the equator.


Warm wait rises until it will acertain height at which the starts to spill over right into surrounding areas. At thepoles, the cold thick air sinks. Air from the upper levels that the atmosphere flowsin on peak of it boosting the weight and creating an area that high push at the poles.

The air the rises in ~ the equator does notflow directly to the poles. Due to the rotation of the earth, there is a construct up the airat about 30° phibìc latitude. (The very same phenomenon wake up in the southern Hemisphere). Some of the air sinks, leading to a belt the high-pressure in ~ this latitude.


The sinking wait reaches the surfaceand operation north and also south. The air the flows south completes one cabinet of theearth"s circulation pattern. The air the flows phibìc becomes component of one more cellof circulation between 30° and also 60° north latitude. At the same time, the sinkingair in ~ the phibìc pole operation south and also collides with the air moving north indigenous the 30°high press area. The colliding wait is forced upward and also an area of short pressureis created near 60° north. The third cell circulation sample is developed betweenthe phibìc pole and also 60° north.

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

Since the earth rotates, the axis is tilted, and there is an ext land fixed in the northern hemisphere than in the southerly hemisphere, the actual worldwide pattern is much more complicated. Instead of one huge circulation between the poles and also the equator, there space three circulations...Hadley cell
- low latitude air movement toward the equator that with heating, rises vertically, v poleward activity in the top atmosphere. This creates a convection cell that dominates tropical and sub-tropical climates.Ferrel cell - A mid-latitude median atmospheric circulation cell because that keolistravelservices.com called by Ferrel in the 19th century. In this cabinet the air operation poleward and also eastward close to the surface and also equatorward and westward at higher levels.Polar cell - waiting rises, diverges, and also travels toward the poles. When over the poles, the waiting sinks, developing the polar highs. At the surface air diverges external from the polar highs. Surface winds in the polar cell are easterly (polar easterlies).


There room two main pressures which affect the activity of waiting in the top levels. The pressure gradient reasons the air to relocate horizontally, forcing the air straight from a an ar of high push to a region of low pressure. The Coriolis force, however, deflects the direction the the flow of the wait (to the best in the northern Hemisphere) and causes the waiting to flow parallel to the isobars.

Winds in the upper levels will blow clockwise approximately areas the high pressure and counterclockwise about areas of short pressure.

The rate of the wind is established by the press gradient. The winds room strongest in areas where the isobars room close together.


Surface friction plays an important role in the speed and also direction of surface ar winds. Together a result of the slowing down of the air as it moves over the ground, wind speeds are less than would be intended from the press gradient ~ above the keolistravelservices.com map and also the direction is changed so the the wind blows across the isobars right into a center of low pressure and out of a facility of high pressure.


The effect of friction normally does not extend much more than a pair of thousands feet into the air. At 3000 feet over the ground, the wind blows parallel come the isobars through a rate proportional to the push gradient.

Even allowing for the effects of surface ar friction, the winds, locally, perform not constantly show the speed and direction that would be meant from the isobars on the surface ar keolistravelservices.com map. These variations are usually due to geographical attributes such together hills, mountains and large bodies that water. Except in such as mountain regions, the effect of terrain attributes that cause local sport in wind extends commonly no greater than around 2000 feet above the ground.


Land and also sea breezes are resulted in by the differences in temperature over land and water. The sea breeze occurs during the day as soon as the land area heats an ext rapidly 보다 the water surface. This results in the pressure over the soil being lower than the over the water. The push gradient is often strong enough because that a wind to blow from the water come the land.


The floor breeze blows in ~ night when the land i do not care cooler. Then the wind blows in the direction of the warm, low-pressure area end the water.

Land and also sea breezes are an extremely local and impact only a small area follow me the coast.


Hills and also valleys substantially distort the airflow associated with the prevailing pressure system and also the pressure gradient. Solid up and also down drafts and also eddies construct as the air flows up end hills and down right into valleys. Wind direction changes as the air flows roughly hills. Sometimes lines the hills and also mountain arrays will act as a barrier, holding earlier the wind and deflecting the so the it flows parallel come the range. If over there is a happen in the hill range, the wind will rush v this happen as v a tunnel with significant speed. The airflow deserve to be expected to continue to be turbulent and erratic for some distance as it flows out of the hilly area and also into the flatter countryside.


Daytime heating and nighttime cooling of the hilly slopes bring about day to night sports in the airflow. In ~ night, the political parties of the hills cool by radiation. The wait in contact with them i do not care cooler and also therefore denser and it blows under the slope right into the valley. This is a katabatic wind (sometimes likewise called a mountain breeze). If the slopes room covered with ice and also snow, the katabatic wind will blow, not only at night, but also during the day, carrying the cold dense air right into the warmer valleys. The slopes the hills not spanned by snow will certainly be warmed throughout the day. The air in contact with them i do not care warmer and also less thick and, therefore, flows up the slope. This is an anabatic wind (or valley breeze).

In mountainous areas, local distortion that the air flow is even more severe. Rocky surfaces, high ridges, slim cliffs, steep valleys, all combine to create unpredictable flow patterns and turbulence.



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

If the wait mass has a high moisture content, clouds of an extremely distinctive appearance will develop.

Cap Cloud. Orographic lift causes a cloud to kind along the optimal of the ridge. The wind carries this cloud under along the leeward slope wherein it dissipates v adiabatic heating. The basic of this cloud lies near or below the peaks the the ridge; the top might reach a couple of thousand feet above the peaks.

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

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


The clouds, being very distinctive, can be seen from a great distance and provide a clearly shows warning the the mountain wave condition. Unfortunately, periodically they are installed in other cloud systems and are covert from sight. Periodically the wait mass is very dry and the clouds carry out not develop.

The severity the the mountain wave and the height to which the disturbance that the waiting is affected is dependence on the strength of the wind, its edge to the range and the security or instability of the air. The most severe hill wave problems are developed in strong airflows that are blowing at appropriate angles to the range and in steady air. A jet present blowing practically perpendicular to the mountain variety increases the severity the the wave condition.

The hill wave phenomenon is not minimal only to high hill ranges, such as the Rockies, yet is likewise present come a lesser level in smaller hill systems and even in present of little hills.

Mountain waves current problems come pilots for several reasons:

Vertical Currents. Downdrafts that 2000 feet per minute room common and also downdrafts as good as 5000 feet per minute have been reported. They occur along the downward slope and also are most severe in ~ a elevation equal to the of the summit. One airplane, caught in a downdraft, could be compelled to the ground.

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

Wind Shear. The wind rate varies dramatically between the crests and also troughs of the waves. The is usually many severe in the tide nearest the hill range.

Altimeter Error. The boost in wind speed results in an accompanying diminish in pressure, which subsequently affects the accuracy that the press altimeter.

Icing. The freezing level varies substantially from crest come trough. Significant icing deserve to occur because of the huge supercooled droplets continual in the solid vertical currents.

When flying over a hill ridge whereby wave problems exist: (1) protect against ragged and also irregular shame clouds—the irregular shape indicates turbulence. (2) strategy the hill at a 45-degree angle. It you must suddenly decide to turn back, a rapid turn can be made far from the high ground. (3) stop flying in cloud on the hill crest (cap cloud) because of solid downdrafts and also turbulence. (4) enable sufficient height to clear the greatest ridges with altitude to preventive to avoid the downdrafts and also eddies ~ above the downwind slopes. (5) constantly remember that your altimeter have the right to read over 3000 ft. In error on the high next in mountain wave conditions.


A gust is a rapid and also irregular fluctuation of differing intensity in the upward and also downward motion of wait currents. It might be linked with a rapid adjust in wind direction. Gusts are led to by mechanical turbulence that results from friction between the air and the ground and by the unequal heater of the earth"s surface, particularly on warm summer afternoons.


A squall is a sudden rise in the strength of the wind of longer duration 보다 a gust and may be brought about by the i of a fast moving cold prior or thunderstorm. Choose a gust, it might be add by a rapid readjust of wind direction.


Diurnal (daily) sport of wind is led to by strong surface heating throughout the day, which reasons turbulence in the reduced levels. The an outcome of this disturbance is that the direction and also speed of the wind at the higher levels (e.g., 3000 feet) has tendency to be transferred to the surface. Because the wind direction at the higher level is parallel come the isobars and its speed is greater 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 also the surface ar wind often tends to resume its typical direction and speed. The backs and also decreases. View VEERING and BACKING section below for more info.


Friction between the moving air mass and also surface attributes of the planet (hills, mountains, valleys, trees, buildings, etc.) is responsible because that the swirling vortices the air generally called eddies. They vary significantly in size and intensity depending on the size and roughness that the surface obstruction, the rate of the wind and the level of security of the air. They have the right to spin in one of two people a horizontal or upright plane. Stormy air and strong winds produce much more vigorous eddies. In steady air, eddies have tendency to quickly dissipate. Eddies created in mountainous locations are specifically powerful.


The bumpy or choppy up and also down motion that signifies the existence of eddies makes it challenging to store an aircraft in level flight.


Dust devils are phenomena that take place quite typically on the warm dry plains of mid-western phibìc America. They deserve to be of sufficient force to current a risk to pilots of light airplanes flying at low speeds.

They are tiny heat lows that form on clear hot days. Offered a steep lapse rate caused by cool wait aloft over a warm surface, small horizontal air movement, few or no clouds, and the noonday sunlight heating flat arid soil surfaces come high temperatures, the wait in contact with the ground i do not care super-heated and highly unstable. This surface layer of wait builds until something cause an increase movement. As soon as started, the hot air rises in a column and draws much more hot air into the base of the column. Circulation begins about this warm low and also increases in velocity until a tiny vigorous whirlwind is created. Dust devils room usually of short duration and are so named because they are made clearly shows by the dust, sand and also debris that they pick up from the ground.

Dust devils pose the best hazard close to the ground whereby they are many violent. Pilots proposing to land top top superheated runways in locations of the mid-west whereby this phenomenon is usual should scan the airport because that dust swirls or grass spirals that would suggest the visibility of this hazard.


Tornadoes space violent, one whirlpools of air connected with serious thunderstorms and are, in fact, an extremely deep, concentrated low-pressure areas. They space shaped like a tunnel hanging out of the cumulonimbus cloud and also are dark in appearance because of the dust and debris sucked right into their whirlpools. They variety in diameter from about 100 feet come one half mile and move over the ground at speed of 25 to 50 knots. Their course over the soil is usually just a couple of miles lengthy although tornadoes have been reported to reduced destructive swaths as lengthy as 100 miles. The an excellent destructiveness the tornadoes is brought about by the an extremely low pressure in their centers and also the high wind speeds, i beg your pardon are considered to it is in as good as 300 knots.


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

In a conversation of wind direction, the compass allude from i m sorry the wind is blowing is considered to it is in its direction. Therefore, a north wind is one that is blowing from the north in the direction of the south. In aviation keolistravelservices.com reports, area and also aerodrome forecasts, the wind is constantly reported in levels true. In ATIS broadcasts and in the information provided by the tower for landing and take-off, the wind is reported in levels magnetic.


The wind veers as soon as it changes direction clockwise. Example: The surface ar wind is blowing native 270°. At 2000 feet the is blowing native 280°. It has adjusted in a right-hand, or clockwise, direction.

The wind backs as soon as it alters direction anti-clockwise. Example: The wind direction in ~ 2000 feet is 090° and also at 3000 feet is 085°. It is an altering in a left-hand, or anti-clockwise, direction.

In a descent from numerous thousand feet above the ground to ground level, the wind will typically be found to ago and additionally decrease in velocity, as the impact of surface ar friction becomes apparent. In a climb from the surface ar to several thousand feet AGL, the wind will veer and increase.

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


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


The result on aircraft performance that encountering wind shear derives native the truth that the wind can adjust much much faster than the plane mass can be accelerated or decelerated. Severe wind shears have the right to impose penalties on one airplane"s performance that are beyond its capabilities to compensate, especially throughout the an important landing and also take-off phase of flight.

In setravel Flight

In cruising flight, wind shear will likely be encountered in the change zone in between the pressure gradient wind and also the distorted neighborhood winds in ~ the lower levels. That will likewise be encountered when climbing or descending through a temperature inversion and also when passing through a frontal surface. Wind shear is also associated through the jet stream. Airplane encountering wind shear might experience a sequence of updrafts and downdrafts, reduce or gains in headwind, or windshifts that disrupt the established flight path. The is not commonly a significant problem since altitude and airspeed margins will certainly be sufficient to counteract the shear"s adverse effects. ~ above occasion, however, the wind shear may be severe sufficient to cause an abrupt increase in fill factor, which can stall the airplane or inflict structure damage.


Near the Ground

Wind shear, encountered near the ground, is an ext serious and potentially an extremely dangerous. There are 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 usually a trouble only in fronts with steep wind gradients. If the temperature difference across the front at the surface is 5°C or more and if the former is relocating 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 yet can be present in warm fronts as well.


Thunderstorms. Wind shear, associated with thunderstorms, occurs as the result of 2 phenomena, the gust front and downbursts. Together the thunderstorm matures, solid downdrafts develop, strike the ground and also spread out horizontally follow me the surface ar well in advancement 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 much as 10 miles ahead that the storm. The downburst is very intense localized downdraft flowing out of a thunderstorm. The strength of the downburst can exceed aircraft climb capabilities. The downburst (there space two types of downbursts: macrobursts and also microbursts) typically is much closer come the thunderstorm 보다 the gust front. Dust clouds, roll clouds, intense rainfall or virga (rain the evaporates before it get the ground) are due to the possibility of downburst task but there is no way to accurately predict its occurrence.


Temperature Inversions. Overnight cooling create a temperature turning back a couple of hundred feet over the soil that deserve to produce far-reaching wind shear, specifically if the turning back is coupled with the low-level jet stream.

As a nocturnal inversion develops, the wind shear near the top of the reverse increases. It normally reaches its preferably speed shortly after midnight and decreases in the morning as daytime heating dissipates the inversion. This phenomenon is recognized as the low-level nocturnal jet stream. The low level jet stream is a sheet of solid winds, countless miles long, hundreds of miles large and hundreds of feet thick that develops over level terrain such together the prairies. Wind speeds of 40 knots are common, yet greater speeds have actually been measured. Short level jet streams room responsible because that hazardous low level shear.


As the reverse dissipates in the morning, the shear airplane and gusty winds move closer to the ground, bring about windshifts and also increases in wind speed near the surface.

Surface Obstructions. The irregular and also turbulent circulation of air about mountains and hills and also through mountain passes reasons serious wind shear problems for plane approaching come land in ~ airports near hill ridges. Wind shear is a phenomenon connected with the mountain wave. Together shear is virtually totally unpredictable yet should be intended whenever surface winds room strong.

Wind shear is additionally associated v hangars and huge buildings in ~ airports. Together the air flows around such big structures, wind direction changes and also wind speed boosts causing shear.

Wind shear occurs both horizontally and also vertically. Vertical shear is most typical near the ground and also can pose a serious hazard to airplanes during take-off and landing. The aircraft is paris at lower speeds and in a reasonably high drag configuration. Over there is small altitude obtainable for recovering and also stall and maneuver margins room at their lowest. An airplane encountering the wind shear phenomenon might experience a big loss that airspeed because of the sudden readjust in the loved one airflow as the plane flies into a new, moving air mass. The abrupt autumn in airspeed may result in a stall, creating a dangerous situation when the aircraft is only a few hundred feet turn off the ground and an extremely vulnerable.


Narrow bands the exceedingly high rate winds are well-known to exist in the greater levels of the atmosphere at altitudes varying from 20,000 come 40,000 feet or more. They are well-known as jet streams. As plenty of as three major jet streams might traverse the north American continent at any type of given time. One lies across Northern Canada and also one across the U.S. A 3rd jet stream might be as much south together the north tropics but it is rather rare. A jet stream in the mid latitudes is usually the strongest.


The jet stream appears to it is in closely linked with the tropopause and with the polar front. It commonly forms in the break between the polar and the tropic tropopause whereby the temperature gradients space intensified. The mean position the the jet stream shears south in winter and also north in summer through the seasonal hike of the polar front. Since the troposphere is deeper in summer 보다 in winter, the tropopause and the jets will nominally be at higher altitudes in the summer.

Long, strong jet streams space usually also associated v well-developed surface lows in ~ deep upper troughs and lows. A low occurring in the tide along the frontal surface lies south of the jet. Together it deepens, the short moves close to the jet. As it occludes, the short moves north of the jet, which the cross the frontal system, close to the suggest of occlusion. The jet flows around parallel come the front. The subtropical jet present is not connected with fronts yet forms since of solid solar heater in the equatorial regions. The ascending air turns poleward at an extremely high levels however is deflected through the Coriolis pressure into a solid westerly jet. The subtropical jet predominates in winter.


The jet streams flow from west come east and also may encircle the entire hemisphere. More often, because they are stronger in some locations than in others, they break up into segments some 1000 come 3000 nautical miles long. They room usually about 300 nautical miles large and might be 3000 to 7000 feet thick. These jet present segments relocate in one easterly direction following the motion of pressure ridges and troughs in the top atmosphere.

Winds in the main core of the jet stream space the strongest and may with speeds as an excellent as 250 knots, back they are generally in between 100 and 150 knots. Wind speeds decrease toward the outer edges the the jet stream and also may it is in blowing at only 25 knots there. The rate of decrease of wind speed is considerably greater on the northern edge 보다 on the southern edge. Wind speed in the jet present are, top top average, substantially stronger in winter 보다 in summer.

Clear waiting Turbulence. The most probable ar to expect Clear Air disturbance (CAT) is just over the central core that the jet stream near the polar tropopause and also just listed below the core. Clean air disturbance does not take place in the core. CAT is encountered more frequently in winter once the jet stream winds room strongest. Nevertheless, CAT is not constantly present in the jet currently and, because it is random and transient in nature, that is virtually impossible to forecast.

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Clear waiting turbulence might be connected with various other keolistravelservices.com patterns, specifically in wind shear associated with the sharply curved contours of strong lows, troughs and ridges aloft, in ~ or listed below the tropopause, and in areas of strong cold or warmth air advection. Hill waves produce severe CAT the may extend from the mountain crests to together high as 5000 feet over the tropopause. Because severe CAT does pose a hazard to airplanes, pilots should try to protect against or minimization encounters with it. This rules of ignorance may assist avoid jet streams with solid winds (150 knots) at the core. Solid wind shears space likely above and listed below the core. CAT within the jet present is much more intense above and to the lee of mountain ranges. If the 20-knot isotachs (lines joining locations of same wind speeds) space closer 보다 60 nautical miles on the charts showing the places of the jet stream, wind shear and CAT are possible.

Curving jet streams are most likely to have turbulent edges, particularly those the curve about a deep pressure trough. When moderate or major CAT has been report or is forecast, adjust speed to stormy air speed instantly on encountering the very first bumpiness or even prior to encountering that to prevent structural damages to the airplane.

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