Chapter 4 | Wind
Differences in temperature create differences in pressure. These pressure differences drive a complex system of winds in a never ending attempt to reach equilibrium. Wind also transports water vapor and spreads fog, clouds, and precipitation. To help you relate wind to pressure patterns and the movement of weather systems, this chapter explains convection and the pressure gradient force, describes the effects of the Coriolis and frictional forces, relates convection and these forces to the general circulation, discusses local and small-scale wind systems, introduces you to wind shear, and associates wind with weather.
When two surfaces are heated unequally, they heat the overlying air unevenly. The warmer* air expands and becomes lighter or less dense than the cool* air. The more dense, cool air is drawn to the ground by its greater gravitational force lifting or forcing the warm air upward much as oil is forced to the top of water when the two are mixed. figure 18 shows the convective process. The rising air spreads and cools, eventually descending to complete the convective circulation. As long as the uneven heating persists, convection maintains a continuous “convective current.”
* Frequently throughout this book, we refer to air as warm, cool, or cold. These terms refer to relative temperatures and not to any fixed temperature reference or to temperatures as they may affect our comfort. For example, compare air at -10° F to air at 0° F; relative to each other, the -10° F air is cool and the 0° F, warm. 90° F would be cool or cold relative to 100° F.
The horizontal air flow in a convective current is “wind.” Convection of both large and small scales accounts for systems ranging from hemispheric circulations down to local eddies. This horizontal flow, wind, is sometimes called “advection.” However, the term “advection” more commonly applies to the transport of atmospheric properties by the wind, i.e., warm advection; cold advection; advection of water vapor, etc.
figure 18. Convective current resulting from uneven heating of air by contrasting surface temperatures. The cool, heavier air forces the warmer air aloft establishing a convective cell. Convection continues as long as the uneven heating persists.
PRESSURE GRADIENT FORCE
Pressure differences must create a force in order to drive the wind. This force is the pressure gradient force. The force is from higher pressure to lower pressure and is perpendicular to isobars or contours. Whenever a pressure difference develops over an area, the pressure gradient force begins moving the air directly across the isobars. The closer the spacing of isobars, the stronger is the pressure gradient force. The stronger the pressure gradient force, the stronger is the wind. Thus, closely spaced isobars mean strong winds; widely spaced isobars mean lighter wind. From a pressure analysis, you can get a general idea of wind speed from contour or isobar spacing.
Because of uneven heating of the Earth, surface pressure is low in warm equatorial regions and high in cold polar regions. A pressure gradient develops from the poles to the Equator. If the Earth did not rotate, this pressure gradient force would be the only force acting on the wind. Circulation would be two giant hemispheric convective currents as shown in figure 19. Cold air would sink at the poles; wind would blow straight from the poles to the Equator; warm air at the Equator would be forced upward; and high level winds would blow directly toward the poles. However, the Earth does rotate; and because of its rotation, this simple circulation is greatly distorted.
figure 19. Circulation as it would be on a nonrotating globe. Intense heating at the Equator lowers the density. More dense air flows from the poles toward the Equator forcing the less dense air aloft where it flows toward the poles. The circulation would be two giant hemispherical convective currents.
A moving mass travels in a straight line until acted on by some outside force. However, if one views the moving mass from a rotating platform, the path of the moving mass relative to his platform appears to be deflected or curved. To illustrate, start rotating the turntable of a record player. Then using a piece of chalk and a ruler, draw a “straight” line from the center to the outer edge of the turntable. To you, the chalk traveled in a straight line. Now stop the turntable; on it, the line spirals outward from the center as shown in figure 20. To a viewer on the turntable, some “apparent” force deflected the chalk to the right.
A similar apparent force deflects moving particles on the earth. Because the Earth is spherical, the deflective force is much more complex than the simple turntable example. Although the force is termed “apparent,” to us on Earth, it is very real. The principle was first explained by a Frenchman, Coriolis, and carries his name—the Coriolis force.
The Coriolis force affects the paths of aircraft; missiles; flying birds; ocean currents; and, most important to the study of weather, air currents. The force deflects air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This book concentrates mostly on deflection to the right in the Northern Hemisphere.
figure 20. Apparent deflective force due to rotation of a horizontal platform. The “space path” is the path taken by a piece of chalk. The “path on the record” is the line traced on the rotating record. Relative to the record, the chalk appeared to curve; in space, it traveled in a straight line.
Coriolis force is at a right angle to wind direction and directly proportional to wind speed. That is, as wind speed increases, Coriolis force increases. At a given latitude, double the wind speed and you double the Coriolis force. Why at a given latitude?
Coriolis force varies with latitude from zero at the Equator to a maximum at the poles. It influences wind direction everywhere except immediately at the Equator; but the effects are more pronounced in middle and high latitudes.
Remember that the pressure gradient force drives the wind and is perpendicular to isobars. When a pressure gradient force is first established, wind begins to blow from higher to lower pressure directly across the isobars. However, the instant air begins moving, Coriolis force deflects it to the right. Soon the wind is deflected a full 90° and is parallel to the isobars or contours. At this time, Coriolis force exactly balances pressure gradient force as shown in figure 21. With the forces in balance, wind will remain parallel to isobars or contours. Surface friction disrupts this balance as we discuss later; but first let's see how Coriolis force distorts the fictitious global circulation shown in figure 19.
figure 21. Effect of Coriolis force on wind relative to isobars. When Coriolis force deflects the wind until it is parallel to the isobars, pressure gradient balances Coriolis force.
THE GENERAL CIRCULATION
As air is forced aloft at the Equator and begins its high-level trek northward, the Coriolis force turns it to the right or to the east as shown in figure 22. Wind becomes westerly at about 30° latitude temporarily blocking further northward movement. Similarly, as air over the poles begins its low-level journey southward toward the Equator, it likewise is deflected to the right and becomes an east wind, halting for a while its southerly progress—also shown in figure 22. As a result, air literally “piles up” at about 30° and 60° latitude in both hemispheres. The added weight of the air increases the pressure into semipermanent high pressure belts. Figures 23 and 24 are maps of mean surface pressure for the months of July and January. The maps show clearly the subtropical high pressure belts near 30° latitude in both the Northern and Southern Hemispheres.
figure 22. In the Northern Hemisphere, Coriolis force turns high level southerly winds to westerlies at about 30° latitude, temporarily halting further northerly progress. Low-level northerly winds from the pole are turned to easterlies, temporarily stopping further southward movement at about 60° latitude. Air tends to “pile up” at these two latitudes creating a void in middle latitudes. The restless atmosphere cannot live with this void; something has to give.
figure 23. Mean world-wide surface pressure distribution in July. In the warm Northern Hemisphere, warm land areas tend to have low pressure, and cool oceanic areas tend to have high pressure. In the cool Southern Hemisphere, the pattern is reversed; cool land areas tend to have high pressure; and water surfaces, low pressure. However, the relationship is not so evident in the Southern Hemisphere because of relatively small amounts of land. The subtropical high pressure belts are clearly evident at about 30° latitude in both hemispheres.
figure 24. Mean world-wide surface pressure distribution in January. In this season, the pattern in figure 23 is reversed. In the cool Northern Hemisphere, cold continental areas are predominantly areas of high pressure while warm oceans tend to be low pressure areas. In the warm Southern Hemisphere, land areas tend to have low pressure; and oceans, high pressure. The subtropical high pressure belts are evident in both hemispheres. Note that the pressure belts shift southward in January and northward in July with the shift in the zone of maximum heating.
The building of these high pressure belts creates a temporary impasse disrupting the simple convective transfer between the Equator and the poles. The restless atmosphere cannot live with this impasse in its effort to reach equilibrium. Something has to give. Huge masses of air begin overturning in middle latitudes to complete the exchange.
Large masses of cold air break through the northern barrier plunging southward toward the Tropics. Large midlatitude storms develop between cold outbreaks and carry warm air northward. The result is a midlatitude band of migratory storms with ever changing weather. figure 25 is an attempt to standardize this chaotic circulation into an average general circulation.
Since pressure differences cause wind, seasonal pressure variations determine to a great extent the areas of these cold air outbreaks and midlatitude storms. But, seasonal pressure variations are largely due to seasonal temperature changes. We have learned that, at the surface, warm temperatures to a great extent determine low pressure and cold temperatures, high pressure. We have also learned that seasonal temperature changes over continents are much greater than over oceans.
During summer, warm continents tend to be areas of low pressure and the relatively cool oceans, high pressure. In winter, the reverse is true—high pressure over the cold continents and low pressure over the relatively warm oceans. Figures 23 and 24 show this seasonal pressure reversal. The same pressure variations occur in the warm and cold seasons of the Southern Hemisphere, although the effect is not as pronounced because of the much larger water areas of the Southern Hemisphere.
Cold outbreaks are strongest in the cold season and are predominantly from cold continental areas. Summer outbreaks are weaker and more likely to originate from cool water surfaces. Since these outbreaks are masses of cool, dense air, they characteristically are high pressure areas.
As the air tries to blow outward from the high pressure, it is deflected to the right by the Coriolis force. Thus, the wind around a high blows clockwise. The high pressure with its associated wind system is an anticyclone.
figure 25. General average circulation in the Northern Hemisphere. Note the three belts of prevailing winds, the polar easterlies, the prevailing westerlies in middle latitudes, and the northeasterly “trade” winds. The belt of prevailing westerlies is a mixing zone between the North Pole and the Equator characterized by migrating storms.
The storms that develop between high pressure systems are characterized by low pressure. As winds try to blow inward toward the center of low pressure, they also are deflected to the right. Thus, the wind around a low is counterclockwise. The low pressure and its wind system is a cyclone. figure 26 shows winds blowing parallel to isobars (contours on upper level charts). The winds are clockwise around highs and counterclockwise around lows.
The high pressure belt at about 30° north latitude forces air outward at the surface to the north and to the south. The northbound air becomes entrained into the midlatitude storms. The southward moving air is again deflected by the Coriolis force becoming the well-known subtropical northeast trade winds. In midlatitudes, high level winds are predominantly from the west and are known as the prevailing westerlies. Polar easterlies dominate lowlevel circulation north of about 60° latitude.
figure 26. Air flow around pressure systems above the friction layer. Wind (black arrows) is parallel to contours and circulates clockwise around high pressure and counterclockwise around low pressure.
These three major wind belts are shown in figure 25. Northeasterly trade winds carry tropical storms from east to west. The prevailing westerlies drive midlatitude storms generally from west to east. Few major storm systems develop in the comparatively small Arctic region; the chief influence of the polar easterlies is their contribution to the development of midlatitude storms.
Our discussion so far has said nothing about friction. Wind flow patterns aloft follow isobars or contours where friction has little effect. We cannot, however, neglect friction near the surface.
Friction between the wind and the terrain surface slows the wind. The rougher the terrain, the greater is the frictional effect. Also, the stronger the wind speed, the greater is the friction. One may not think of friction as a force, but it is a very real and effective force always acting opposite to wind direction.
As frictional force slows the windspeed, Coriolis force decreases. However, friction does not affect pressure gradient force. Pressure gradient and Coriolis forces are no longer in balance. The stronger pressure gradient force turns the wind at an angle across the isobars toward lower pressure until the three forces balance as shown in figure 27. Frictional and Coriolis forces combine to just balance pressure gradient force. figure 28 shows how surface wind spirals outward from high pressure into low pressure crossing isobars at an angle.
figure 27. Surface friction slows the wind and reduces Coriolis force but does not affect pressure gradient force; winds near the surface are deflected across the isobars toward lower pressure.
figure 28. Circulation around pressure systems at the surface. Wind spirals outward from high pressure and inward to low pressure, crossing isobars at an angle.
The angle of surface wind to isobars is about 10° over water increasing with roughness of terrain. In mountainous regions, one often has difficulty relating surface wind to pressure gradient because of immense friction and also because of local terrain effects on pressure.
THE JET STREAM
A discussion of the general circulation is incomplete when it does not mention the “jet stream.” Winds on the average increase with height throughout the troposphere culminating in a maximum near the level of the tropopause. These maximum winds tend to be further concentrated in narrow bands. A jet stream, then, is a narrow band of strong winds meandering through the atmosphere at a level near the tropopause. Since it is of interest primarily to high level flight, further discussion of the jet stream is reserved for chapter 13, “High Altitude Weather.”
LOCAL AND SMALL SCALE WINDS
Until now, we have dealt only with the general circulation and major wind systems. Local terrain features such as mountains and shore lines influence local winds and weather.
MOUNTAIN AND VALLEY WINDS
In the daytime, air next to a mountain slope is heated by contact with the ground as it receives radiation from the sun. This air usually becomes warmer than air at the same altitude but farther from the slope.
Colder, denser air in the surroundings settles downward and forces the warmer air near the ground up the mountain slope. This wind is a “valley wind” so called because' the air is flowing up out of the valley.
At night, the air in contact with the mountain slope is cooled by terrestrial radiation and becomes heavier than the surrounding air. It sinks along the slope, producing the “mountain wind” which flows like water down the mountain slope. Mountain winds are usually stronger than valley winds, especially in winter. The mountain wind often continues down the more gentle slopes of canyons and valleys, and in such cases takes the name “drainage wind.” It can become quite strong over some terrain conditions and in extreme cases can become hazardous when flowing through canyon restrictions as discussed in chapter 9.
A katabatic wind is any wind blowing down an incline when the incline is influential in causing the wind. Thus, the mountain wind is a katabatic wind. Any katabatic wind originates because cold, heavy air spills down sloping terrain displacing warmer, less dense air ahead of it. Air is heated and dried as it flows down slope as we will study in later chapters. Sometimes the descending air becomes warmer than the air it replaces.
Many katabatic winds recurring in local areas have been given colorful names to highlight their dramatic, local effect. Some of these are the Bora, a cold northerly wind blowing from the Alps to the Mediterranean coast; the Chinook, figure 29, a warm wind down the east slope of the Rocky Mountains often reaching hundreds of miles into the high plains; the Taku, a cold wind in Alaska blowing off the Taku glacier; and the Santa Ana, a warm wind descending from the Sierras into the Santa Ana Valley of California.
figure 29. The “Chinook” is a katabatic (downslope) wind. Air cools as it moves upslope and warms as it blows downslope. The Chinook occasionally produces dramatic warming over the plains just east of the Rocky Mountains.
LAND AND SEA BREEZES
As frequently stated earlier, land surfaces warm and cool more rapidly than do water surfaces; therefore, land is warmer than the sea during the day; wind blows from the cool water to warm land—the “sea breeze” so called because it blows from the sea. At night, the wind reverses, blows from cool land to warmer water, and creates a “land breeze.” figure 30 diagrams land and sea breezes.
Land and sea breezes develop only when the overall pressure gradient is weak. Wind with a stronger pressure gradient mixes the air so rapidly that local temperature and pressure gradients do not develop along the shore line.
figure 30. Land and sea breezes. At night, cool air from the land flows toward warmer water—the land breeze. During the day, wind blows from the water to the warmer land—the sea breeze.
Rubbing two objects against each other creates friction. If the objects are solid, no exchange of mass occurs between the two. However, if the objects are fluid currents, friction creates eddies along a common shallow mixing zone, and a mass transfer takes place in the shallow mixing layer. This zone of induced eddies and mixing is a shear zone. figure 31 shows two adjacent currents of air and their accompanying shear zone. Chapter 9 relates wind shear to turbulence.
figure 31. Wind shear. Air currents of differing velocities create friction or “shear” between them. Mixing in the shear zone results in a snarl of eddies and whirls.
WIND, PRESSURE SYSTEMS, AND WEATHER
We already have shown that wind speed is proportional to the spacing of isobars or contours on a weather map. However, with the same spacing, wind speed at the surface will be less than aloft because of surface friction.
You also can determine wind direction from a weather map. If you face along an isobar or contour with lower pressure on your left, wind will be blowing in the direction you are facing. On a surface map, wind will cross the isobar at an angle toward lower pressure; on an upper air chart, it will be parallel to the contour.
Wind blows counterclockwise (Northern Hemisphere) around a low and clockwise around a high. At the surface where winds cross the isobars at an angle, you can see a transport of air from high to low pressure. Although winds are virtually parallel to contours on an upper air chart, there still is a slow transport of air from high to low pressure.
At the surface when air converges into a low, it cannot go outward against the pressure gradient, nor can it go downward into the ground; it must go upward.* Therefore, a low or trough is an area of rising air.
* You may recall that earlier we said air “piles up” in the vicinity of 30° latitude increasing pressure and forming the subtropical high pressure belt. Why, then, does not air flowing into a low or trough increase pressure and fill the system? Dynamic forces maintain the low or trough; and these forces differ from the forces that maintain the subtropical high.
Rising air is conducive to cloudiness and precipitation; thus we have the general association of low pressure—bad weather. Reasons for the inclement weather are developed in later chapters.
By similar reasoning, air moving out of a high or ridge depletes the quantity of air. Highs and ridges, therefore, are areas of descending air. Descending air favors dissipation of cloudiness; hence the association, high pressure-good weather.
Many times weather is more closely associated with an upper air pattern than with features shown by the surface map. Although features on the two charts are related, they seldom are identical. A weak surface system often loses its identity in the upper air pattern, while another system may be more evident on the upper air chart than on the surface map.
Widespread cloudiness and precipitation often develop in advance of an upper trough or low. A line of showers and thunderstorms is not uncommon with a trough aloft even though the surface pressure pattern shows little or no cause for the development.
On the other hand, downward motion in a high or ridge places a “cap” on convection, preventing any upward motion. Air may become stagnant in a high, trap moisture and contamination in low levels, and restrict ceiling and visibility. Low stratus, fog, haze, and smoke are not uncommon in high pressure areas. However, a high or ridge aloft with moderate surface winds most often produces good flying weather.
Highs and lows tend to lean from the surface into the upper atmosphere. Due to this slope, winds aloft often blow across the associated surface systems. Upper winds tend to steer surface systems in the general direction of the upper wind flow.
An intense, cold, low pressure vortex leans less than does a weaker system. The intense low becomes oriented almost vertically and is clearly evident on both surface and upper air charts. Upper winds encircle the surface low and do not blow across it. Thus, the storm moves very slowly and usually causes an extensive and persistent area of clouds, precipitation, strong winds, and generally adverse flying weather. The term cold low sometimes used by the weatherman describes such a system.
A contrasting analogy to the cold low is the thermal low. A dry, sunny region becomes quite warm from intense surface heating thus generating a surface low pressure area. The warm air is carried to high levels by convection, but cloudiness is scant because of lack of moisture. Since in warm air, pressure decreases slowly with altitude, the warm surface low is not evident at upper levels. Unlike the cold low, the thermal low is relatively shallow with weak pressure gradients and no well defined cyclonic circulation. It generally supports good flying weather. However, during the heat of the day, one must be alert for high density altitude and convective turbulence.
We have cited three exceptions to the low pressure—bad weather, high pressure—good weather rule: (1) cloudiness and precipitation with an upper air trough or low not evident on the surface chart; (2) the contaminated high; and (3) the thermal low. As this book progresses, you can further relate weather systems more specifically to flight operations.