7-1 CHAPTER 7. WIND
7.1 Introduction. Wind is the air in motion relative to the surface of the Earth. Although we cannot actually see the air moving, we can measure its motion by the force that it applies on objects. For example, leaves rustling or trees swaying on a windy day indicate that the wind is blowing. Winds are a major factor to both weather and aircraft.
Winds cause the formation, dissipation, and redistribution of weather. Winds also affect aircraft during all phases of flight. This chapter discusses how winds are named and the origin of wind.
7.2 Naming of the Wind. Wind is named according to the direction from which it is blowing. For example, a west wind indicates the wind is blowing from the west to the east. In aviation, 36 points of the compass are normally used to represent the direction from which the wind is blowing. For example, north winds come from 360°, east from 90°, south from 180°, and west from 270°.
7.3 Forces That Affect the Wind. Three primary forces affect the flow of wind: Pressure Gradient Force (PGF), Coriolis force, and friction.
7.3.1 Pressure Gradient Force (PGF). Wind is driven by pressure differences which create a force called the Pressure Gradient Force (PGF). Whenever a pressure difference develops over an area, the PGF makes the wind blow in an attempt to equalize pressure differences. This force is identified by height contour gradients on constant pressure charts and by isobar gradients on surface charts.
PGF is directed from higher height/pressure to lower height/pressure and is perpendicular to contours/isobars. Whenever a pressure difference develops over an area, the PGF begins moving the air directly across the contours/isobars.
Figure 7-1. Direction of Pressure Gradient Force
PGF is directed across contours/isobars towards lower height/pressure.
7-2 Wind speed is directly proportional to the PGF, which itself is directly proportional to the contour/isobar gradient. Closely spaced contours/isobars indicate strong winds, while widely spaced contours/isobars mean lighter wind. From a pressure analysis, you can get a general idea of wind speed from contour/isobar spacing.
Figure 7-2. Magnitude of Pressure Gradient Force
On the left panel, the contours/isobars are widely spaced apart, PGF is weak, and the wind speed is weak. On the right panel, the contours/isobars are more closely spaced, the PGF is stronger, and the wind speed is stronger.
The wind would flow from high to low pressure if the PGF was the only force acting on it. However, because of the Earth’s rotation, there is a second force called the Coriolis force that affects the direction of wind flow.
7.3.2 Coriolis Force. 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, consider a turntable. If one used a pencil and a ruler to draw a straight line from the center to the outer edge of the turntable, the pencil will have traveled in a straight line.
However, stopping the turntable, it is evident that the line spirals outward from the center (see Figure 7-3). To a viewer on the turntable, some apparent force deflected the pencil to the right.
Figure 7-3. Illustration of Coriolis Force
7-3 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 the Frenchman Gaspard-Gustave de Coriolis, and now carries his name—the Coriolis force.
Coriolis force affects all moving objects. The force deflects air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
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.
Figure 7-4. Coriolis Force Variations Across the Earth
Coriolis force deflects moving objects to the right of their path in the Northern Hemisphere and to the left of their path in the Southern Hemisphere. Coriolis deflection is maximized at the poles and zero at the Equator.
7-4 Figure 7-5. Coriolis Force Magnitude Variations with Wind Speed
Coriolis force magnitude is directly proportional to wind speed. Wind speed is twice as strong in the right panel;
thus, the Coriolis force is doubled.
7.3.3 Friction Force. Friction between the wind and the terrain surface slows the wind.
The rougher the terrain, the greater the frictional effect. Also, the stronger the wind speed, the greater 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.
Figure 7-6. Friction Force Magnitude Variations with Terrain Roughness
Friction force magnitude is directly proportional to terrain roughness. Even though the wind speed is the same in both panels, the terrain is rougher in the right panel; thus, the friction force is stronger.
Figure 7-7. Friction Force Magnitude Variations with Wind Speed
Friction force magnitude is directly proportional to wind speed. Wind speed is twice as strong in the right panel;
thus, the friction force is doubled.
The frictional drag of the ground normally decreases with height and becomes insignificant above the lowest few thousand feet. However, this may vary somewhat since both strong winds and rough terrain extend the friction layer to higher altitudes.
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7.4 Upper Air Wind. In the atmosphere above the friction layer (lowest few thousand feet), only PGF and Coriolis force affect the horizontal motion of air. Remember that the PGF drives the wind and is oriented perpendicular to height contours. When a PGF is first established, wind begins to blow from higher to lower heights directly across the contours. 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 contours. At this time, Coriolis force exactly balances PGF, as shown in Figure 7-8. With the forces in balance, wind will remain parallel to contours. This is called the geostrophic wind.
Figure 7-8. Geostrophic Wind
Figure 7-9. Upper Air Wind Flow
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7.5 Surface Wind. At the surface of the Earth, all three forces come into play. As frictional force slows the wind speed, Coriolis force decreases. However, friction does not affect PGF. PGF and Coriolis force are no longer in balance. The stronger PGF turns the wind at an angle across the isobars toward lower pressure until the three forces balance, as shown in Figure 7-10.
Figure 7-10. Surface Wind Forces
The angle of surface wind to isobars is about 10° over water, increasing to as high as 45° over rugged terrain. The end result is, in the Northern Hemisphere, the surface wind spirals clockwise and outward from high pressure, and counterclockwise and inward into low pressure (see Figure 7-11). 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.
Figure 7-11. Surface Wind Flow
8-1 CHAPTER 8. GLOBAL CIRCULATIONS AND JET STREAMS
8.1 Non-Rotating Earth Circulation System. Global circulations explain how air and storm systems travel over the Earth’s surface. The global circulation would be simple (and the weather boring) if the Earth did not rotate, the rotation was not tilted relative to the sun, and had no water.
Without those factors, the ground and atmosphere directly beneath the sun would be subject to more of the sun’s heat than anywhere else on the planet. The result would be the Equator becoming very hot, with the hot air rising into the upper atmosphere.
That hot air would then move toward the poles, where it would become very cold and sink, returning to the Equator (see Figure 8-1). One large area of high pressure would be at each of the poles, with a large belt of low pressure around the Equator.
Figure 8-1. Non-Rotating, Non-Tilted, Waterless, Earth Circulation System
8.2 Rotating Earth Circulation System. However, since the Earth rotates, the axis is tilted, and there is more land mass in the Northern Hemisphere than in the Southern Hemisphere, the actual global pattern is much more complicated.
Instead of one large circulation between the poles and the Equator, there are three circulations (see Figure 8-2):
• Hadley cell—Low-latitude air movement toward the Equator that, with heating, rises vertically with poleward movement in the upper atmosphere. This forms a convection cell that dominates tropical and subtropical climates.
• Ferrel cell—A mid-latitude mean atmospheric circulation cell for weather named by William Ferrel in the 19th century. In this cell, the air flows poleward and eastward near the surface, and equatorward and westward at higher levels.
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• Polar cell—Air rises, diverges, and travels toward the poles. Once over the poles, the air sinks, forming the polar highs. At the surface, air diverges outward from the polar highs. Surface winds in the polar cell are easterly (polar easterlies).
Figure 8-2. Earth Circulation System
Between each of these circulation cells are bands of high and low pressure at the surface.
The high pressure band is located about 30° N/S latitude and at each pole. Low pressure bands are found at the Equator and 50°-60° N/S.
Usually, fair and dry/hot weather is associated with high pressure, with rainy and stormy weather associated with low pressure. The results of these circulations become evident on a globe. Consider the number of deserts located along the 30° N/S latitude around the world compared to the region between 50°-60° N/S latitude. The higher latitudes, especially near the west coast of continents, tend to have more precipitation due to more storms moving around the Earth at these latitudes.
8.3 Jet Streams.
8.3.1 Introduction. Jet streams are relatively narrow bands of strong wind in the upper levels of the atmosphere. The winds blow from west to east in jet streams, but the flow often meanders southward and northward in waves. Jet streams follow the boundaries between
8-3 hot and cold air. Since these hot and cold air boundaries are most pronounced in winter, jet streams are the strongest for both the Northern and Southern Hemisphere winters.
8.3.2 Direction of Wind Flow. Why do the jet stream winds blow from west to east? As stated in the previous section, if the Earth was not rotating, the warm air would rise at the Equator and move toward both the poles. The Earth’s rotation divides this circulation into three cells. Likewise, the Earth’s rotation is responsible for the jet stream.
The motion of the air is not directly north and south, but rather is affected by the momentum the air has as it moves away from the Equator and how fast a location on or above the Earth moves relative to the Earth’s axis.
An object’s speed relative to the Earth’s axis depends on its location. Someone standing on the Equator is moving much faster than someone standing on a 45° latitude line.
In Figure 8-3, the person at the position on the Equator arrives at the yellow line sooner than the other two. Someone standing on a pole is not moving at all (except that he or she would be slowly spinning). The speed of the rotation is great enough to cause a person to weigh one pound less at the Equator than he or she would at the North or South Pole.
Figure 8-3. Speed Relative to the Earth’s Axis Versus Latitude
The momentum of air as it travels around the Earth is conserved, which means as the air that is over the Equator starts moving toward one of the poles, it keeps its eastward motion constant. The Earth below the air, however, moves slower, as that air travels toward the poles. The result is that the air moves faster and faster in an easterly direction (relative to the Earth’s surface below) the farther it moves from the Equator.
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8.3.3 Location. In addition, with the three cell circulations mentioned previously, the regions around 30° N/S and 50°-60° N/S are areas where temperature changes are the greatest.
As the difference in temperature between the two locations increases, the strength of the wind increases. Therefore, the regions around 30° N/S and 50°-60° N/S are also regions where the wind in the upper atmosphere is the strongest.
Figure 8-4. Three Cell Circulations and Jet Stream Location
The 50°-60° N/S region is where the polar jet is located with the subtropical jet located around 30° N. Jet streams vary in height of 4 to 8 miles and can reach speeds of more than 275 miles per hour (239 knots /442 kilometers per hour).
Figure 8-5. Polar and Subtropical Jet Streams
The actual appearance of jet streams results from the complex interaction between many variables, such as the location of high and low pressure systems, warm and cold air, and seasonal changes. They meander around the globe, dipping and rising in altitude/latitude, splitting at times and forming eddies, and even disappearing altogether to appear somewhere else.
8-5 Jet streams also follow the sun, in that as the sun’s elevation increases each day in the spring, the jet streams shift north moving into Canada by summer. As autumn approaches and the sun’s elevation decreases, the jet stream moves south into the United States, helping to bring cooler air to the country.
Figure 8-6. Jet Stream Wind Speeds
Also, the jet stream is often indicated by a line on maps, and shown by television meteorologists. The line generally points to the location of the strongest wind. In reality, jet streams are typically much wider. They are less a distinct location, and more a region where winds increase toward a core of highest speed.
One way of visualizing this is to consider a river. The river’s current is generally the strongest in the center, with decreasing strength as one approaches the river’s bank. It can be said that jet streams are rivers of air.
9-1 CHAPTER 9. LOCAL WINDS
9.1 Description. Local winds are small-scale wind field systems driven by diurnal heating or cooling of the ground. Air temperature differences develop over adjacent surfaces.
Air in contact with the ground heats during the day and cools at night. Low-level pressure gradients develop with higher pressure over the cooler, denser air, and lower pressure over the warmer, less dense air (see Figure 9-1).
Figure 9-1. Local Wind Circulation
Low-level winds develop in the direction of the Pressure Gradient Force (PGF). Coriolis force is insignificant because the circulation’s dimension (less than 100 miles) and life span (less than 12 hours) are too short for significant Coriolis deflection. Thus, the wind generally blows from a high-pressure cool surface to a low-pressure warm surface.
Air rises over the warmer surface and sinks over the cooler surface. A local wind circulation is easiest to identify when synoptic-scale wind patterns are weak.
Local winds include: sea breeze, land breeze, lake breeze, lake effect, valley breeze, mountain-plains wind circulation, and mountain breeze.