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.
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9.2 Hazards. Local winds can produce aviation weather hazards. Turbulence and shifting surface winds are common. Clouds and precipitation (including thunderstorms) can develop in the rising air over the warmer surface given sufficient moisture and lift.
9.3 Sea Breeze. A sea breeze (see Figure 9-2) is a coastal local wind that blows from sea to land, and caused by temperature differences when the sea surface is colder than the adjacent land. Sea breezes usually blow on relatively calm, sunny, summer days.
Figure 9-2. Sea Breeze
Air above the land becomes warmer (less dense) than air above the water. This is because land heats up faster than water. Low-level pressure gradients develop with lower pressure over the warmer land and higher pressure over the cooler water.
Low-level winds develop in the direction of the PGF. Thus, the wind blows from the water to the land. The air rises over land and sinks over water. Clouds (and precipitation) may develop in the rising air over land with cloud dissipation over the sinking air offshore.
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9.3.1 Sea Breeze Front. A sea breeze front (see Figure 9-3) is the horizontal discontinuity in temperature and humidity that marks the leading edge of the intrusion of cooler, moister marine air associated with a sea breeze. It often produces a wind shift and enhanced cumulus clouds along its leading edge. Cumuliform clouds may be absent if the air mass being lifted over land is dry or stable.
Figure 9-3. Sea Breeze Front
A sea breeze front’s position and movement are influenced by coastline shape, low-level wind direction and speed, and temperature difference between land and sea surface. This temperature difference can be affected by the presence of cloud cover over land and the diurnal cycle. The depth of convection is usually too shallow for precipitation to develop.
However, sea breeze fronts can be a lifting mechanism for shower and thunderstorm development.
9.3.2 Effects of Coastline Shape. Locally, the shape of the coastline plays an important role in the development of convection along sea breezes (see Figure 9-4). A narrow peninsula or island is generally an area of strong convective development during the late morning or early afternoon. This is because the sea breezes formed along opposing shores merge near the peninsula’s or island’s center.
9-4 Figure 9-4. Effects of Coastline Shape on a Sea Breeze
Convergence occurs where sea breezes merge from opposite directions. Stronger lift may be sufficient to initiate showers and thunderstorms if the air mass is sufficiently moist and unstable.
9.4 Land Breeze. A land breeze (see Figure 9-5) is a coastal breeze blowing from land to sea caused by the temperature difference when the sea surface is warmer than the adjacent land. Land breezes usually occur at night and during early morning.
9-5 Figure 9-5. Land Breeze
Air above the land becomes cooler (denser) than air above the water due to conduction.
This is because land cools faster than water. Low-level pressure gradients develop with higher pressure over the cooler land and lower pressure over the warmer water.
Low-level winds develop in the direction of the PGF. Thus, the wind blows from the land to the water. The land breeze is usually weaker than the sea breeze. The air rises over water and sinks over land. Clouds and precipitation may develop in the rising air over the water.
9.5 Lake Breeze. A lake breeze (see Figure 9-6) is a local wind that blows from the surface of a large lake onto the shores during the afternoon and is caused by the temperature difference when the lake surface is colder than the adjacent land. The lake breeze is similar in origin to the sea breeze, and is common in the Great Lakes. Both occur during the warm season, primarily spring and summer. Both are easiest to detect in light synoptic wind conditions.
9-6 Figure 9-6. Lake Breeze
As with sea breezes, thunderstorms are favored in the upward motion branch of the lake breeze circulation. This is especially true where breezes from adjacent lakes collide.
The strength of the lake breeze circulation is affected by a lake’s depth. A shallow lake
(e.g., Lake Erie and Lake St. Clair) warms up rapidly and is less effective as the source of a lake breeze in summer than a deep lake (e.g., the other Great Lakes).
9-7 Figure 9-7. Sea Breeze/Lake Breeze Example
The sinking air behind the lake breeze inhibits clouds over Lake Ontario and Lake Erie and for miles inland (National Aeronautics and Space Administration (NASA)).
9.6 Valley Breeze. A valley breeze (see Figure 9-8) is a wind that ascends a mountain valley during the day. Air in contact with the sloping terrain becomes warmer (less dense) than air above the valley. This is because the air in contact with the sloping terrain heats up faster than air above the valley.
Figure 9-8. Valley Breeze