Climate Controls and Radiation Balance

Equatorial regions receive a net surplus and polar regions receive a net deficit of solar radiation because of differences in solar angle and beam dissipation at the poles and the equator. This heat imbalance between the equator and the poles drives the ocean–atmosphere circulation.

Heat is transported in the atmosphere primarily through convection, conduction, and advection. Convection and conduction are important in vertical atmospheric heat transport. Latent and sensible heating is the key mechanism by which convective and conductive transport are enacted. Horizontal heat transport is achieved primarily through migration of air masses and through eddy circulation.

The average global circulation on a simplified basis consists of three circulation cells. This structure is found in both hemispheres. The cell that straddles the tropics (0 degrees to approximately 30 degrees), known as the Hadley cell, is characterized by rising motion on its ascending limb along the equator and sinking motion on its descending limb at the subtropics (approximately 30 degrees). A second cell, known as the Ferrel cell, has an ascending limb at the midlatitudes and descending limb at the subtropics. The third cell, known as the polar cell, is characterized by rising motion at the midlatitudes and subsidence at the poles.

Such a general circulation structure results in a climate with intense convective precipitation in the regions along the rising limb of the Hadley cell. This region is identified as the intertropical convergence zone (ITCZ) because it is a zone where intense heating leads to convective motion and low pressure. Low-level or surface convergence and upper-level divergence result in convective precipitation in this zone. Regions along the descending limb of the Hadley cell—approximately 30 degrees north and 30 degrees south—tend to be warm and dry as subsiding air warms and dries out the air column. It is no coincidence that deserts are found at this latitude. The midlatitudes (40 degrees to 50 degrees)—the rising limb of the Ferrel cell—have cells of low pressure and receive precipitation from storms and frontal systems. Polar regions along the subsiding limb of the polar cell are characterized by cold and dry climates.

The three-division structure results in surface wind distribution characterized by winds blowing from the east (easterly) (trade winds) out of the subtropical high-pressure zone to the low-pressure zone at the equator; winds from the west (westerly) out of the subtropical highs to the midlatitude low-pressure zone; and polar easterlies flowing from the polar high-pressure zone to the midlatitude low (Figure 42-1). Such an east–west wind direction, as opposed to a north–south wind direction, prevails because of the Coriolis effect, which acts on the pressure gradient force and deflects winds to the right in the northern hemisphere and to the left in the southern hemisphere.

FIGURE 42-1 General circulation of the atmosphere.

(Courtesy Deborah Mioduszewski.)

The tricell structure is only a simplified representation of the general atmospheric circulation. In reality, the Ferrel cell does not persist throughout the year, as does the Hadley cell. The pressure gradient at the polar front is so intense that it results in eddies that are instrumental in poleward heat transport, particularly during the winter, when the equator–pole pressure gradient is at a maximum.

The midlatitudes are the regions most influenced by air masses. Eddy circulation is predominant at the boundary—known as the polar front—between the Ferrel cell and the polar cell.

Atmospheric Profile

Midlatitude and Polar Climates

Most weather systems, such as midlatitude cyclones and thunderstorms, form at the frontal boundaries between air masses of different densities and are steered by upper-level winds, typically to the east in the midlatitudes and west above about 60 degrees. This is in contrast to tropical cyclones, which are not influenced by air masses and only form over the ocean. Midlatitude cyclones are a powerful means by which air can be transported across latitudes. These differing air masses meet at boundaries called fronts.

Cold and warm fronts are typically oriented around a cyclone as shown in Figure 42-2. The colder, typically polar air mass advances equatorward behind the cyclone at the cold front, while the warmer air mass advances poleward ahead of it behind the warm front. Typically, the faster-moving cold front catches up to the warm front as the cyclone matures and “occludes,” and the cyclone begins to lose its strength as the temperature differences from which it derives its energy dissipate. Occasionally neither air mass will be able to dislodge the other, and the front will become stationary. Stationary fronts can be conducive to prolonged periods of precipitation.

FIGURE 42-2 Midlatitude cyclone model. L, Low pressure.

(Courtesy Deborah Mioduszewski.)

These areas of low pressure have favored storm tracks that shift during the year and as larger-scale atmospheric conditions change. Often atmospheric patterns emerge where cyclones take the same track, moving through the same areas for up to several weeks. Conversely, locations removed from these storm tracks may experience persistent fair weather associated with an accompanying anticyclone. Cyclones often form and move along the polar front, where there is a sharp contrast in temperature and moisture. During winter, the precise track of a cyclone will determine whether rain, snow, or a mixture will fall in a given location, with snow more likely on the north side of the cyclone and rain on the south side.

These cyclones bring the most noticeable changes to sensible weather in the mid- and high latitudes. In addition to the precipitation they bring, which ranges from drizzle and fog to severe thunderstorms, temperature and humidity levels vary greatly in different parts of the storm. Warm and moist air is advected in behind the warm front with a shift in the wind, causing extremely uncomfortable weather in the summer and thaws in the winter. Colder, dryer, and commonly breezy or blustery weather accompanies the passage of a cold front as air from higher latitudes is advected in behind a departing cyclone.

Subtropical and Tropical Climates

The subtropics lie north and south of the Hadley cell, roughly at 35 degrees north and 35 degrees south. The subtropics are characterized by warm and dry climates.

Tropical climates are found along the rising limb of the Hadley cell. Temperature is high throughout the year, with the average coldest temperature not dropping below 18° C (64.4° F). Convective processes and convective precipitation dominate the climate of the tropics but are not the sole climatic phenomena found there. Monsoons, tropical depressions, and cyclones account for much of the precipitation delivered over the tropics.

Monsoons

The term monsoon refers to the seasonal reversal in atmospheric circulation and associated precipitation in tropical and subtropical regions. Reversal of wind direction takes place with seasonal migration of the ITCZ, which induces a temperature difference between the land surface and ocean. The land–ocean temperature difference induces a land–ocean pressure gradient that drives the monsoonal winds. Regional components of the monsoon include the tropical and subtropical regions of Asia, Australia, Africa, South America, and southwestern North America. The onset of monsoonal rains varies among and within these regions.

tropical cyclones

Intense convective heating, particularly in the tropics, creates regions of low atmospheric pressure. Such regions of low pressure are characterized by rain-bearing clouds and windy conditions. If low-pressure systems form over warm oceans of the tropics and subtropics at least 5 degrees from the equator, they have the potential to develop into cyclones, which are intense, rotating low-pressure systems. Winds flow counterclockwise in the northern hemisphere and clockwise in the southern hemisphere around cyclones. Cyclones, hurricanes, and typhoons all refer to the same phenomenon. The norm is to use typhoons to identify severe cyclones in East Asia; hurricanes for severe cyclones in North America, and cyclones for severe cyclones that form over the Indian Ocean region. Cyclones form in areas of intense low pressure in the tropics and subtropics. Tropical cyclones are known for their destructive strength—often due to strong winds but also due to storm-surge flooding that occurs along coastal regions. Tropical cyclones form in all the major ocean basins, except in the South Atlantic and the Southeast Pacific. Minimum factors needed for tropical cyclones to develop are sea surface temperatures exceeding 28° C (82.4° F) and a minimal amount of vertical wind shear. Extratropical cyclones form at frontal boundaries. After formation, tropical cyclones tend to move westward and poleward. Some cyclones, particularly in the midlatitudes, recurve and enter the westerlies. Tropical cyclones tend to dissipate on making landfall. Cyclones are classified by their intensity as follows:

These are further categorized using the Saffir-Simpson scale, ranging from category 1 to category 5 (most severe) (Table 42-1).

TABLE 42-1 Saffir-Simpson Scale of Tropical Cyclone Intensity

Thunderstorms

Thunderstorms result from atmospheric instability due to intense convective heating processes in the tropics and convergence of air masses of different temperatures in midlatitudes, particularly in continental climates. Cumulonimbus clouds are characteristic features of thunderstorms, as are electrical activity (lightning and thunder), wind gusts, and heavy rain. Hail may be encountered in some thunderstorms. Heavy thunderstorms in upstream regions of river catchments with sharp elevation gradients may induce flash flooding in downstream regions.

One of the major dangers associated with thunderstorms is lightning. Lightning can strike more than 20 miles away from the parent thunderstorm, which may appear far enough away to present the illusion that there is no risk. When a thunderstorm is in the area, it is advisable to seek shelter away from high ground, water, and open spaces. Distance to a lightning strike can be calculated by counting the number of seconds between the flash of lightning and the thunder, keeping in mind that sound can travel a mile in 4.5 seconds.

Arid Climates

Arid climates are characterized by the presence of very little moisture. These are found most often along subtropical high-pressure zones or in the interiors of large continents where the influence of moisture-laden oceanic frontal systems is limited. Deserts are characteristic ecosystems of arid climates. Extreme heat, high diurnal temperature ranges, wind, and often fire danger are important elements of desert climates.

Mountain Climates

The lifting of air because of the presence of mountains is known as orographic lift, which results in cooling of the air. If sufficient moisture is present, condensation takes place. Such convection can induce thunderstorm development on the windward side of the mountain (Figure 42-3).

FIGURE 42-3 Orographic precipitation.

(Courtesy Deborah Mioduszewski.)

In the presence of steep topography, convective processes may be limited to the windward side because the mountain ridge can form a wet–dry divide. Air flowing over a mountain ridge and downward along the leeward side warms and dries through the process of adiabatic compression. Thus leeward winds are characteristically dry and warm. Such winds are known internationally as foehn winds, and as Chinooks in the United States. The Santa Ana winds that blow into Los Angeles are downslope winds.

Wind direction typically reverses twice a day in mountain climates. During the day, wind flows upslope from the valley toward the mountain. At night, winds blow downslope from the mountain toward the valley. Horizontal temperature differences that develop in complex terrain produce diurnal mountain winds. Overnight and into the early morning, cold air accumulates in valleys and causes a temperature inversion. By morning, the valley bottoms are colder than are mountain slopes and a pressure gradient develops that drives upslope winds. Convective heating during the day dissipates the temperature inversion in the valleys.

Varied topography in mountain ranges results in a myriad of microclimates. Factors such as exposure to sunshine, slope direction, elevation, and windward or leeward side of major weather systems determine the varied characteristics of microclimates found in mountainous regions.

Marine/Coastal Climates

The difference in heat capacity between land and ocean initiates a diurnal sea–land breeze circulation. During the day, land heats up much faster than does the ocean, establishing a land–ocean temperature difference that causes a pressure gradient. Wind blows toward the lower pressure over land to equalize the pressure difference. At night, land surfaces cool rapidly compared with adjacent oceans. The cycle is reversed at night, with winds blowing from the land toward the ocean to dispel the ocean–land temperature (and hence pressure) gradient (Figure 42-4).

FIGURE 42-4 Land–sea breeze circulation. H, High pressure; L, low pressure.

(Courtesy Deborah Mioduszewski.)

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