Name the principal controls of temperature and use examples to describe their effects.

A temperature control is any factor that causes temperatures to vary. The primary control of temperature is latitude. Recall from Chapter 2 that latitude determines the annual variations in Sun angle and length of daylight, which causes warmer temperatures in the tropics and colder temperatures at the poles. As the Sun’s vertical rays migrate during the year, they produce the seasonal temperature changes we observe each year. Figure 3.4 shows the annual temperature cycle for several different locations and reminds us of the importance of latitude as a control of temperature and seasonal temperature variations.

But latitude is not the only control of temperature. If it were, we would expect all places along the same line of latitude to have identical temperatures, which is clearly not the case. For instance, Eureka, California, and New York City are both coastal cities at about the same latitude, and both have the same annual mean temperature of 11°C (about 52°F). Yet New York City averages 9°C (17°F) warmer than Eureka in July and 9°C (17°F) colder than Eureka in January. To explain these differences and countless others, we must acknowledge that factors other than latitude strongly influence temperature.

Elevation

Recall that temperatures on average decrease with an increase in altitude in the troposphere. As a result, some mountaintops are snow covered year-round (Figure 3.5). Two cities in Ecuador, Quito and Guayaquil, demonstrate the influence of altitude on mean temperature (Figure 3.6). Both cities are located near the equator and relatively close to one another, but the annual mean temperature at Guayaquil is about 26°C (80°F), compared with Quito’s mean of half that value, 13°C (56°F). What explains the difference? Guayaquil is located near sea level, whereas Quito is high in the Andes Mountains, at 2800 meters (9200 feet).

Elevation significantly affects not only mean temperatures, but also the daily temperature range. Temperatures drop as altitude increases, and atmospheric pressure and density also diminish. Because air is thinner at high altitudes, the overlying atmosphere absorbs, reflects, and scatters a smaller portion of the incoming solar radiation. Consequently, with an increase in altitude, the intensity of solar radiation increases, resulting in rapid daytime heating. Conversely, nighttime cooling also occurs at an accelerated rate. Therefore, observing stations located high in the mountains generally have a greater daily temperature range than do stations at lower elevations.

Elevation clearly modifies temperatures, but the annual temperature range (difference between the warmest and coldest monthly mean temperatures) is not strongly affected. The monthly mean temperatures are lower throughout the year at higher elevations, but the annual temperature range would be similar to a location at lower elevation, all other factors being equal.

Land and Water

Recall that the heating of Earth’s surface controls, to a large degree, the heating of the air above it. Therefore, to understand variations in air temperature, we must understand the variations in heating properties of different surfaces—soil, water bodies, forests, ice, and so on. Different land surfaces reflect and absorb varying amounts of incoming solar energy, which in turn cause variations in the temperature of the air above. The greatest contrast, however, is not between different land surfaces but rather between land and water, as illustrated in Figure 3.7. This satellite image shows surface temperatures in portions of California, Nevada, and the adjacent Pacific Ocean on the afternoon of May 2, 2004, during a spring heat wave. Land-surface temperatures, shown mainly in red, are clearly much higher than water-surface temperatures. The peaks of the Sierra Nevada, still capped with snow, form a cool blue band down the eastern side of California.

For large bodies of land and water, such as those shown in Figure 3.7, land heats more rapidly and to higher temperatures than water and, conversely, cools more rapidly and to lower temperatures than water. Why do land and water heat and cool differently? Several factors are responsible:

• An important reason that the surface temperature of water rises and falls much more slowly than the surface temperature of land is that water is highly mobile. As water is heated, convective flow distributes the heat through a considerably larger mass. In contrast, heat does not penetrate deeply into soil or rock; it remains concentrated near the surface. Obviously, no mixing can occur on land because it is not fluid. Instead, heat must be transferred by the slow process of conduction. During winter, the shallow layer of rock and soil that was heated in summer cools rapidly. Water bodies, in contrast, cool slowly as they draw on the reserve of heat stored within. As the water surface cools, the chilled surface water, which is dense, sinks and is replaced from below by warmer water, which is less dense. Consequently, a larger mass of water must cool before the temperature at the surface will drop appreciably.

• Because land surfaces are opaque, heat is absorbed only at the surface. This fact is easily demonstrated at a beach on a hot summer afternoon by comparing the surface temperature of the sand to the temperature just a few centimeters beneath the surface. Water, being more transparent, allows some solar radiation to penetrate to a depth of several meters.

• The specific heat—the amount of heat needed to raise the temperature of 1 gram of a substance by 1°C—is more than three times greater for water than for land. Thus, considerably more heat is required to raise the temperature of water the same amount as an equal volume of land.

• Evaporation (a cooling process) from water bodies is greater than from land surfaces. Energy is required to evaporate water. When energy is used for evaporation (latent heat), it does not raise the temperature (sensible heat).

Collectively, these factors cause water to warm more slowly, store greater quantities of heat, and cool more slowly than land.

Monthly temperature data for two cities demonstrate the moderating influence of a large water body and the more extreme seasonal temperatures associated with land-locked locations (Figure 3.8). Vancouver, British Columbia, is a maritime city located along the windward Pacific coast, whereas Winnipeg, Manitoba, is far from the influence of water. Both cities are at about the same latitude and thus experience similar Sun angles and lengths of daylight throughout the year. Vancouver, however, has a mean January temperature that is 20°C (36°F) warmer than Winnipeg’s and a July mean temperature that is 2.6°C (4.7°F) cooler than Winnipeg’s. The key to Vancouver’s moderate year-round climate is the influence of the Pacific Ocean.

On a different scale, the moderating influence of water may also be demonstrated when temperature variations in the Northern and Southern Hemispheres are compared. The views of Earth in Figure 3.9 show the uneven distribution of land and water over the globe. Water covers 61 percent of the Northern Hemisphere; land represents the remaining 39 percent. However, the figures for the Southern Hemisphere (81 percent water and 19 percent land) reveal why it is correctly called the “water hemisphere.” Table 3.1 portrays the considerably smaller annual temperature ranges in the water-dominated Southern Hemisphere compared with the Northern Hemisphere.

Ocean Currents

The Gulf Stream is an important surface current in the Atlantic Ocean that flows northward along the East Coast of the United States (Figure 3.10). Surface currents like this one are set in motion by the wind. At the water surface, energy is passed from moving air to the water through friction. The resulting drag exerted by winds blowing steadily across the ocean causes the surface layer of water to move. Thus, major horizontal movements of surface waters are closely related to the circulation of the atmosphere.

The atmospheric circulation that produces the ocean currents is driven by the unequal heating of Earth by the Sun. Recall from Chapter 2 that there is a net gain of incoming solar radiation in the tropics and a net loss at the poles. Because the tropics are not becoming progressively warmer, nor are the polar regions becoming colder, there must be a large-scale transfer of heat from areas of excess heat energy to areas of deficit. This is indeed the case.

Ocean water movements account for about one-quarter of this total heat transport, and winds account for the remaining three-quarters (Figure 3.11).

Surface-ocean currents have an important effect on climate. The moderating effect of poleward-moving warm ocean currents is well known. The North Atlantic Drift, an extension of the warm Gulf Stream, keeps wintertime temperatures in Great Britain and much of Western Europe warmer than would be expected for their latitudes (see Figure 3.11).

In contrast to warm ocean currents, the effects of which are most apparent during the winter, cold currents exert their greatest influence in the tropics or, during the summer months, in the middle latitudes. For example, the cool Peru Current off the west coast of South America moderates the tropical heat along this coast. As shown in Figure 3.12, Arica, Chile, a town adjacent to the cold Peru current, is about 8°C (13°F) cooler in summer than Rio de Janeiro, Brazil. Closer to home, the cold California Current keeps summer temperatures in subtropical coastal southern California 6°C (about 11°F) cooler on average than temperatures recorded at east coast stations at the same latitude.

Geographic Position and Prevailing Wind Direction

The effects of differential heating of land and water can be blown inland by the prevailing wind at a particular location. A coastal station where prevailing winds blow from the ocean onto the shore (a windward coast) observes considerably different temperatures from those observed by a coastal location where prevailing winds blow from the land toward the ocean (a leeward coast). In the first situation, the windward coast experiences the full moderating influence of the ocean—cooler summers and milder winters. A station on a leeward coast at the same latitude would experience a much larger annual temperature range. Eureka, California, and New York City, two cities mentioned earlier, illustrate this aspect of geographic position (Figure 3.13). The prevailing winds in the midlatitudes flow from west to east and are called the westerlies. Because Eureka is strongly influenced by prevailing westerly winds from the ocean and New York City is not, the annual temperature range at Eureka is much smaller.

If there is an ocean current along the coast, then the moderating effect of the ocean current can also be blown inland. Because of the prevailing westerly winds, the moderating effects of the North Atlantic Drift are carried far inland. For example, the January mean in London is almost 5°C (8°F) higher than in New York City, which lies 13° latitude farther south than London.

Prevailing winds can also affect temperatures if the winds blow across a mountain range, where the mountains create a barrier from the moderating effects of the ocean. Seattle and Spokane, both in the state of Washington, illustrate this aspect of geographic position. Although Spokane is only about 360 kilometers (225 miles) east of Seattle, the towering Cascade Range effectively cuts off Spokane from the moderating influence of the Pacific Ocean. Consequently, Seattle’s temperatures show a marked marine influence, whereas Spokane’s are more typically continental (Figure 3.14). Seattle is 7°C (13°F) warmer than Spokane in January and 4°C (7°F) cooler than Spokane in July. The annual temperature range at Spokane is 11°C (nearly 20°F) greater than in Seattle.

Mountains not only act as barriers to airflow, but also influence temperatures in other ways. The windward (upslope) side of a mountain tends to be cooler, while the leeward (downslope) side tends to be warmer. One example of the influence of mountains on temperature is found in the southwestern United States. Winds blowing onshore along the California coast keep temperatures relatively moderate. These winds then travel up and over a large mountain belt and descend into eastern California, Nevada, and Arizona. The air coming down the mountain warms by compression as it descends. Consequently, the air reaching the leeward side of the mountain is much warmer than the air on the windward side. The adiabatic processes that produces this phenomenon will be discussed in more detail in the next chapter.

Albedo Variations

Recall that albedo refers to the fraction of radiation reflected by an object. Also recall that solar radiation reflected back to space is lost to Earth and does not play a role in heating Earth’s surface and atmosphere. Thus, any increase in albedo reduces the amount of energy available to heat the atmosphere. Conversely, a decrease in albedo means an increase in the quantity of energy absorbed by Earth’s surface and available to heat the atmosphere.

Cloud Cover

You may have noticed that clear days are often warmer than cloudy ones. This demonstrates that cloud cover is an important control of temperature because clouds reflect some of the sunlight that strikes them. Because cloud cover reduces the amount of incoming solar radiation, daytime temperatures are lower than if the sky were clear (Figure 3.15). The albedo of clouds depends on the thickness of the cloud cover and can vary from 25 to 90 percent (see Figure 2.17).

At night, clouds have the opposite effect. They absorb outgoing Earth radiation and emit a portion of it back toward the surface. Thus, nighttime air temperatures do not drop as dramatically as they would on a clear night. The effect of cloud cover is to reduce the daily temperature range by lowering the daytime maximum and raising the nighttime minimum, as illustrated by the graph in Figure 3.15.

Extensive periods of cloud cover can also reduce temperatures sufficiently in some locations to disrupt the seasonal cycle of temperatures. For example, each year much of southern Asia experiences an extended period of heavy monsoon rains. (This pattern is associated with the monsoon circulation and is discussed in Chapter 7.) The graph for Yangon, Myanmar (Burma), illustrates this pattern (Figure 3.16). Notice that the highest monthly mean temperatures occur in April and May, before the summer solstice, rather than in July and August as normally occurs at most stations in the Northern Hemisphere. This is because the extensive cloud cover during the summer months, when we would usually expect temperatures to climb, increases the albedo of the region and reduces incoming solar radiation at the surface. As a result, the highest monthly mean temperatures occur in late spring, when the skies are still relatively clear.

Influence of Snow and Ice

Snow- and ice-covered surfaces have high albedos. This is why snow covering the ground keeps daytime temperatures on a sunny day cooler than they would be otherwise. This idea is illustrated in Figure 3.17. The incoming solar energy is reflected by the snow and lost rather than absorbed by the ground and heating the lower atmosphere.

Large portions of the Arctic Ocean are covered by sea ice—frozen seawater that floats because it is less dense than liquid water. As you would expect, the area covered by sea ice changes with the seasons, expanding in winter and contracting in summer. Monitoring since the late 1970s has also shown that, over the years, the area covered by sea ice is shrinking. Thus, broad zones that were once covered by highly reflective ice are being replaced by the darker ocean surface that reflects less and absorbs more. The lowering of albedo in the Arctic is contributing to rising temperatures in this region, a topic that will be addressed in more detail in Chapter 14.

Other Factors Influencing Temperature

Many local, as well as weather-related, factors influence the temperature of a location. One local factor is the type of surface that predominates. For example, areas that are heavily vegetated tend to have cooler average summer temperatures than sparsely vegetated arid regions. Shade and transpiration by plants absorb large quantities of heat from the surface. This idea is exemplified by cities in the eastern United States, such as Atlanta, Georgia, that have significantly higher summer temperatures than the surrounding forested rural areas. This phenomenon, called the urban heat island, is considered in more detail in Box 3.2.

A related factor, the amount of water vapor in the air, influences daily temperature range because it is one of the atmosphere’s important heat-absorbing gases. When the air is dry and the sky is clear, surface heat readily escapes at night, and the temperature can fall rapidly. By contrast, when the air is humid, absorption of outgoing longwave radiation by water vapor slows nighttime cooling.

Atmospheric circulation patterns strongly influence the movement of warm and cold air across a region, which directly affects temperatures. These large-scale circulation patterns are also associated with weather systems that bring cloud cover and precipitation, which reduce incoming solar radiation and result in cooler temperatures. These important influences of temperature will be considered in the following chapters.

• specific heat: The amount of heat needed to raise 1 gram of a substance 1°C at sea-level atmospheric pressure.

• temperature controls: Factors that cause variations in temperature from place to place, such as latitude and altitude.

• Controls of temperature are factors that cause temperature to vary at a given location. The primary control of temperature is latitude because it determines the annual variations in Sun angle and length of daylight.

• Altitude is an easily visualized control: The higher up you go in elevation, the colder it gets; therefore, mountains are cooler than adjacent lowlands.

• Land and water heat and cool differently. Land areas experience greater temperature extremes than water-dominated areas.

• Poleward-moving warm ocean currents moderate winter temperatures, while equatorward movement of cold currents moderate summer temperatures.

• The effect of cloud cover is to reduce the daily temperature range by lowering the daytime maximum and raising the night-time minimum.

• Geographic position also affects temperature; for example, mountains act as barriers to marine influence, and windward coastal locations tend to experience cooler summers and milder winters than leeward coastal locations. Also, the windward (upslope) side of a mountain tends to be cooler, while the leeward (downslope) side tends to be warmer.