A temperature control is any factor that causes temperature to vary from place to place and from time to time. Earlier in this chapter, we examined the most important cause for temperature variations—differences in the receipt of solar radiation. Because variations in Sun angle and length of daylight depend on latitude, they are responsible for warm temperatures in the tropics and colder temperatures at more poleward locations. Of course, seasonal temperature changes at a given latitude occur as the Sun’s vertical rays migrate toward and away from a place during the year. The figure below ▼ reminds us of the importance of latitude as a control of temperature.

But latitude is not the only control of temperature; if it were, we would expect all places along the same parallel of latitude to have identical temperatures. This is clearly not the case. For example, Eureka, California, and New York City are both coastal cities at about the same latitude, and both have an annual mean temperature of 11°C (52°F). However, New York City is 9°C (16°F) warmer than Eureka in July and  cooler in January. In another example, two cities in Ecuador—Quito and Guayaquil—are relatively close to each other, yet the annual mean temperatures of these two cities differ by 10°C (18°F). To explain these situations and countless others, we must realize that factors other than latitude also exert a strong influence on temperature. In the next sections, we examine these other factors, which include differential heating of land and water, altitude, geographic position, cloud cover and albedo, and ocean currents.

Land and Water

The heating of Earth’s surface directly influences the heating of the air above it. Therefore, to understand variations in air temperature, we must understand the variations in heating properties of the different surfaces that Earth presents to the Sun—soil, water, trees, ice, and so on. Different land surfaces 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 between land and water. The figure below ▼ illustrates this idea nicely. This satellite image shows surface temperatures in portions of Nevada, California, and the adjacent Pacific Ocean on the afternoon of May 2, 2004, during a spring heat wave. Land-surface temperatures are clearly much higher than water-surface temperatures.

The image shows the extreme high surface temperatures in southern California and Nevada in dark red. Surface temperatures in the Pacific Ocean are much lower. The peaks of the Sierra Nevada, still capped with snow, form a cool blue line down the eastern side of California.

In side-by-side areas of land and water, such as those shown in the figure above ▲, land heats more rapidly and to higher temperatures than water, and it cools more rapidly and to lower temperatures than water. Variations in air temperatures, therefore, are much greater over land than over water.

Why do land and water heat and cool differently? Several factors are responsible:

• The specific heat (the amount of energy needed to raise the temperature of 1 gram of a substance by 1°C) is far greater for water than for land. Thus, water requires a great deal more heat to raise its temperature the same amount than does an equal quantity of land.

• Land surfaces are opaque, so heat is absorbed only at the surface. Water, being more transparent, allows heat to penetrate to a depth of many meters.

• The water that is heated often mixes with water below, thus distributing the heat through an even larger mass.

• Evaporation (a cooling process) from water bodies is greater than that from land surfaces.

Evaporation (a cooling process) from water bodies is greater than that from land surfaces.

Monthly temperature data for two cities will demonstrate the moderating influence of a large water body and the extremes associated with land ▼. Vancouver, British Columbia, is located along the windward Pacific coast, whereas Winnipeg, Manitoba, is in a continental position far from the influence of water. The two cities are at about the same latitude and, thus, experience similar Sun angles and lengths of daylight. Winnipeg, however, has a mean January temperature that is 20°C lower than Vancouver’s. Conversely, Winnipeg’s July mean is 2.6°C higher than Vancouver’s. Although their latitudes are nearly the same, Winnipeg, which has no water influence, experiences much greater temperature extremes than does Vancouver. The key to Vancouver’s moderate year-round climate is 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. In the Northern Hemisphere, 61 percent is covered by water, and land accounts for the remaining 39 percent. However, in the Southern Hemisphere, 81 percent is covered by water and 19 percent by land. The Southern Hemisphere is correctly called the water hemisphere ▼. Table 2 portrays the considerably smaller annual temperature variations in the water-dominated Southern Hemisphere as compared with the Northern Hemisphere.

Table 2 portrays the considerably smaller annual temperature variations in the water-dominated Southern Hemisphere as compared with the Northern Hemisphere.

Altitude

The two cities in Ecuador mentioned earlier—Quito and Guayaquil—demonstrate the influence of altitude on mean temperatures. Although both cities are near the equator and not far apart, the annual mean temperature at Guayaquil is 25°C (77°F), as compared to Quito’s mean of 13°C (55°F). The difference is explained largely by the difference in the cities’ elevations: Guayaquil is only 12 meters (40 feet) above sea level, whereas Quito is high in the Andes Mountains, at 2800 meters (9200 feet). The figure below ▼ provides another example.

Monthly mean temperatures for Concepción and La Paz, Bolivia

Both cities have nearly the same latitude (about 16 degrees south). However, because La Paz is high in the Andes, at 4103 meters (13,461 feet), it experiences much cooler temperatures than Concepción, which is at an elevation of 490 meters (1608 feet.)

Recall that temperatures drop an average of 6.5°C per kilometer in the troposphere; thus, cooler temperatures are to be expected at greater heights. Yet the magnitude of the difference is not explained completely by the normal lapse rate. If the normal lapse rate is used, we would expect Quito to be about 18°C cooler than Guayaquil, but the difference is only 12°C. The fact that high-altitude places, such as Quito, are warmer than the value calculated using the normal lapse rate results from the absorption and reradiation of solar energy by the ground surface.

Geographic Position

The geographic setting can greatly influence the temperatures experienced at a specific location. A coastal location where prevailing winds blow from the ocean onto the shore (a windward coast) experiences considerably different temperatures than does a coastal location where the prevailing winds blow from the land toward the ocean (a leeward coast). In the first situation, the windward coast will experience the full moderating influence of the ocean—cool summers and mild winters—compared to an inland location at the same latitude.

A leeward coast, on the other hand, will have a more continental temperature pattern because the winds do not carry the ocean’s influence onshore. Eureka, California, and New York City, the two cities mentioned earlier, illustrate this aspect of geographic position. The annual temperature range at New York City is 19°C (34°F) greater than Eureka’s ▼.

Seattle and Spokane, both in the state of Washington, illustrate a second aspect of geographic position: mountains that act as barriers. Although Spokane is only about 360 kilometers (220 miles) east of Seattle, the towering Cascade Range separates the cities. Consequently, Seattle’s temperatures show a marked marine influence, but Spokane’s are more typically continental ▼. Spokane is 7°C (13°F) cooler than Seattle in January and 4°C (7°F) warmer than Seattle in July. The annual range at Spokane is 11°C (20°F) greater than at Seattle. The Cascade Range effectively cuts off Spokane from the moderating influence of the Pacific Ocean.

Cloud Cover and Albedo

You may have noticed that clear days are often warmer than cloudy ones and that clear nights are usually cooler than cloudy ones. Cloud cover is another factor that influences temperature in the lower atmosphere. Studies using satellite images show that at any particular time, about half of our planet is covered by clouds. Cloud cover is important because many clouds have a high albedo; therefore, clouds reflect a significant portion of the sunlight that strikes them back into space (refer to Figure 16.24). By reducing the amount of incoming solar radiation, clouds reduce daytime temperatures.

At night, clouds have the opposite effect as during daylight: They act as a blanket, absorbing radiation emitted by Earth’s surface and reradiating a portion of it back to the surface. Consequently, some of the heat that otherwise would have been lost remains near the ground. Thus, nighttime air temperatures do not drop as low 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 ▼.

Clouds are not the only phenomenon that increase albedo and, thereby, reduces air temperatures. We also recognize that snow- and ice-covered surfaces have high albedos. This is one reason mountain glaciers do not melt away in the summer, and it is why snow may still be present on a mild spring day. In addition, during the winter, when snow covers the ground, daytime maximums on a sunny day are less than they otherwise would be because energy that the land would have absorbed and used to heat the air is reflected and lost.

• Controls of temperature are factors that cause temperature to vary from place to place and from time to time. Latitude (Earth–Sun relationships) is one example. Ocean currents provide another example.

• Unequal heating of land and water is a temperature control. Because land and water heat and cool differently, land areas experience greater temperature extremes than do water-dominated areas.

• Altitude is an easy-to-visualize control: The higher up you go, the colder it gets; therefore, mountains are cooler than adjacent lowlands.

• Geographic position as a temperature control involves interrelated factors, such as mountains acting as barriers to marine influence and a place being on a windward coast or a leeward coast.

• leeward coast: A coast where the prevailing winds blow from the land toward the ocean.

• specific heat: the amount of heat energy required to raise the temperature of one unit of mass (1 kg or 1 g) of a substance by one degree Celsius (°C) or Kelvin (K)

• temperature control: A factor that causes temperature to vary from place to place or from time to time.

• windward coast: A coast where the prevailing winds blow from the ocean toward the land.