2.3 Surface Temperature Patterns

Describe Earth’s surface temperature patterns and explain what causes them.

What controls the overall air temperature regime of any given location on Earth? Death Valley, California, is always hot in summer; in fact, it holds the record for the highest air temperature ever officially recorded on Earth: 56°C (134°F) on July 10, 1913, in the shade! Earth’s lowest official air temperature, −89°C (−129°F), was recorded in Vostok Station, Antarctica, at a scientific research base in July 1983. Why is Death Valley so hot? Why is Antarctica so cold?

Recording air temperatures gives us critical data for answering these questions as well as for monitoring climate change. Temperature data are compiled by the Global Historical Climatology Network (GHCN) and are recorded daily at over 75,000 meteorological stations in 180 countries and territories (Figure 2.15). For each station, an average daily temperature is calculated from maximum and minimum daily temperature measurements. Average monthly and average annual temperatures are then derived from these calculations.

Figure 2.15

Map of global temperature stations. This world map shows the locations of official GHCN meteorological stations that record air temperatures. Station density decreases in remote areas and in less economically developed countries. Ships and aircraft supplement the temperature data for sparsely covered regions.
(Global Historical Climatology Network, NOAA/NESDIS/NCDC)

The difference between the average maximum and average minimum temperatures over a year at a location is its annual temperature range, or seasonality. Earth’s maximum temperature range (the difference between the highest temperature in Death Valley and the lowest temperature in Antarctica) is therefore 145°C (265°F). In this section we use the average annual temperature and the annual temperature range to explore geographic patterns of temperature.

Average Annual Temperature Patterns

The average annual temperature at any given location is controlled mainly by elevation and latitude. The average annual temperature is lower at higher elevations and higher latitudes.

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Elevation: Colder in the Mountains

Question 2.10

Does it snow in Hawai’i?

It often snows in Hawai’i at the highest elevations (over 3,000 m or 10,000 ft), where it is always very cold. Hawai’i’s lowest recorded temperature is −11°C (12°F).

The troposphere is heated mostly by Earth’s surface. In the troposphere, temperature decreases with altitude. In Section 1.3, we learned that there is an average environmental lapse rate of 6.5°C per 1,000 m (or 3.6°F per 1,000 ft). Mountains that protrude high into the troposphere are always cooler than surrounding lowland regions. High mountains are often snowcapped in summer, even in tropical locations, such as Hawai’i, where adjacent areas of low elevation are warm. Figure 2.16 illustrates this point with a satellite thermal image.

Figure 2.16

Land surface temperature and elevation. This satellite thermal infrared image shows average January land surface temperatures for South Asia during the period 2001–2010. Reds and oranges, which depict warm temperatures, are found mainly at low elevations, and blues, which show cold temperatures, are found mainly in mountainous regions.
(NASA)

Latitude: Colder near the Poles

Because Earth is spherical, temperature generally decreases away from the equator as sunlight becomes more diffuse (see Figure 2.5). Sunlight is increasingly diffuse outside the tropics, resulting in lower surface temperatures at higher latitudes (Figure 2.17).

Figure 2.17

Latitude-temperature relationship. (A) For every 145 km traveled poleward, the average surface temperature (averaged here across both hemispheres) decreases 1°C (1°F per 50 mi) (red dashed line) between 20 and 80 degrees latitude. The effects of elevation have been removed from this graph. The gray areas show the potential variation around the observed average temperature (black line). (B) As a general rule, average annual temperatures are highest near the equator and decrease toward higher latitudes. Antarctica is colder than the Arctic because of ocean currents and the high elevation of the Antarctic ice sheet. Notice too that in mountainous areas at low latitudes, the effect of elevation prevails, lowering temperatures even on the equator. In some areas, such as the Sahara in northern Africa, temperatures increase away from the equator because the persistently cloudless skies at slightly higher latitudes result in intense sunlight.

Patterns of Seasonality

Imagine you live in a city where the average January temperature is −47°C (−53°F). It is so cold that you must leave your car running all day and all night during the winter months to avoid an engine seized up through freezing. On a cold day, boiling water thrown into the air comes down as ice crystals, and if your house were not heated, there would be no liquid water in it. Food containing water would be frozen solid. Come summer, the average temperature at the same location is roughly 15°C (59°F), and past heat waves have reached the upper thirties (Celsius; nineties Fahrenheit). Such is life for the 1,400 citizens of Verkhoyansk, Russia. The city has an average annual temperature range of 62°C (−47°C to 15°C) or 112°F (−53°F to 59°F).

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Figure 2.18 gives a quick visual summary of the temperature ranges of two contrasting places, Verkhoyansk (67° north latitude), and Singapore (1° north latitude). The difference in seasonality between Verkhoyansk and Singapore can be explained by two factors: latitude and proximity to the oceans.

Figure 2.18

Seasonality in Verkhoyansk and Singapore. (A) Verkhoyansk, Russia, has among the highest annual temperature ranges of any city on the planet, as this climate diagram shows. This person (inset) in Verkhoyansk has thrown boiling water into −47°C (−53°F) air. The water has crystallized into ice before reaching the ground. (B) In contrast to Verkhoyansk, Singapore’s average monthly temperatures vary by only a few degrees. Every month brings warm tropical temperatures.
(Bryan and Cherry Alexander/Science Source)

As a general rule, high latitudes have a greater annual temperature range than low latitudes. In addition, continental (inland) locations have a greater annual temperature range than maritime (ocean-influenced) locations. This increase in seasonality with distance from the oceans is called the continental effect.

continental effect

The increase in seasonality with distance from the oceans.

As a general pattern, locations near the coast have maritime climates, with a relatively low annual temperature range. In contrast, locations far inland have continental climates, with a relatively high annual temperature range. The relationship among latitude, continentality, and annual temperature range is illustrated in Figure 2.19.

Figure 2.19

GEO-GRAPHIC Latitude and continentality influence seasonality. This map shows average January ground surface temperatures during the period 2001–2010. The data were collected by NASA’s satellite Terra.

What Causes the Continental Effect?

Why do inland regions have greater seasonality than coastal regions? There are four main factors that cause the continental effect: (1) the specific heat of water, (2) the evaporation of water, (3) the mixing of water, and (4) the transparency of water.

You may have noticed at the beach that the sand becomes warm in the afternoon sunlight, but the water does not—it is still cool and refreshing. If you were to go to that same beach early in the morning before sunrise, you might find that the sand is cooler than the water. The water hardly changes temperature over a 24-hour period, but the sand varies from warm during the day to cool at night. Land is quick to heat up and quick to cool down, but sea-water’s temperature changes much less. In addition, compared with the seasonal swings of temperature on land, the temperature of water changes relatively little between summer and winter. Figure 2.20 shows the differences in daytime heating between land and water using satellite thermal imagery.

Figure 2.20

Land and water heating. Land and surface water temperatures stand in sharp contrast in this thermal infrared image, made during a heat wave in the western United States on May 2, 2004. The land has heated up quickly, but the water has not. Notice that the inland mountains in California and in eastern Nevada (far right) have remained cooler because they are at higher elevations.
(NASA)

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As a general pattern, continents become warmer in summer than oceans at the same latitude. In winter, continents become colder than oceans. Figure 2.21 illustrates this phenomenon using isotherms, lines of equal temperature.

Figure 2.21

Seasonal temperature patterns. (A) In summer, large landmasses heat up, causing isotherms to bend toward the poles. Locations 1 and 3, which are over water, are between 10°C and 15°C (50°F and 59°F). At the same latitude over land (location 2), the temperature is up to 10°C warmer. (B) In winter, isotherms bend toward the equator because landmasses are relatively cold. Location 2 (over land) is 10°C to 15°C colder than locations 1 and 3 (over water) at the same latitude.

The cause of these temperature differences between land and water is the difference in their heat capacities. Heat capacity is the amount of heat that must be absorbed to change the temperature of an object. Specific heat, which is the heat required to raise the temperature of any object or material by a given amount, is a measure of heat capacity. Water’s specific heat is higher than those of most materials that make up landmasses, and the continental effect results mainly from this difference.

specific heat

The heat required to raise the temperature of any object or material by a given amount.

Different materials have different specific heat values, as shown in Table 2.2. For example, it takes 1 calorie of energy to raise the temperature of 1 g of water 1°C. Therefore, water has a specific heat of 1. However, it takes only about one-fifth of a calorie to raise 1 g of dry sand 1°C. Dry sand, therefore, has a specific heat of approximately 0.2, one-fifth that of water. If we added 1 calorie to 1 g of sand, its temperature would increase by about 5°C. In other words, water can absorb five times more energy than dry sand. Thus, when the same amount of heat is applied, land heats up more than water.

Evaporation, water transparency, and water mixing are also important in moderating the temperature of water. Evaporation cools water and prevents it from becoming warmer than it would otherwise become. Because land has relatively little water to evaporate, land heats up more in sunlight than oceans do.

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Table : TABLE 2.2 AT A GLANCE: Specific Heat Properties of Different Materials

SUBSTANCE

SPECIFIC HEAT (Cal/g × °C)

Pure water

        1

Wet mud

        0.6

Dry sandy clay

        0.33

Dry quartz sand

        0.19

Transparency is also important. Because water is transparent, sunlight can pass through several hundred meters of water and warm it. Land is opaque, and sunlight does not penetrate it. Similarly, convection mixes the water warmed by sunlight with cooler water at greater depths. Land is rigid and cannot mix, so the land surface heats up faster and to a higher temperature.

Ocean Currents and Seasonality

Warm ocean currents originating in the tropics carry immense amounts of heat toward the poles. When warm ocean currents reach high latitudes, some of their heat is transferred to the atmosphere. A good example of such an ocean-atmosphere heat transfer in the Northern Hemisphere is the Gulf Stream, which flows from the tip of Florida along the coast of eastern North America and into the North Atlantic Ocean. The warm water transfers its heat to the atmosphere in the North Atlantic, warming regions downwind.

The Gulf Stream transports more water than all the rivers of the world combined. Some 30 million m3 (1 billion ft3) of water pass by Florida each second. This amount increases to some 150 million m3 (5.3 billion ft3) per second by the time the current reaches Nova Scotia, Canada. As a comparison, the Amazon River, the largest river in the world, has an average flow of about 200,000 m3 (7.1 million ft3) per second.

Warm ocean currents raise the average annual temperature and reduce the annual temperature range. Cold currents influence temperatures less. The Gulf Stream’s effect on annual temperature ranges can be seen when coastal stations at the same latitude are graphed and compared (Figure 2.22).

Figure 2.22

Ocean currents influence average annual temperatures. The Gulf Stream warms northern Europe as it delivers its tropical heat to the North Atlantic Ocean and then to the atmosphere. The ocean current off the coast of Alaska is much colder than the Gulf Stream. Tromsø, Norway, and Barrow, Alaska, are at the same latitude (roughly 71° north latitude) and are both coastal towns at sea level. Tromsø’s winters are less severe than Barrow’s because of the heat delivered to the atmosphere by the Gulf Stream. This satellite thermal infrared image was made December 13, 2013. Light gray shows areas with no data.
(Data from NASA)

Because it transports so much heat, the Gulf Stream has many effects on physical geography, from hurricane formation to the behavior of the global climate system. We will examine the important effects of ocean currents and sea surface temperatures on storms in North America in Section 5.1.

Prevailing Wind and Seasonality

New York, New York, and Crescent City, California, are both coastal cities located at 40° north latitude. We might expect, therefore, that both would have maritime climates, but they do not. Although the average annual temperature for both cities is about 12°C (53°F), these two cities have very different annual temperature ranges. New York, on the east coast, has a continental climate. Crescent City, in California, has a maritime climate. As shown in Figure 2.23, the direction of the prevailing winds causes this pattern.

Figure 2.23

Effect of prevailing winds on seasonality. New York, New York, is dominated by prevailing winds that originate in the interior of the North American continent. Although New York is coastal, the air masses it receives and, consequently, its annual temperature range are continental. Crescent City, California, in contrast, is dominated by prevailing winds that originate over the Pacific Ocean and, consequently, it has a maritime climate. The background map shows average January temperatures during the period 2001–2010.
(NASA)

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Generally, because the prevailing wind direction is from the west, west coasts have maritime climates and east coasts have continental climates. This pattern is strongest at midlatitudes. It is weakened or nonexistent in the Southern Hemisphere (because landmasses there are small), polar regions (because the water is very cold), and in the tropics (because both land and sea are warm).

Many factors combine to control temperatures at any given location on the planet, but some play a larger role than others, depending on the geographic location. Figure 2.24 shows the global patterns of Earth’s annual temperature ranges and summarizes the main factors determining those patterns.

Figure 2.24

Global annual temperature range map. When we sum up the factors that control seasonality, the geographic pattern that emerges is controlled by latitude, the continental effect, ocean currents, and prevailing winds.