7.8: Global Climate Zones

Climate in a given region is defined by several factors, including the average annual air temperature, seasonal range of temperatures, range of temperatures between day and night, extent and persistence of cloud cover, and annual precipitation and its seasonal distribution. Each factor varies with latitude and location in relation to land, mountain chains, and ocean. The factors also vary from year to year. Hence, most climate zones are not separate and distinct, but merge into one another. The demarcation lines drawn between the zones in Figure 7-23 are therefore only approximations.

Figure 7-23. The climate zones of the oceans are arranged in bands that generally follow a latitudinal pattern and that are related primarily to ocean surface water temperature (see Figure 7-14). Terrestrial climate and vegetation zones follow a more complex pattern that reflects the presence of mountain chains, the prevailing direction of air mass movements, and the proximity to the oceans, all of which affect the distribution of rainfall and seasonal temperature ranges.

Ocean Climate Zones

Climate zones over the oceans are generally latitudinal bands that parallel the average ocean surface temperatures shown in Figure 7-14.

Polar ocean zones are covered with ice for much of the year, and ocean surface temperatures remain at or close to freezing all year (Fig. 7-24a) because of the heat-buffering effect of ice (Chap. 5, CC5). These zones have low precipitation and generally light winds (except where land and ocean interact). In subpolar zones, sea ice forms in winter but melts each year. Despite low precipitation amounts in the polar and subpolar zones, ocean surface salinity, particularly in the Arctic Ocean, is low. Surface water salinity is lowered during the continual freeze-and-thaw cycle that creates sea ice. As water freezes, ice exclusion causes dissolved salts to be excluded from the ice. The salts are left in the remaining water which raises the salinity and the resulting cold, high-salinity surface water sinks because its density is greater than the water below (CC1). When sea ice melts in the spring and summer, the non-salty water released from the melting ice mixes with the upper layer of ocean water and lowers its salinity. In the Arctic Ocean, freshwater runoff from the continents also contributes to low surface-water salinity.

Figure 7-24. Seasonal and latitudinal variation in temperatures at the Earth’s surface. (a) Average ocean surface water temperature. (b) Average temperature over land (or ice-covered ocean). Seasonal variation is much greater over land than over ocean, especially at high latitudes.

Temperate ocean zones are located in the zones of strong westerly winds (Fig. 7-11). They have high precipitation (Fig. 7-17) and are subject to strong storms called “extratropical cyclones.” These storms form especially in winter at the atmospheric polar fronts, which are locations where the polar and Ferrel cells meet (Fig. 7-10). The storms travel eastward and toward lower latitudes. Extratropical cyclones are described later in this chapter.

Subtropical ocean zones are located at the divergence between the atmospheric Hadley and Ferrel cells (Fig. 7-10). In these zones, winds are generally light, skies are usually clear, and precipitation is low. Many of the world’s most desirable beach vacation areas are on coasts in these zones.

Trade wind zones are generally dry with limited cloud cover, but they are subject to persistent winds. Ocean surface water evaporation is very high, and thus, salinity tends to be high (Figs. 7-15, 7-16). In the equatorial region, ocean surface waters are warm (Fig. 7-14), evaporation is high (Fig. 7-16a), winds are light (Fig. 7-11), clouds are persistent, and rainfall is high and continuous throughout the year (Fig. 7-17).

Land Climate Zones

Land (terrestrial) climate zones are not arranged into the same type of orderly latitudinal bands that characterize ocean climate zones (Fig. 7-23). The reasons for this difference include differences in thermal properties between land and water and the effects of mountains on air mass movements.

The average ocean surface temperature at any latitude changes little between seasons (Fig. 7-24a) because of the high heat capacity of water (Chap. 5, CC5). In contrast, soil and rocks have a much lower heat capacity. Consequently, seasonal changes in solar intensity cause substantial seasonal changes in average land-surface temperature (Fig. 7-24b). Seasonal changes are small in equatorial regions and increase with increasing latitude. The reason is that the annual range of daily total solar energy received at the Earth’s surface increases with the distance from the equator.

Figure 7-24b shows the temperatures at continental locations far from oceanic influence. Daily and seasonal temperature changes are less in coastal locations than in regions far from the oceans at the same latitude (Fig. 7-25). The ocean provides a source of relatively warm air in winter and cool air in summer. Although the coastal air mass moderates climates in all coastal locations, the distance inland to which this effect reaches on a specific coast is determined by the prevailing wind direction and the location and height of mountain ranges. 

Figure 7-25. Annual variation in monthly average air temperature for Portland, Oregon; Sheridan, Wyoming; and Boston, Massachusetts. (a) Daily high temperatures. (b) Daily low temperatures. These cities are at approximately the same latitude; however, their climates are very different. Portland has a maritime climate with a small annual temperature range; Sheridan has a continental climate with a large annual temperature range; and Boston has a more continental climate than would be expected from its coastal location, because the climatic winds there blow from the continent toward the ocean.

If climatic winds blow onshore, coastal-ocean air can moderate temperatures and enhance precipitation many hundreds of miles beyond the coast. For example, warm coastal air that flows eastward across Europe is unimpeded by mountains. Hence, the moderate marine climate extends farther inland in western Europe than in the western parts of other continents, such as North and South America, where coastal mountains impede the westward flow of warm, moist air from the Pacific Ocean (Fig. 7-23). In the latter regions, the mountain effect (described in more detail later in this chapter) removes the principal source of the air mass’s heat: its water vapor.

If climatic winds blow offshore, the moderating influence of the ocean is reduced. For example, Boston, Massachusetts, has a wider temperature range than Portland, Oregon (Fig. 7-25). Although both are coastal cities at about the same latitude, Boston is in a region of offshore climatic winds, while Portland is in a region of onshore climatic winds.

Coastal land climates are also affected by ocean currents. For example, Scandinavia and Alaska are at about the same latitude, but winters are much more severe in Alaska than in Scandinavia. In winter, most of Scandinavia is in the westerly wind zone and the temperate ocean climate zone, whereas most of Alaska is in the polar easterly wind zone and the subpolar ocean climate zone (Figs. 7-11, 7-23). In the North Atlantic Ocean, the Gulf Stream Current carries warm water from Florida north and west across the ocean to the seas around Scandinavia. The warm water provides heat energy through evaporation to moderate the Scandinavian atmosphere in winter. In fact, enough heat energy is provided to cause the westerly wind zone to extend into the region throughout the year (Fig. 7-11). A similar warm current, the Kuroshio Current, flows north and west from south of Japan toward Alaska (Chap. 10). However, the Aleutian Island arc deflects this warm current and it does not reach the Bering Sea on the west coast of Alaska.

The terrestrial climate zones shown in Figure 7-23 are defined primarily by seasonal temperature ranges and precipitation rates. In any given location, these parameters are controlled by the interaction of several factors, including ocean water temperatures, climatic winds, distance from the ocean, and the locations of mountain chains. A complete description of the zones is beyond the scope of this text, but the knowledge gained in this chapter can be used to explore the distribution of terrestrial climate zones depicted in Figure 7-23.