Land heats and cools about four times faster than water. Therefore, the location of continents and oceans greatly influences Earths pattern of air temperature as well as the sources and direc-tion of movement of air masses.

Coastal areas assume the temperature characteristics of the land or water that is on their windward side. In latitudes of prevailing westerly winds, for example, west coasts of continents have oceanic temperatures and east coasts have continental temperatures. These temperatures are determined by the wind flow.

Since the upper layer of the ocean is nearly always in a state of mixing, heat losses or heat gains occurring at the surface are distributed throughout a large volume of water. This mixing process sharply reduces air temperature contrasts between day and night and between winter and summer over oceanic areas.

Over land, there is almost no redistribution of heat by turbulence; also, the effect of conduc-tion is negligible. Thus strong seasonal and diurnal contrasts exist in the interiors of continents. Dur-ing the winter, a large part of the incident solar radiation is reflected back toward space by the snow cover that extends over large portions of the northern continents. For this reason, the northern continents serve as source regions for dry polar air.

The large temperature difference between the land and water surfaces, which reverses between the two seasons, determines the seasonal weather patterns to a great extent.

In unit 3, figures 3-1-4A and 3-1-4B, the isotherms over the Northern Hemisphere are more closely spaced and parallel in winter than in summ-er. In the Southern Hemisphere, the temperature gradient does not have as great a seasonal change as it does in the Northern Hemisphere. These conditions are due to the unequal distribution of land and water on the two hemispheres. Since the Southern Hemisphere has less land and more water surface than the Northern Hemisphere, the change due to the greater water surface is less with consequently more nearly uniform isotherms. Also, the con-tinents of the Southern Hemisphere taper toward the poles and do not extend as far poleward as do those in the Northern Hemisphere. The nature of the surface affects local heat distribution. Color, texture, and vegetation influence the rate of heating and cooling. Gen-erally, dry surfaces heat and cool faster than moist surfaces. For instance, plowed fields, sandy beaches, and paved roads become hotter than sur-rounding meadows and wooded areas during the day. During the night, however, the situation is reversed.

The distribution of water vapor and clouds is another important factor influencing air temperature. Although areas with a high percent-age of cloud cover have a high degree of reflec-tivity, the energy which is not reflected is easily trapped in the lower layers due to the greenhouse effect. Thus, areas of high moisture content have relatively high temperature.

The higher mean temperature of the Northern Hemisphere is an effect not only of its higher percentage of land, but also of the fact that its oceans are also warmer than those in the Southern Hemisphere. This is partly due to the movement of warm equatorial waters from the Southern Hemisphere into the Northern Hemisphere caused by the southeast trades crossing the equator. Another factor conducive to higher mean temperatures in the Northern Hemisphere is the partial protection of its oceans from cold polar waters and arctic ice by land barriers. There is no such barrier between the antarctic region and the southern oceans.

Climates over land may vary radically within very short distances because of the elevation and variations in land forms. Therefore, topography plays an extremely important role in determining the climate of a region.

The height of an area above sea level exerts a considerable influence on its climate. For instance, the climate at the equator in the high Andes of South America is quite different from that found a few feet above sea level at the same latitude. All climatic values are affected by surface elevation.

An important influence on climate is moun-tainous terrain, especially the long, high chains of mountains that act as climatic divides. These obstacles deflect the tracks of cyclones and block the passage of air masses at the lower levels. If the pressure gradients are strong enough to force the air masses over the mountains, the forced ascent and descent modifies the air masses to a great extent, thus modifying the climate on both the windward and leeward sides of the range. The orientation of the mountain range may block certain air masses and prevent them from reaching the lee side of the mountains. For example, the Himalayas and the Alps, which have east-west orientations, prevent polar air masses from advancing southward. Therefore, the cli-mates of India and Italy are warmer in winter than are other locations of the same latitude. The coastal ranges in North America, running in a north-south line, prevent the passage of un-modified maritime air masses to the lee side.

The most noted effect of mountains is the distribution of precipitation. The precipitation values, level for level, are much higher on the windward side than on the leeward side. In regions where the prevailing circulation flows against a mountain barrier, the amount of precipitation increases more or less uniformly with elevation on the windward side of the range. This steady increase normally occurs up to elevations of about 10,000 feet. However, in the trade wind zone (such as at the Hawaiian islands), precipita-tion increases only to about 3,000 feet and then decreases gradually. Even with this decrease in amount, more rain is received at 6,000 feet than at sea level.

Another important topographical feature is the presence of lakes. The lake effect can be notable for large unfrozen bodies of water. The lee sides of lakes show considerable diurnal and annual modification in the form of more moderate temperatures; increased moisture, clouds, and precipitation; and increased winds (due to less friction) and land and sea breeze effects.

Ocean currents play a significant role in con-trolling the climate of certain regions. Ocean currents transport heat moving cold polar waters equatorward into warmer waters and moving warm equatorial waters poleward into cooler waters.

Currents are driven by the major wind systems; therefore, cold southward-moving cur-rents flow along the west coasts of continents, and warm northward moving currents flow along the east coasts of continents. This is true in both hemispheres. Basically, this results in cooler climates along the west coasts and warmer climates along the east coasts.

A brief explanation of the effects of ocean currents is presented here.

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