Before we examine the annual and daily rhythms of air temperature change, we need to introduce the fact that land and water surfaces have quite different properties in absorption and radiation of heat. The general law may be stated as follows. Land surfaces are rapidly and intensely heated under the sun's rays, whereas water surfaces are only slowly and moderately heated. On the other hand, land surfaces cool off more rapidly and reach much lower temperatures than water surfaces when solar radiation is cut off. Temperature contrasts are therefore great over land areas, but only moderate over water areas. It is further true that the larger the mass of land, the greater are seasonal temperature contrasts. Because the heating of ground and water surfaces controls heating of the atmosphere above, the same observations apply to air temperature as to surface temperature.
If heat is being applied equally to both substances, the ground will attain a high temperature long before the water will; specific heat of the water is said to be great, that of rock or dry soil to be small.
Over the oceans there are two differences.
(1) Maximum and minimum temperatures are reached about a month later than on land-in August and February, respectively because water bodies heat or cool much more slowly than land areas.
(2) The yearly range is less than over land, following the law of temperature differences between land and water surfaces.
Coastal regions are usually influenced by the oceans to the extent that maximum and minimum temperatures occur later than in the interior.
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