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Being a fluid, water is able to freely circulate. Surface water that has been warmed by the sun can mix with cooler water at depth, thus keeping the body of water cool. Land can not circulate warmer surface layers with cooler ones below, resulting in high surface temperatures and higher air temperatures than these over water. Finally, the specific heat of water is higher than that of land. Specific heat is the amount of energy required to raise the temperature of one gram of substance through one degree Celsius. Water has a specific heat that is five times greater than land. This means that it takes five times more energy to heat one gram of water than one gram of land. So if adjacent land and water receive the same amount of insolation, the water will warm much slower than land, and give up its heat much more slowly than land.
Figure 5.6 Comparison of seasonal temperatures for three midlatitude sites Combining the above differences between the heating of land and water, a few observations can be made. First, air temperatures are usually lower over a water surface than over adjacent land. As this cooler maritime air comes ashore, air temperatures over land will be reduced. Because the water's temperature doesn't fluctuate as much as that of land, temperature ranges are smaller over water and coastal locations than those in the interior. Finally, seasonal temperature lags are longer, by about a month, for coastal locations than their interior counterparts. These temperature characteristics are shown in Figure 5.6 above. San Francisco, being located on the west coast of the United States, is influenced by the moderating effect of the Pacific Ocean. Wichita is in the center of the country is controlled by the thermal characteristics of land. Norfolk on the east coast is influenced by both the westerly winds from the continent and the moderating influence of the Atlantic Ocean. Thus the west coast location has the smallest temperature range (and longest seasonal lag), the interior location has the largest temperature range and the east coast location a moderate range. |
