AWH 5-10

Chapter 5, Heat and Temperature 5-10 Figure 5-6. Specific Heat Capacity: Water versus Sand The difference in the specific heat of various materials is one of the primary reasons why the temperature of a body of water, such as a lake or the ocean, is less variable with time than the surface temperature of land. Water heats up more slowly than land during the day and during the summer and cools down more slowly at night and during the winter. Thus, a body of water exhibits greater resistance to temperature change (called thermal inertia) than does a land mass. Heat flow differences are another reason why water bodies warm up and cool down more slowly than land. Incoming solar radiation penetrates water to significant depths but can only heat the top skin layer of soil and rock. Also, since water is a fluid, its heat energy can be circulated through great volumes and depths via convection. Water temperature changes occur to depths of 6 meters (m) (20 ft) or more on a daily basis, and 200 m to 600 m (650 ft to 1950 ft) annually. The process is more problematic over land since heat must be transferred via the slow process of conduction. Land temperature changes occur to depths of only 10 centimeters (cm) [4 inches (in)] on a daily basis and 15 m (50 ft) or less annually. Water is much more resistant to temperature changes than land. It warms up and cools down more slowly than land and helps to moderate nearby air temperature. This is why islands and localities located immediately downwind from the ocean or a large lake (maritime locations) exhibit smaller diurnal and seasonal temperature variations than localities well inland (continental locations). Figure 5-7 illustrates this effect. Although both cities are at approximately the same latitude, the temperature is far less variable in San Francisco (maritime) than St. Louis (continental).