Heat, Land and Sea
   If you ever spend a vacation day at the beach, or on the shore of a large lake, you may notice a phenomenon that can help you remember how high and low pressure, and warm and cold air, translate into wind direction. Early in the day, beach and ocean may be at just about the same temperature, and any winds will be light.

   In the afternoon, however, you may begin to experience a "sea breeze", a cooling wind from the ocean blowing up over the surf and into your face. This happens because during the day the land heats up more rapidly than the ocean, which absorbs heat more slowly, but--as we'll see--retains that heat for longer, once it's absorbed. The land, perhaps, may reach 80° F, while the ocean is only 65°-- a little chilly for swimming! Most of the Sun's heat passes right through Earth's atmosphere: only about 19% is absorbed by clouds and the atmosphere. Earth's surface--assuming this is a great beach vacation and there are no clouds--absorbs about 51%, and warms up rapidly. The land radiates heat back into the air above it, warming up the layers right above it. The warm air rises up through convection, and cooler air from the ocean flows in to take its place.

   At night the reverse occurs, and the result (at least in late spring and summer, and close to the coast) is what's called a "land breeze." The ocean cools off slower than the land. Now the land's temperature may fall to 55° F, while the water is still 65°. But the same principles apply day or night--air flows from a cooler to a warmer region--though now the direction is reversed. The wind blows from the land to the ocean.

   The seasonal consequences of being close to a body of water can be seen in the effect of oceans on winter temperatures. The United Kingdom, that relatively small island nation, situated relatively far north compared to most of the United States, has warmer winters than places of similar latitude in North America because of the Gulf Stream, a persistent current of warmer tropical water flowing north-east across the Atlantic.

   Even America's East Coast, though sometimes very cold, is on average less chilly than the frigid Midwest, in the middle of the Great Plains, where there's no moderating effect of nearby water. Of course, local circumstances vary greatly and modify the general rule: Chicago, the "windy city", posts freezing temperatures despite being on a body of water: the frozen air sweeping down from Canada is hardly warmed at all by Lake Erie

   You can experiment with these phenomena, and see how sand and water heat up and cool off at different rates by doing Activity 1.4.1 Air in Motion: High and Low Pressure, and Activity 1.4.3 The Effect of Temperature on the Density of Air, in the Teacher's Guide.