Читать книгу Weather For Dummies - John D. Cox - Страница 59

Heat that is moving from the surface of water or land warms the air just above it in a process of direct transfer known as conduction. This is the way the icy cold of a glass or the boiling heat from a cup travels up the handle of a metal spoon, for example. And just around the metal spoon handle, a thin layer of air is absorbing some of the heat.

From this thin layer of air at the surface, the heat energy finds its way into higher levels of the atmosphere through a process known as convection, the vertical mixing of liquid or gas of different temperatures. Convection is what happens when a pot of water boils.

Some of this air mixing happens through the mechanical forcing of wind. This is referred to as forced convection. Blowing near the surface, swirling eddies in the flowing air carry the heat up into the sky. Two general rules apply: the faster the wind, the greater this kind of convection. Also, the more uneven the surface — the bigger and more numerous the eddies — the greater this kind of mixing.

Another kind of vertical mixing known as free convection depends on buoyancy — the ability of warmer air to rise in cooler air. In the atmosphere, a kind of bubble of warm air is formed near the surface and floats up to higher altitude, above the cooler, denser air around it, much like a hot-air balloon would do. As it rises higher and higher, the bubble of air expands, and as it expands, it cools. This kind of rising and falling of air of different temperatures and densities is going on all the time.

The process of free convection can be especially noticeable on a warm summer afternoon. The Sun is heating the ground and the heat from the ground is quickly warming the air just above it. Before long, a rising column of warm expanding air is formed. These are the thermal updrafts that soaring birds ride on a warm day.

If conditions are right, if the air bubble contains enough moisture and the surrounding air is colder than the bubble of air, a cloud can eventually form when the rising air gets cold enough for its water vapor to condense into tiny liquid droplets or even ice crystals. (For more about cloud formation, see Chapter 6.) This condensation process gives off still more heat, called latent heat. This latent heat plays a major role in the in the formation of clouds and storms. (See the sidebar, “How to cause a storm.”)