Читать книгу Know Your Clouds - Tim Harris - Страница 7

A “tablecloth” of orographic cloud sits on Cape Town’s Table Mountain

A parcel of air is a little like a sponge: it can hold water vapour. Warm air can hold more than cool air, but whatever its temperature it can only accommodate so much before it becomes saturated. Then, tiny water droplets condense around even tinier particles of dust or salt and form cloud. One of two things can cause the air to become saturated: either the amount of water vapour in the air parcel increases, or its temperature falls. For example, if air moves over the ocean or a lake, it picks up evaporating water vapour as it goes. Warm air will pick up more than cold air. If this air then rises, for example if it reaches land and passes over a mountain, it will cool. If the cooling continues, the air eventually reaches the point (its dew point) where its water vapour condenses—and cloud develops.

Three primary types of uplift cause cloud to form. Orographic uplift happens when wind pushes air over hills and mountains. Frontal uplift occurs when a wedge of air undercuts another, pushing it higher. Convectional uplift takes place when the ground is heated and causes the air in contact with it to become warmer and rise vertically. Of course, none of these scenarios will produce cloud if the air is bone dry. However, if it is moist, one form of cloud or another will form sooner or later.

The type of cloud that develops depends on whether orographic, frontal, or convective processes are dominant; how much water vapour the air contains; how quickly the air temperature decreases with altitude; and the wind speed.