How Weather Works

水plays a major role in weather, despite making up such a small fraction of the atmosphere. In some areas, the local atmosphere may contain as much as 4 percent water, while other regions have no atmospheric water at all. As water can exist as a solid, liquid or gas under normal atmospheric conditions, it participates in thehydrologic cycle. In this cycle, water evaporates from the ocean in the form of water vapor and eventually returns to land and sea in the form ofprecipitation.

You can't see water vapor, but it quickly becomes visible when it cools andcondenses对一些东西。如果你ever noticed moisture beads on the windows of a warm car on a cold day, you've seen condensation in action. Warm air vapor touches the cold window and the vapor turns back to a liquid. Clouds form along similar lines. The atmosphere is full of tiny dust particles calledcondensation nuclei, which come from volcanic eruptions, dust storms,firesand pollution. When water vapor condenses, it clings to these microscopic specks. If there's enough cooling water vapor in the air, these accumulate by the trillions to formclouds. If temperatures are cold enough, the water turns to ice around the condensation nuclei. For a more in-depth look at clouds, readHow Clouds Work.

In a windless world, these water droplets would descend right back down to the surface, butEarth'scomplex upper air winds keep the clouds afloat, moving them across vast distances and altering their shape in the process. If too much water condenses around a particle or if the air temperature drops, the water will fall back to the surface. Liquid particles fall in the form ofrain, while frozen particles fall assnow. If the rain freezes as it falls, it becomesfreezing rain. In some cases, rain ascends to higher, chilly altitudes by an updraft; the particles freeze, then return to Earth in the form of ahailstone.

Clouds come in various shapes and sizes and occur at varying altitudes. They can even gather on the ground in the form offog. This occurs when warm, moist air close to the ground either cools rapidly or becomes oversaturated with water vapor.

But as you know, Earth's most substantial cloud formations occur in the air. On the next page, we'll look at how all that water vapor gets up so high.