Jason Macdonald, of King's High School, asks :-
Cullen Crossan, of Balclutha,, asked:-
Why does it rain? What causes rain?
Tony Trewinnard, a meteorologist with Blue Skies Weather, responded.
The atmosphere has an ability to hold water vapour, but this ability depends upon temperature (and a few other things).
The warmer air is, the more moisture it can hold. Water vapour evaporates from most surfaces in the world, but especially from the oceans. After it evaporates, it gets carried all over the globe by winds. As the winds travel over colder surfaces the air cools. Since cooler air can't hold as much moisture, if the air gets cold enough the moisture will condense, becoming visible as the tiny droplets that make up clouds. If the air cools some more, some of these droplets will amalgamate and become bigger drops. Once these drops are too heavy for vertical air currents to support them, they fall out of the sky as rain.
It rains because the amount of moisture in the air is greater than the air's capacity to hold that moisture. Often clouds form in the rising air due to the movement of winds over mountains and hills, or due to heating of the earth's surface by the sun (causing air currents like the `thermals' that glider pilots use). If the air keeps rising over very high mountains such as the Southern Alps or from very strong thermals (like the sort that produce summer thunderstorms) the continually cooling air will be forced to loose much of its moisture in the form of heavy rain.