Why it Rains?

Rainclouds can form through one of three main processes.

Orographic rainclouds form over hills and mountains as moist air rises and cools forming clouds. Rain falls on the windward slopes, while the leeward side, in the rain-shadow of the mountains, remains dry.

Frontal rainclouds form when two air masses meet.

Convective rainclouds occur when air is buoyant and able to rise to the upper troposphere. Rising thermals of moist air are cooled and form small clouds. As the Sun continues to heat, more clouds form which grow bigger, expanding upward and sideways, to form a larger cloud which increases in height and size. Mature clouds will have strong upcurrents.

Rain needs a Nucleus

Rain Water can condense in pure air using air molecules as a condensation nucleus. Most, however, of the condensation nuclei that cause clouds and rain are naturally occurring particles of dust, salt or smoke. Without dust, salt and so on, there would probably be many, many fewer clouds on planet Earth. Artificially created particles can also have an effect. Most drops are about 1 mm (0.05 in) in diameter. Raindrops generally grow inside clouds by coalescence. Small raindrops collide and join together forming bigger drops that are heavy enough to fall to the surface. Lots of the rain that falls to Earth is in fact melted snow. Tiny water droplets freeze into ice particles near the top of the cloud. The ice particles then grow into snowflakes and melt to form raindrops. This process is called the Bergeron-Findeisen process. However, in the tropics and equatorial regions the whole rain-forming process occurs below the freezing level.