Cloud formation and rainfall

The main way in which water vapour in the atmosphere can be converted to clouds, and hence potentially fall as rain, is for air temperature to be reduced.

When the cooling air reaches its dew-point temperature, water vapour begins to condense as water droplets or ice crystals, becoming visible as clouds. The dew-point temperature is usually reached by lifting air into the colder, higher levels of the atmosphere.
See also Humidity

This lifting can happen in several ways:

Convective rainfall:
When the earth's surface is heated, air near the ground expands and rises by buoyancy above the cooler and therefore heavier air around it. As it rises it cools; if the dew-point temperature is reached, condensation of water vapour releases heat, which causes the air to rise further.

So on a hot summer afternoon--especially in the tropics where the air is moist, surface heating is intense and condensation is abundant--you may see towering anvil-topped clouds. Often 10 or more kilometres high, these thunderstorms may generate intense rainfall.

Orographic rainfall:
Air may be forced to rise when land barriers such as mountain ranges lie in the path of extensive air masses.

The ideal conditions are when winds off a warm ocean meet a relatively continuous mountain range close to the coast, at right angles. Prime examples are parts of the west coasts of North and South America and the north-east coast of Australia.

On the leeward side of such mountains, descending air depleted of its moisture is characteristically dry, sometimes causing desert conditions.

Frontal rainfall:
When moist air moves into a low-pressure area or a front, conditions favour development of clouds and rain. Because of the slower ascent and cooling along gently inclined surfaces, this type of rainfall is less violent than in thunderstorms. It is usually also more widespread and protracted. The dull grey overcast skies and drizzly precipitation of the cooler months in the middle latitudes, which produce some of the most unpleasant weather of these seasons, are usually associated with fronts and low pressure systems.