What Causes Air to Rise?

There are four ways in which the air can be forced to rise, thus causing the water droplets to condense. We will look at each of these in turn.

1. The air is heated from the ground.

This tends to occur mainly in the tropics, where the air in contact with the ground is warmer than the air above, so it will rise. As a result, sometimes huge thunder–clouds (called cumulonimbus clouds) form and the associated rainfall can be very heavy. This type of rainfall is called convection rain.

2. The air is forced to cross a mountain barrier.

As an air mass crosses a mountain barrier, it is forced to rise. If the mountains are sufficiently high and the air mass contains enough moisture, clouds will form and rain will fall on the windward side (the side closest to where the air mass is coming form). This rainfall is known as orographic rain, meaning "caused by the shape of the land". The air mass, now drier and warmer, will continue to flow down the other side of the mountains (the leeward side).

3.    Air rises as it enters an area of flow pressure.

As air rises, other air must replace it which, in turn, also rises. This is a complex process. As the air rises, it moves in a spiral, but if enough air enters an area of flow pressure, clouds will form and rain may fall.

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