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
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 suf ciently
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 ow down the other side of the
mountains (the leeward side).
3. Air rises as it enters an area of ow 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 ow pressure, clouds will form
and rain may fall.