Download Convectional Rain 1) On a warm day the sunshine heats up the

Convectional Rain
1) On a warm day the sunshine heats up the ground. Air above the warmed ground also becomes
warmer, and it begins to rise (because warm air is less dense than the air around it). As it warms up it
absorbs more water because, as you should remember, warm air can hold more water than cool air.
2) The atmosphere gets cooler as you go higher - by roughly one degree centigrade cooler for every
100m of altitude in dry air - so as the warm air rises it becomes cooled again by the colder air around it.
3) Eventually the air reaches a height where the temperature forces the water vapour in the warm air to
start condensing. This is called the condensation point, and is where the clouds begin to form. The
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typical cloud formed this way is called a cumulus cloud, or a cumulonimbus cloud when it has a grey
bottom and 'looks like rain'.
4)The rain forming process starts, and usually leads to very heavy rain, perhaps with thunder and
lightening. Convectional rain is often experienced at the end of a hot summer day and associated with
torrential downpours with large drops and towering dark grey clouds.
In Summer months, look out for clouds forming over fields of corn, large carparks and concentrations
of buildings. All these features heat up quickly and cause warmed air to rise. If you are lucky, you may
be able to watch a towering cloud forming in a matter or ten minutes or so.
Frontal Rain
1)Two bodies of air are involved - an area of relatively warm air and an area of relatively cool air.
2)The warmer air is less dense, so when it meets the cooler air it rises up over the cooler air mass. The
cooler air stays where it is, and lies underneath the warmer air.
3)As the warmer air rises over the cold air it starts to cool.
4) As the air cools, water vapour is precipitated and the cloud forming process begins, leading to rain.
Sky's are typically grey and cloud covers almost all the sky.
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Relief Rain
1) A physical obstruction of some kind is needed, so that warm moist air is forced to rise up over it.
Mountain ranges, big hills and even cliffs along the coast be large enough to force the air to rise.
2) As the warm air rises over the obstruction it cools and clouds form. Rain falls from the clouds, or if
the droplets don't grow large enough, fog may form over the hill tops. Fog is basically just clouds at
ground level.
3)The air passes over the obstruction and can sink again, gaining warmth as it does so. This air is drier
than it was before it lost water as rain, so any clouds left will evaporate again, leaving clear skies. This
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area has only a little rain because the cloud making process isn't working. Because there is so little rain,
the area is said to be in a 'rain shadow'.
Relief rain is common in upland and mountainous areas where is can lead to extraordinary local rainfall
patterns. It's common for one side of a mountain to be in warm sunshine, yet only a few hundred
meters away it's raining on the other side of the mountain ridge. This can be important to the
development of settlements, agriculture and tourism. For example, in summer most people want to be
on the sunny side of a mountain with little precipitation, but in winter, when rain is replaced by snow,
they want to be on the side where the precipitation does fall.
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