How does it rain?
Almost all the air around us is moist. We all know that clouds are made from water, but how exactly? The water condensing in the clouds has to become heavy enough to fall to Earth. The tiny droplets alone just aren’t heavy enough to fall. To fall from the sky, these droplets need to become heavier, and they do this by colliding with other droplets, growing bigger and heavier, and they are able to fall as rain. Some may get caught in upward blowing winds and get blown back into the clouds for a while, but once they are heavy enough to overcome the force of the wind, they will fall to earth. It will keep raining as long as the conditions are right to make the clouds and let the water droplets grow heavy enough to fall.
There are 3 types of rainfall:
On a warm day the ground is heated by the sun, and the air above the ground also warms, and begins to rise. The higher you go the cooler it gets, so as this warm air rises it cools because of the colder air around it. 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 typical cloud formed this way is called a cumulus cloud, or a cumulonimbus cloud when it has a grey bottom, these are the type of clouds that people often look at the sky and say it ‘looks like rain’. 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
Both warm and cold air are needed for this kind of rain. 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. As the warmer air rises over the cold air it starts to cool down, and as it cools water vapour is precipitated and the cloud forming process begins, leading to rain. The sky will generally be grey, and will cover almost all of the sky.
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. Relief rain needs a physical obstruction of some kind, so that warm moist air is forced to rise up over it. Mountain ranges, big hills and even cliffs along the coast can be large enough to force the air to rise. 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. 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 area has only a little rain because the cloud making process isn’t working