When warm wet air rises, it cools and water vapour condenses to form clouds. A cloud is made of small drops of water or ice crystals, depending on its height and how cold its surrounding air is. To form rain, water vapour needs what's called a condensation nucleus, which can be tiny particles of dust, or pollen, swept up high into the atmosphere. When the condensing droplets that form the cloud get large and heavy enough to overcome the upward pressure of convection, they begin to fall.
Rain is restricted to drops of water that fall from a cloud. They have a typically diameter of at least 0.5 mm. No matter what the intensity of rain is the size of the drop rarely exceeds about 5 mm. Larger drops do not survive as the process of surface tension which holds the drop together is exceeded by the frictional drag of air and therefore larger drops break apart into smaller ones.
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Rain is restricted to drops of water that fall from a cloud. They have a typically diameter of at least 0.5 mm. No matter what the intensity of rain is the size of the drop rarely exceeds about 5 mm. Larger drops do not survive as the process of surface tension which holds the drop together is exceeded by the frictional drag of air and therefore larger drops break apart into smaller ones.
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