Rain and Drizzle

If you watch raindrops failing on a clear glass window it is immediately noticeable that they vary considerably in size. A spattering of rain will show up as individual drops, but a downpour soon develops a stream of water down the glass.

Rain is actually classified as water drops larger than 0.5 mm in diameter, whereas smaller drops are classified as drizzle. The difference is purely one of drop size rather than intensity of precipitation. Drizzle usually comes from sheets of low shallow cloud, whereas rain is more likely from deeper clouds. Drizzle, with its many small drops, will cut down the visibility more than the equivalent amount of water falling as rain. Also heavy drizzle is more wetting than slight rain.

When air rises, it cools and water vapour in it condenses into tiny droplets of water to form a cloud. Condensation usually occurs around small particles called cloud condensation nuclei. The motion of air within the cloud causes the water drops to collide and larger drops tend to grow at the expense of the smaller ones (in a process called coalescence). If water droplets continue being developed within the cloud, such as in moist air rising over a hill, they eventually start falling out as drizzle. In deeper clouds, where the up draughts are more vigorous, the water droplets become larger before entering a region of the cloud where there is a compensating downdraught and fall as rain.

This explains precipitation from cloud that is composed entirely of water, but another process is at work when a cloud contains ice crystals. The water inside a cloud does not start to freeze at 0 °C, but at a much lower temperature. In the meantime, it exists as supercooled water. When the temperature falls to -40 °C, all water turns to ice, but between about -10 °C and -40 °C, the cloud consists of a mixture of supercooled water and ice crystals. A scientist called Bergeron demonstrated that water vapour condenses more readily (a process known as sublimation) on to ice crystals than on to supercooled water.

Aggregation of the ice crystals occurs as they move into areas of cloud where the temperature is above -25 °C. Accretion also occurs as water droplets crystallize on coming into contact with the ice crystals. These snowflakes eventually begin to fall, being precipitated out as rain when the air temperature is above about 3 °C.