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rain forms 
By international convention, precipitation in the form of rain is classified according to the quantity that falls.
heavy rain
Precipitation that results in over 7.6 mm accumulation per hour.
moderate rain
Precipitation that results in 2.5 to 7.6 mm accumulation per hour.
light rain
Precipitation of water drops over 0.5 mm in diameter; it results in accumulations of 2.5 mm per hour.
drizzle
Uniform continuous precipitation of slow-falling water droplets between 0.2 and 0.5 mm in diameter.
