According to NOAA, drizzle is fairly uniform fine precipitation with drop diameters less than 0.5 millimeters. The drops are very close together. Drizzle floats with air currents, like fog, but falls to the ground. Its intensity is determined by visibility:

Drizzle Intensity Visibility
light more than ½ statute mile
moderate ¼ to ½ mile
heavy less than ¼ mile

Glossary: D's.
Retrieved 1 January 2010.

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