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Drizzle is liquid precipitation with small droplet size. In order to be classified as drizzle rather than as rain, the droplets must be less than 0.5mm in diameter; however drizzle droplets are significantly larger than cloud droplets. For an observer who cannot readily distinguish droplet size, rain droplets may be observed to splash as they hit a puddle and drizzle droplets generally do not.
Drizzle most often falls from Stratus clouds which often have very low cloud bases, sometimes touching the ground to form fog. The presence of drizzle in a weather forecast usually indicates significantly reduced visibility, even in the absence of fog, as the small droplets fall slowly and obscure vision. At temperatures between 0°C and -10°C droplets can remain as supercooled water to form freezing drizzle. In Arctic and Antarctic environments, where the air is clean and there are fewer condensation nuclei, freezing drizzle can occur at even lower temperatures. Whilst accretion rates on aircraft do not approach those experienced during an episode of severe rain ice, vigilance is appropriate if the temperature of the surfaces of an aircraft is below the freezing point and it flies into drizzle. When air or surface temperature is below freezing and drizzle is present black ice may readily form on unprotected runways.
Airline and business jet operators have reported heavy freezing during drizzle at temperatures as high as 15°C, due to cold fuel.
Take-off is prohibited with freezing drizzle present. Generally, rime ice is associated with freezing drizzle, however, clear ice with freezing precipitation can never be ruled out, despite droplet size.
In METARs and TAFs, drizzle is abbreviated to DZ and freezing drizzle as FZDZ. .
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