If a raindrop is less than 0.5mm in diameter, it’s drizzle – and if a drizzle particle is over 0.5mm, it’s a raindrop. That’s following the definitions of the National Weather Service Observing Handbook No. 8, Aviation Weather Observations for Supplementary Aviation Weather Reporting Stations (SAWRS), Manual Observations, October 1996. But quantifying drizzle is not as straightforward as simply collecting raindrops in a disdrometer, [see photo] because drizzle doesn’t (necessarily) ‘fall’ in a predictable way, as rain (sometimes) does, In short, Disdrometers have difficulty with drizzle :
“Commonly used disdrometers tend not to accurately measure concentrations of very small drops in the raindrop size distribution (DSD), either through truncation of the DSD at the small-drop end or because of large uncertainties on these measurements. Recent studies have shown that, as a result of these inaccuracies, many if not most ground-based disdrometers do not capture the ‘drizzle mode’ of precipitation, which consists of large concentrations of small drops and is often separated from the main part of the DSD by a shoulder region.”
– explains a report in the Journal of Applied Meteorology and Climatology, Jan. 2019. The authors propose a solution to such problems :
“We present a technique for reconstructing the drizzle mode of the DSD from ‘incomplete’ measurements in which the drizzle mode is not present.”
See: Reconstructing the Drizzle Mode of the Raindrop Size Distribution Using Double-Moment Normalization, Journal of Applied Meteorology and Climatology, Jan. 2019
Note: If the water droplet size is below 20 micrometers (or so) it’s not drizzle, it’s fog. [Ref : Pruppacher H., Klett J. (2010) Microstructure of Atmospheric Clouds and Precipitation. In: Microphysics of Clouds and Precipitation. Atmospheric and Oceanographic Sciences Library, vol 18.]
BONUS: The internationally recognized symbols for drizzle(s) can be found here.
Photo: Courtesy Wikipedia