Rain drop dizes and their equivalents

While rain drop size varies based on rain intensity and rainfall speed, one standardized laboratory drop of H2O is defined as 0.05 ml or about Ø 2.3 mm (1 ml = 20 drops), Rain drop sizes found in nature vary widely as shown below:

• Drizzle raindrop sizes can be very small, 0.00004 ml or Ø 0.2 to 0.5 mm, where they are not able to produce any measurable readings in most rain gauges. (AMS glossary: raindrop)

• Torrential downpours in tropical regions can produce very large raindrops of even 2 ml or Ø 8 mm equivalent diameter. Raindrops of such large sizes have very high fall speeds and are rare because they usually break up into small drops before reaching the ground. (AMS glossary: raindrop)

• Most measurable rainfall drop sizes will range from 0.001 ml (Ø 0.6 mm) to 0.3 ml (Ø 4 mm). In terms of equivalent standardized laboratory drop sizes, this ranges from 1/50th of a laboratory drop size to a drop 40 times larger in volume than a standardized laboratory drop. References: Microphysics of Clouds and Precipitation by Pruppacher and Klett, (1978, Reidel, Boston), pp. 316-19 and Bad Meteorology: Raindrops are shaped like teardrops. NOT REALLY.

What is the speed of a falling rain drop

Rain drop maximum speed is dependent on multiple factors which include air temperature, water drop temperature, air density and atmospheric pressure. These factors affect the falling rain drop’s Reynolds number in air along with the water surface tension and viscosity which determines shape and breakup velocity of a drop. To keep things simple, the maximum (terminal) speed of a rain drop increases with rain drop size. Its mass increases with the cube of drop size and air resistance force increases more slowly with the square of rain drop size.

For typical rain drop sizes, falling speeds will range as shown below:

• Drizzle drops fall roughly at speeds 0.7 to 2 m/s (2 to 7 ft/s) for drop sizes of Ø 0.2 to Ø 0.5 mm diameter.

• Rain drops fall at speeds roughly 2 to 9 m/s (7 to 30 ft/s) for drop sizes of Ø 0.6 to Ø 4 mm diameter.

• Larger drop sizes greater than 4 or 5 mm have been observed to fall at around 9 m/s (30 ft/s) and larger drops at up to 13 m/s (42 ft/s) but only in rare occurrences.

What is the shape of a falling raindrop

• Raindrop Shape & Size Calculator: How Much Rain is in Each Rain Drop?

Other references

1. Book called Microphysics of Clouds and Precipitation by Pruppacher and Klett, (1978, Reidel, Boston), pp. 316-19

2. Beard, K.V. Terminal Velocity and Shape of Cloud and Precipitation Drops [pdf]. Journal of the Atmospheric Sciences (May 1976): 851-864.

3. Holladay, April. Falling raindrops hit 5 to 20 mph speeds. Wonderquest. Albuquerque: 19 Dec 2001.

4. https://en.wikipedia.org/wiki/Drop_(unit)

5. Raindrop Size Distribution Characteristics of Summer and Winter Season Rainfall Over North Taiwan
   Balaji Kumar Seela, Jayalakshmi Janapati, Pay-Liam Lin, Pao K. Wang, and Meng-Tze Lee