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. 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. Its mass increases with the cube of drop size and air resistance force increases more slowly with the square of rain drop size.įor typical rain drop sizes, falling speeds will range as shown below:ĭrizzle 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. To keep things simple, the maximum (terminal) speed of a rain drop increases with rain drop size. 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. Rain drop maximum speed is dependent on multiple factors which include air temperature, water drop temperature, air density and atmospheric pressure. 316-19 and Bad Meteorology: Raindrops are shaped like teardrops. References: Microphysics of Clouds and Precipitation by Pruppacher and Klett, (1978, Reidel, Boston), pp. 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. Most measurable rainfall drop sizes will range from 0.001 ml (Ø 0.6 mm) to 0.3 ml (Ø 4 mm). 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. Torrential downpours in tropical regions can produce very large raindrops of even 2 ml or Ø 8 mm equivalent diameter. 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:ĭrizzle 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.
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