Raindrops of anything but the smallest sizes (< Ø0.7 mm) are not spherical. As found in literature, the characteristic raindrop diameter is the theoretical diameter a raindrop would have if it was perfectly spherical with the same exact volume of water as the real non-spherical drop.

Temperature and the amount of air dissolved in rainwater (or dissolved air pollutants) affect each water droplet’s density, which in turn affects its mass and weight and it’s vertical fall speed (velocity). Vertical fall speed, in turn, affects its exact shape since the balance of aerodynamic and surface tension forces changes with increasing fall speed and increasing air temperature as the drop nears the ground. Aerodynamic forces increase with the square of airspeed, while surface tension forces decrease with increasing temperature.

Raindrops with an equivalent diameter of Ø5 mm or larger may be prone to break up into smaller drops as their fall velocity increases.

The following web calculator computes rain drop mass based on size and also the raindrop shape and volume. It is based on the following scientific research papers, which characterized raindrop shapes and sizes using a 2D video distrometer (optical rain measurement technique).

REFERENCES:

1. Huang, Gwo-Jong & Bringi, V.N. & Thurai, M. & Shimomai, Toyoshi & Kozu, T.. (2007). DROP SHAPE AND ORIENTATION ANGLE DISTRIBUTIONS IN RAIN FROM 2-D VIDEO DISDROMETER.

2. Beard, K. V., & Kubesh, R. J. (1991). Laboratory Measurements of Small Raindrop Distortion. Part 2: Oscillation Frequencies and Modes, Journal of Atmospheric Sciences, 48(20), 2245-2264. Retrieved Jan 9, 2021, from https://journals.ametsoc.org/view/journals/atsc/48/20/1520-0469_1991_048_2245_lmosrd_2_0_co_2.xml