The final droplet size distribution is an . The number of droplets with diameter between and per unit volume of space is . This is commonly referred to as the Marshall–Palmer law after the researchers who first characterized it. The parameters are somewhat temperature-dependent, and the slope also scales with the rate of rainfall (d in centimeters and R in millimetres per hour).
Intensity and duration of rainfall are usually inversely related, i.e., high intensity storms are likely to be of short duration and low intensity storms can have a long duration.
The effect leads to increased rainfall, both in amounts and intensity, downwind of cities. is also causing changes in the precipitation pattern globally, including wetter conditions across eastern and drier conditions in the tropics. Antarctica is the driest continent. The globally averaged annual precipitation over land is 715 mm (28.1 in), but over the whole Earth it is much higher at 990 mm (39 in). classification systems such as the system use average annual rainfall to help differentiate between differing climate regimes. Rainfall is measured using . Rainfall amounts can be estimated by .
Deviations can occur for small droplets and during different rainfall conditions. The distribution tends to fit averaged rainfall, while instantaneous size spectra often deviate and have been modeled as . The distribution has an upper limit due to droplet fragmentation.