Saturday, July 28, 2012

Rain from Clouds

Water is continually evaporating and condensing in the sky. Clouds contain water vapor and small drops of condensed water (cloud droplets). Most of the condensed water in clouds does not fall as precipitation

Potential rain forms when water droplets condense on dust, salt, or smoke particles, which act as a cloud seeding nucleus.

When the water droplets become large enough that their fall speed exceeds the updraft speed in the cloud, they fall toward the ground. If the water drops do not evaporate as they travel earthward, they reach the ground as rain.

Virga, oil on canvas by Crystal Foreman Brown
Virga is the term for visible streaks or shafts of water that fall from clouds but evaporate before reaching the ground.
After the last of the rain had fallen from the sky and come to earth — leaving the sky clear and the earth damp and gleaming — the world below grew joyful in the cool left by the rain, and the greater clarity of life that returned with the blue of the heavens furnished each soul with its own sky.
~ Fernando Pessoa (1888-1935) from The Book of Disquiet.

Rain Drop Size

Size, fall velocity, and particle density (drops per square foot of air) vary for different types of rain (shown below).

Median diameter
Fall Velocity
Drops per sec.
per sq. ft.
Light rain.041.2415.726
Moderate rain.151.6018.746
Heavy rain.602.0522.046
Excessive rain1.602.4024.076
Source: Lull, H.W., 1959, Soil Compaction on Forest and Range Lands, U.S. Dept. of Agriculture, Forestry Service, Misc. Publication No.768

Rain is grace; rain is the sky descending to the earth; without rain, there would be no life.
~ John Updike (1932-2009)