Oklahoma Climatological Survey







The hydrologic cycle is a continuous process by which water is transported from the oceans to the atmosphere to the land and back to the sea. One of the most important processes in the hydrologic cycle is precipitation.



Figure 1 - Comparison of Typical Sizes of Cloud Droplets and Precipitation


  • Formation of precipitation requires the growth of cloud droplets (~0.005 cm diameter) to the size of rain droplets (~0.5 cm).


  • The temperature of the cloud, the layer of air below the cloud, and the earth's surface will dictate the type of precipitation that will be formed.

    Temperatures inside a "warm cloud" are above 0 deg C; hence, warm clouds are composed completely of liquid water. Temperatures in "cold clouds" must be below 0 deg C and also may be above 0 deg C; hence, cold clouds typically are composed of a mixture of liquid water and ice crystals.

  • The following are the most typical types of precipitation:
    • Rain- drops have diameters greater than 0.5 millimeters. Rain falls from all types of clouds.
    • Drizzle- drops have diameters less than 0.5 millimeters. Drizzle falls from stratiform clouds.
    • Hail- frozen water in balls from 5 to 75 millimeters in diameter. Hail grows by trips through liquid water and frozen regions of convective storms.
    • Sleet - has diameter less than 0.5 millimeters. Sleet starts as rain or snow, melts as it falls, and then re-freezes in a relatively thick layer of air temperatures below freezing.
    • Freezing Rain- rain which falls through a frozen layer near the ground and then freezes on impact with a frozen surface.
    • Snow - up to 10 millimeters in diameter. Snowflakes are groups of ice crystals which do not melt because either the freezing level is close to the ground or the air temperature is below freezing all of the way to the ground.





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