2.6 Precipitation

Precipitation is defined as the depth of rainfall, or the water equivalent of snow, sleet and hail falling during a given measurement period. It may be in the form of rain, snow, sleet or hail, or in minor forms such as dew and hoar frost, but existing theories do not yet satisfactorily account for all the observed characteristics. In tropical climates, precipitation occurs as a result of the gradual coalescence of the tiny condensed droplets as they collide within the cloud layer. In cooler climates, the formation of ice crystals in the upper levels of the cloud in turn is followed by crystal growth at the expense of water droplets, and this results in snow or hail. When sufficient growth has occurred, the large water drops or ice crystals have a larger ratio of mass to surface area than the small water drops or ice crystals.

Self-assessment question

Verify the above statement that large drops have a larger mass surface area ratio than small ones, by considering two rain drops of equivalent diameter 1 mm and 3 mm, respectively. Take the density of the rainwater to be 998 kg m⁻³.

Note: For a sphere of radius r, the and the surface area = 4 π r².

Answer

Carrying out similar calculations for the 3 mm raindrops, we get

Thus, the large drop has a larger ratio of mass to surface area.

The increased mass to surface area ratio allows the large drops to fall more rapidly than the small drops and they separate from the cloud. The small drops with very low velocity remain in the air currents associated with the cloud.

The large-scale cooling needed to give significant amounts of precipitation is achieved by lifting the air. There are three lifting processes (Figure 3) and these are used to describe the type of precipitation. They are:

• frontal (or cyclonic) – resulting from warm moist air meeting;

• orographic – upward deflection of warm air streams by mountains;

• convective (or convectional) – uplift associated with local heating by solar radiation.

Lifting processes leading to precipitation:

Figure 3

In the hydrological cycle, the total volume of water remains constant. The volume of water precipitated must necessarily balance the volume evaporated (when considered over a sufficiently long period of time to make variation in atmospheric storage of moisture insignificant).