How precipitation is measured - Precipitation is generally stated in units of inches or centimeters that fall per unit of time. 'One inch of rainfall, for example, is a quantity sufficient to cover the ground to a depth of one inch, provided that none is lost by runoff, evaporation, or sinking into the ground.
A simple form of rain gauge can be operated by setting out a straight -sided, flat-bottomed pan and measuring the depth to which water accumulates during a particular period. Unless this period is short, however, evaporation seriously upsets the results. Furthermore, very small amounts of rainfall, such as 0.1 in. (0.25 cm), would make too thin a layer to be accurately measured. To avoid this difficulty, as well as to reduce evaporation loss, good rain gauges are made in the form of a cylinder whose base is a funnel leading into a narrow tube. A small amount of rainfall will fill the narrow pipe to a considerable height, thus making it easy to read accurately, once a simple scale has been provided for the pipe. This gauge requires frequent emptying unless it is equipped with automatic devices for this purpose.
Snowfall is measured by melting a sample column of snow and reducing it to an equivalent in water. Thus, rainfall and snowfall records may be combined for purposes of comparison.
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