A rain gauge is an instrument to measure rainfall, which is measured in mm. Occasionally the rain fall is also reported in centimeters or in inches. What the rainfall measure is telling us is the height of rain water collected in a vessel that has vertical walls and has uniform cross section from bottom to the top. The purpose of the rain gauge is simple; to measure how much rain has fallen in the last 24hrs. The gauge is reset every 24hrs for accuracy and if rainfall exceeds expectations a graph is plotted to measure how much rain should fall on average for the following few days.