Isohyets

Isohyets are used to depict areas of equal precipitation. The areas along the lines show wehre equal precipitation is occurring. This can be useful so you can see where areas of low precipitation are. Also it can show how water is being distributed across an area and any measures necessary to counteract that.

http://www.bbc.co.uk/blogs/weather/ianfergusson/2010/06/formula-one-weather-forecast-c-1.shtml