Aridity describes the relative lack of rain or snowfall to an area. Arid areas generally receive less than 10″ of rain per year on average. In arid areas, plant and animal life forms must be adapted to living without significant moisture. A semi-arid area receives more precipitation annually than the arid areas, between 10″ and 20″ per year, yet is still considered relatively dry. Colorado, as a whole, is considered to be semi-arid, since the entire state receives on average approximately 17″ of precipitation annually.