How Much Water Is in an Inch of Rain?

An inch of rain is usually defined as the amount of rain that must fall in order for the ground in a defined geographical area to be covered to a depth of 1 inch (2.54 cm). By measuring rainfall as related to a defined area, it is possible to estimate the total amount of water that fell in that area. For example, 1 inch (2.54 cm) of rain in Washington, D.C., translates to more than 1 billion gallons (about 4 billion liters) of water. By contrast, 1 inch (2.54 cm) of rain in New York City would come to more than 5.2 billion gallons (more than 19 billion liters).

More facts about rainfall:

  • Louisiana is the wettest state in the continental United States with an average rainfall of 56 inches (142.2 cm). The driest state in the U.S. is Nevada, with an average of less than 10 inches (25.4 cm) annually.

  • The driest place in the world is Wadi Halfa in the Sudan, which experiences an average of 0.1 inches (2.54 mm) of rainfall each year. Cherrapunji, India, is considered the wettest place in the world, although the location receives little to no rain for half of the year. The other six months are distinguished by frequent monsoons.

  • A heavy rain is considered to be rainfall that is in excess of 0.3 inches (0.8 cm) per hour.

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