ASK TOM WHY: How many gallons of water come down when 1 inch of rain falls across 1 acre?

Dear Tom,

How many gallons of water come down when 1 inch of rain falls across 1 acre?

—Larry Cabai, Elgin

 
Dear Larry,

Because we usually perceive rainwater in terms of drops, it’s difficult to comprehend the immense volume of water that falls when it rains. Yet, when the staggering numbers of individual raindrops are added together, the number of gallons of water that comes down in rain is immense.

One inch of rain across a single acre yields 27,143 gallons of water, and over a square mile it’s 17.4 million gallons. Across the area within Chicago’s city limits, the number of gallons is a staggering 3.97 billion gallons weighing 33 billion pounds. For Cook County, the comparable values are 16.4 billion gallons of water weighing 137 billion pounds.

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