Hydraulic engineers in the United States often use, as a units of volume of water, the acre-foot, defined as the volume of water that will cover 1...

The acre-foot is defined as the volume of water that will cover one acre of land to a depth of one foot.

One acre is equal to 43650 square feet. One foot is equal to 12 inches. The thunderstorm caused 2 inches of rain to fall on a town with area 45 square kilometer.

One square kilometer is equal to `1*10^6` square meter. One square meter is equal to `3.28084^2` square feet = 10.7639 square feet.

45 square kilometer is equal to 45*10^6*10.7639 or 484.376*10^6 square feet.

The volume of water that fell on the town is equal to 484.376*10^6/43560*(2/12) = 1853.29 acre feet.

The volume of water that fell during the thunderstorm in acre-feet is 1853.29.