Precipitation that drives Minnesota growing season comes during thunderstorms

The first wild grapes are ripe. Dozens of bird species relish this fruit.

Raccoons, eastern chipmunks and gray squirrels take green acorns from bur oak trees.

Flocks of Canada geese are flying and honking. The adults have been flightless since molting wing feathers in mid-June, and the young from this year's hatch also are flying.

Most Minnesota lakes have near-surface temperatures in the 70s to low 80s, perfect for swimming and tubing. When on a picnic or out in a boat, the fly that gives us the painful bites about our ankles is the stable fly, not a housefly. House flies have mouth parts that don't allow them to bite, though they are the infamous germ carriers.

When we see parched corn and soybean fields, walk on crunchy lawns, watch ponds drying up, lake levels dropping, and streams with little water, most of us react by saying "hat we need now are some good all-day soaking rains." That can happen, but much of our growing season precipitation occurs during thunderstorms.

In an average year, thunderstorms will develop on 45 days in southern Minnesota and on about 30 days in northern Minnesota. The majority occur from May through September when 65 to 75 % of our annual precipitation usually falls. Thunderstorms can bring high winds, hail, heavy downpours causing flooding and tornadoes, but most often they bring moderate amounts of rain to sustain natural communities, agriculture, and our city lawns and gardens.

About 90 % of the moisture that falls on Minnesota is carried from ocean sources by moist air masses. The greatest amount of it comes from the Gulf of Mexico but a small portion originates in the Pacific Ocean. Local moisture sources, such as evaporation off lakes and transpiration from vegetation, are of less importance. We know that the annual precipitation in Minnesota varies from about 20 inches in the northwest to 32 inches in the southeast, and we are aware that our precious water resources are used over and over again because of the hydrologic, or water, cycle. So when it's dry and our landscape needs a good drink, we keep hoping that atmospheric conditions will give us a good rain. One inch would be great.

Looking at the numbers, it's interesting to see what an inch of rain amounts to on an acre of land. An acre of ground contains 43,560 square feet — that's about one football field of surface area. Rainfall of a single inch in depth over an acre would mean a total of 6,272,640 cubic inches of water. This is equal to 3,630 cubic feet of water. A cubic foot of water weighs about 62.4 pounds, so the weight of an inch over one acre of land would be 226,512 pounds. The weight of one U.S. gallon of pure water is 8.345 pounds. Therefore a rainfall of an inch over one acre of ground would mean 27,143 gallons of water.

Jim Gilbert has taught and worked as a naturalist for more than 50 years.