For the Minnesota crop-growing season, the month of August is an important “filling period” for some major crops. Corn uses most of August physiologically to fill out the kernels on the ear, soybeans use the month for seed growth within the pods, and sugar beets see some of the largest gains in weight during August. So, the heat and moisture provided by the weather in August is very important to the yield potential of these major crops.

Twenty years ago, a persistently cold weather pattern brought us the coolest August in state history. Most climate stations reported a mean monthly temperature that was 5 degrees F to 8 degrees F below normal. There were relatively few days that brought maximum temperatures above 80 degrees F, and as a result, Growing Degree Days (a measure of the heat suitable for development of the corn crop) were 20% to 25% below normal as well. Portions of the Red River Valley as well as west-central Minnesota saw only one day bring 80-degree F temperatures.

Mid-month, there were four-day periods of record-setting low temperatures, one occurring Aug. 10-13 and another occurring Aug. 19-22. During these periods, daily maximum temperatures only reached the upper 50s to low 60s, about 20 degrees F to 25 degrees F below normal. Many nighttime low temperatures fell into the upper 30s to lower 40s.

There were numerous frosts in northern counties, and on the morning of Aug. 21, temperatures even dipped into the 20s in some northern areas, where Tower (St. Louis County) reported an all-time state record low temperature for the date of just 23 degrees F.

Elsewhere, there were light frosts reported in some important agricultural counties such as Stearns, Sherburne, Polk, Lac qui Parle, Chippewa, Traverse, Becker, Clay and Murray. Some farmers reported frost burn on some corn and soybean canopies. By the end of the month, crops had made relatively little progress compared with their normal August development toward maturity, and some farmers were worried about further frost damage occurring in September.

Nice recovery

Fortunately, the weather pattern changed, and September 2004 brought average monthly temperatures that were 4 degrees F to 6 degrees F above normal. Growing Degree Days for crop maturation were 12% to 15% above normal, helping to speed crops along toward physiological maturity, and farmers began harvesting crops by the first week of October. The potential yield reductions from such a record-setting cold August were thwarted by a favorable autumn weather pattern.

More on Minnesota’s weather history is available in my book, “Minnesota Weather Almanac: Second Edition,” available at most bookstores or through the Minnesota Historical Society Press.