In 1947, a Minnesota farmer growing corn would have been optimistic to expect yields between 45 and 55 bushels per acre. This expectation might have been governed by the availability of new hybrid seeds, as well as better management and availability of fertilizer.

The start of the 1947 growing season in Minnesota was helped by favorable weather for early planting, as well as adequate to surplus rainfall during the spring and early summer. These weather conditions would have provoked optimism for most corn growers. However, August of that year brought a dramatic shift in the weather pattern for Minnesota, and corn yield prospects were greatly reduced.

August of 1947 was the hottest in state history.

Average August mean temperature across the state is usually in the low to mid 60s F, plus or minus 3 or 4 degrees In 1947, the average temperature ranged from the low 70s to upper 70s F, departing from the long-term average by 5 degrees above normal to an astounding 9 degrees above normal. Many counties in the state reported 12 to 16 days with high temperatures of 90 degrees or above, and some reported four or five days when the afternoon temperature exceeded 100 degrees.

The following readings still represent all-time statewide record highs for their respective dates:

Daily highs above 100 degrees for half of state’s counties

Of Minnesota’s 87 counties, 44 reported at least one day with a high temperature of 100 degrees or greater. Hundreds of new daily record-high maximum and minimum temperatures were reported during the month from the Minnesota climate station network managed by the National Weather Service. On a number of nights, the temperatures never dropped below the low 80s or high 70s.

This put the corn crop under a great deal of stress during the critical grain-filling period. Corn stands wilted and sloughed dead leaves. In western counties, the heat stress was compounded by some moisture stress, as some areas reported less than 1 inch of rainfall for the month.

By the harvest season, growers’ expectations for good yields had wilted, too. Average 1947 corn yields around the state ranged from 33 to 39 bushels per acre, about 25% to 35% less than farmers had hoped for.

A similar August weather pattern that brought excess heat occurred across Minnesota in 1983, bringing similar corn yield reductions — 85 bushels per acre at harvest, versus a goal of 110 bushels per acre at the start of the planting season.

Since then, better management and more resilient corn hybrids have helped buffer the adverse effects of August weather anomalies, including both heat and drought stress. August heat stress in both 2003 and 2010 reduced corn yields in Minnesota, but not as dramatically as those August months in earlier climate history.

Seeley is an Extension professor emeritus of meteorology and climatology at the University of Minnesota.