The “dog days of summer” are usually associated with the greatest and most persistent heat of the year. They’re characterized by occasional thunderstorms (sometimes severe) and high dew points (often over 70 degrees F). The phrase’s origin is both ancient and astrological: The Greeks and Romans observed that one of the brightest stars — Sirius, the Dog Star, in the constellation Canis Major (Latin for “greater dog”) — rose in conjunction with the sun during the six weeks of midsummer. The usual hot and sultry weather at this time of year depleted people’s energy and wilted vegetation. These undesirable conditions were attributed to the "evil" effects of Sirius.

In Minnesota, the dog days commonly occur between mid-July and early September, while western European nations acknowledge the dog days from the first week of July to the second week of August. For most locations in Minnesota, especially the agricultural counties in southern and western sections of the state, the highest probability of 90-degree-F temperatures occurs during the dog days.

Daily extreme maximum temperatures for the second half of July in Minnesota have ranged from 107 degrees to 114 degrees. In August, they have ranged from 100 degrees to 110 degrees, and in the first week of September, they have ranged from 101 degrees to 105 degrees.

In addition, the highest frequency of 70-degree dew points also occurs during the dog days. The most extreme dew points for the second half of July have ranged from 76 degrees to 84 degrees; for August, they have ranged from 74 degrees to 80 degrees; and for the first week of September, they have ranged from 74 degrees to 78 degrees.

Dog days usher in extreme heat

As a consequence of the high daily temperatures and greater water vapor content, most of the extreme heat index values (the combined effects of heat and humidity on the human body) measured historically in Minnesota have occurred during the dog days as well. July has delivered heat index values ranging from 110 degrees to 130 degrees, while early August has delivered heat index values as high as 112 degrees. Many farmers may recall experiences at the Minnesota State Fair in late August, when the heat index has ranged from 105 degrees to 108 degrees.

In years of drought, the dog days usually still deliver high temperatures — but in addition, often bring welcome thunderstorm rains to alleviate the moisture stress that crops have endured earlier in the summer. Let’s hope that is the case this year.

More on Minnesota’s weather history is available in my new book, the Minnesota Weather Almanac, second edition, available at most bookstores or through the Minnesota Historical Society Press.

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