In July the central part of North America heats up, with sunny skies and south-to-west winds producing the hottest temperatures of the year. Historically July is the warmest and sunniest month of the year everywhere in the Upper Midwest except for a few locations along Lake Superior.
The peak of summer in the Twin Cities area is July 26, which is statistically our warmest day of the year if we use pioneer records going back to 1819 at Fort Snelling. Note that this is well after the longest daylight period of the year, the Summer Solstice, on June 21. Thanks to high sun angle and warmer land, water temperatures peak around the same time as the hot summer air we breathe.
A good share of Minnesota's lakes are regarded as warm water lakes so their surface temperatures can be expected to rise to 80 degrees or higher at the end of July.
Swimmers are advised to avoid water temperatures 70 degrees or cooler, but that shouldn't be a problem this season. Last year with our early spring and hot early summer, water temperatures rose to 84 degrees on Lake Waconia by July 4. At the same time Leech Lake was measuring at 82 degrees. On August 9, 2010, the surface temperature of Lake Waconia hit 90 degrees. Wow! I take water temperatures 1 foot below the surface in at least 5 feet of water.
Jim Gilbert's Nature Notes are heard on WCCO Radio Sundays at 7:15 a.m. His observations have been part of the Minnesota Weatherguide Environment Calendars since 1977, and he is the author of five books on nature in Minnesota. He taught and worked as a naturalist for 50 years.