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Just Ask Us: Why isn’t the hottest day of the year on the summer solstice?

August 6, 2018

Q: Why isn’t the hottest day of the year at the summer solstice, when the day is longest?

A: Just like the hottest time in a day isn’t at high noon, the hottest days of the year are also not when the length of day is longest, according to the National Weather Service.

The summer solstice, the time of year when the Earth’s axis is tilted most toward the sun, is generally from June 20 to 22 for those of us in the Northern Hemisphere, but the highest temperatures generally come about three weeks later.

“There is a lag between the longest day of the year and the warmest average temperatures for most mid- to high-latitude locations,” the Weather Service explains on its website. “This lag is due to the time required for the ground and water to heat up.”

The warming continues for weeks until the sun becomes lower in the sky and days significantly shorter because the axis is moving away from the sun as the Earth travels around it.

On the other side of the year, during winter, when the Northern Hemisphere is tilted away from the sun, the coldest temperatures in the Midwest usually come around mid-January, according to the National Centers for Environmental Information.

The delay in the coldest temperature from the winter solstice, Dec. 21, is likely caused by the average snowfall, which reflects the sun’s rays and continues the region’s cooling. Areas without snowfall, like most of the American West, have their coldest days in December.

— Shelley K. MeschSend questions to: justaskus@madison.com; Just Ask Us, P.O. Box 8058, Madison, WI 53708.