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This time of year, shorter days are hotter

I was mowing and weeding my lawn the other evening when I nearly had to quit, job unfinished, because it had gotten dark already.

It was only 8:45 p.m. What happened to daylight until 9:30 p.m, and how does it seem like summer is already slipping away? For you warm- (or hot-) weather lovers, the good news is that there is still plenty of summer season – and heat – left, though the days are growing steadily shorter.

During the week of the summer solstice — June 20 — we were enjoying 16 hours of daylight, though average highs for that time of year were only in the mid-70s. Fast forward to Aug. 1, we’re down to about 14 hours 50 minutes of daylight, yet average highs are in the mid-80s and on that day Spokane broke a record with a high of 101 degrees (it was 99 degrees at the Coeur d’Alene airport).

Why don’t the hottest days of the year coincide with the longest days? The following logic can also be used to understand why the hottest part of the day isn’t at noon, when the sun is highest in the sky, but is instead between 3 and 6 p.m., depending on the time of year.

Think of the earth’s temperature as an account experiencing daily gains and losses. The earth is always losing heat energy, as it is radiated and lost into space. At some point during the year, the amount of heat energy we receive from the sun is in balance with the heat being lost. Subsequent days will see average temperatures climbing. Past the summer solstice, we continue to see more heat energy coming in than being lost each day, resulting in a net gain to our heat account even as the days start to grow shorter.

Eventually, the tide shifts, and the amount of heat coming in does not make up for what is being lost. Our heat account begins to see net losses, and average temperatures start to fall. The net losses also continue well past the winter solstice, so that the coldest temperatures of the season also occur after the “shortest day” of the year. In Spokane average daily temperatures begin their climb about Jan. 10. During the warm season, average daily temperatures don’t start falling until around Aug. 14.

A check at the latest long-term forecast from the Climate Prediction Center shows that from now through October, temperatures have an equal chance at being above- or below-normal, though precipitation is expected to be below-normal for the period. In Spokane, average temperatures for July came in more than 3 degrees above normal, while precipitation fell short of average by 0.28 inches.

Michelle Boss can be reached at weatherboss@ comcast.net

 

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