Living Columns & Blogs

Why did this summer lack really hot days?

Each spring, I get asked to make a forecast for the upcoming summer season. While I have become comfortable with identifying factors for a winter forecast, making a prognostication for the summer season is not as easy.

In my early career, I did hear one thing that holds true: “The seeds of summer are planted in the middle and latter part of spring.” In other words, the amount of rainfall we receive in that time period can dictate the direction the summer will try to go. This year, a wet April and May pointed to a summer without extremely high temperatures. In fact, we only reached 90 degrees once.

While many of us find reasons to complain about summer’s heat, we don’t really live in a hot place. We have our share of muggy nights (and it is getting muggier, a topic I should cover in the future), but at our elevation it’s hard to reach temperatures above 90 degrees. It’s that much rarer to reach the 100-degree mark, reaching it only nine days since the late 1800s.

Why? Simply, it’s because of our elevation and the abundance of ground moisture. Obviously there’s nothing that can change the first, but the latter of these two does vary a great deal from year to year. And for us to reach these extremely high temperatures we need to be quite dry. The summer with the most days reaching at or above 90 degrees was 1988, when we surpassed this mark a whopping 25 times. It was a summer where we received measurable rain on only 25 days, which is the fifth least on record. On the other side, there have been 16 years where we haven’t reached 90 degrees and those years had one thing in common — a lot of rain and/or a lot of days with rainfall.

So why does rain make a difference? Part of the answer can be that rain comes with a pattern that promotes instability and more clouds, which can keep daytime temperatures down some. But the bigger reason has to do with the amount of moisture available for evaporation. It takes at least 500 calories of energy to evaporate a gram of water. When you keep in mind that it only takes one calorie to warm a gram of water by one degree, you realize that’s a lot of energy.

When this evaporation occurs, it robs the energy from the surrounding environment. This is why you feel so much cooler when you get out of the shower than when you get in. Therefore, when we get a lot of rain leading into the summer, there is more moisture in the ground to evaporate, which keeps us cooler. The resultant water vapor also tends to help fuel more shower and thunderstorm activity, which then keeps the ground wet. Given our current weather pattern, we will have to wait until next year before extreme heat.

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