Why have dew points been jumping up in the evening?

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CEDAR RAPIDS, Iowa (KCRG-TV9) - Saturday through Monday evening, the dew point temperature spiked at the Eastern Iowa Airport. The most logical explanation is evapotranspiration at work yet again!

If you are a plant, the day is pretty quiet around sunrise, but soon the process of evapotranspiration starts releasing more water from plants into the air. Note the dew points climbed quickly through 9 a.m. After this time, we usually start to notice the wind a bit more as the lower atmosphere begins mixing. In this case, a wind speed of 8 to 12 mph was observed, leaving the dew point around 68 to 72 degrees. By the early evening, the dew point spiked to the middle 70s.

Why did this occur at that time? As the sun goes down, the wind speed tends to decrease, so it won’t mix the air as much as it did earlier in the day. Meanwhile, the plants are still giving off plenty of moisture, and the first few hundred feet of the atmosphere (where we are) are extremely moist. As the plants shut down for the night, the dew points will drop off substantially. In our area, this occurs around 10 p.m. This explains why the early evening can be much more humid in the Corn Belt.

This process is robust enough in August that it can lead to foggy mornings if the night remains clear and calm. Did you know that August is our second-foggiest month in eastern Iowa? We’ll touch on that in a future Weather Water Cooler blog post.

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