Things Get Hotter When the Sun Goes Down

Saturday, May 19th was the rare night that was hotter than the day that preceded it.

This phenomenon, while uncommon, is not unheard of. Because almost all heat on land comes from the sun, you might think it would take a volcanic eruption or some other extreme event to make it warmer at night, but this Saturday it was a combination of two different but everyday processes: one is heat shielding and the other is shifting weather systems.

Heat shielding occurs in cities that have a high density of concrete and asphalt, which creates what Science and Technology calls “urban heat islands.” These materials hold heat longer and release it over a long period of time, so cities are often warmer than their surrounding rural areas where green spaces let off heat more quickly.

Large bodies of water (like Lake Michigan) can also cause warm nights because water holds heat longer than earth.  Even an outdoor pool can warm the air around it in the early evening after a hot day.  In Chicago, people often think of this process as having the opposite effect, making it “cooler by the lake.” That coolness is generally caused by the breezes coming off the water or by water staying cooler after cold weather in the same way that it retains heat after warm weather. In the early fall it is sometimes warmer by the lake.

As for Saturday’s warm night, the lake and the asphalt may explain some of the extra heat, but that’s just one of two processes going on. By itself, the urban heat shield doesn’t explain why it actually got hotter at night. It makes sense that cities would be closer to their daytime temperatures at night than rural areas, but they should be at least a little cooler.

The second process has to do with normal patterns in the weather. The air in a given place has certain characteristics, like temperature, moisture level and density. In a three dimensional space where the air shares those properties, the air can be thought of as one mass, and the weather in that air mass is generally the same.

These air masses are continually moving. Denser, high-pressure air masses expand and push out into less dense low-pressure air masses. The border between two weather systems is called a front.  If the weather changes dramatically it is because a front has passed through the area and is now in a new weather system.

So going back to Saturday, we had a warm air mass pushing into the area coupled with heat being released by the asphalt and concrete of the city, creating a warmer night than the day it followed.


This article was originally published in Extra Newspaper.  It’s available in English and Spanish on their website.