Why Are Low Temperatures So Tricky To Forecast??

Ever been driving around at night with your car windows down and notice it suddenly get colder? Ever notice how much warmer it stays in the city of Chicago compared to the far western suburbs?  For instance, Friday morning the low temperature at the Sugar Grove Airport near Aurora was 37 degrees while just about 40 miles away the low temperature at Northerly Island near downtown Chicago was a relatively balmy 58 degrees. Temperature spreads this large, or ever larger, at night are not uncommon when skies are clear and winds are light.

The map below is an infrared satellite image, which measures the temperatures being emitted from the earth. Lighter blue colors are areas where temperatures are colder, while darker blue colors and light purples are where warmer temperatures are being detected.

night time satellite image showing temperatures

It is surprising just how many features show up clearly on this satellite image, including the warmer temperatures in the more urban and suburbanized areas, the warmer temperatures over the lake, and even a couple of small hot spots near the nuclear cooling plants! The image below is the same satellite picture with many of those features highlighted...

night time satellite image showing temperatures

The pool of warmer temperatures at night that commonly occurs near big cities is often referred to as the "urban heat island" effect. You'll notice on the image below that the temperatures get progressively warmer the closer you get to downtown Chicago, with a small hot spot right near "the loop". So what causes this effect? Well, concrete and asphalt are particularly effective in absorbing heat, much more so than vegetation. If you sat a rock and a green leaf you picked from a tree out in the summer sun in the afternoon for an hour or two, and then felt them, which would be hotter? If you guessed the rock, you're correct, the rock is going to be much more effective at absorbing energy from the sun's rays and the rock will get hotter.

Now imagine this effect magnified of hundreds of square miles of a major metropolitan area, where there is mile upon mile of concrete and asphalt absorbing heat on a sunny day. Once the sun sets the air will begin to cool quickly, however in areas where most of the surface is comprised of buildings, roads, and other infrastructure that was heated up by the sun all day, the temperature fall will be slower as all of the heat that was absorbed by these objects is slowly released thereby warming up (or more accurately stated, slowing the rate of cooling) of the air above it. More rural areas with less concrete and more vegetation will have much less heat absorbed into the ground and therefore temperatures will be able to fall more quickly at night.

More information about urban heat islands can be found on the following pages the Environmental Protection Agency (EPA) has put together...

  Urban Heat Island Basics

  Heat Island Impacts

  Heat Island Video Segments

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