An urban heat island has the power to split an incoming storm system in two or keep a city and its suburbs significantly hotter than surrounding areas, especially once the sun goes down, meteorologists say.
Chicago is one such heat island — and this week the magnitude of the island’s impact on our weather will be on display, said Accuweather.com meteorologist Brett Edwards.
“It’s going to be oppressive, to say the least,” Edwards said of the heat wave the city is in for beginning Thursday. Records dating to the 1930s and ’40s could be met or broken Friday and Saturday if the high temperatures forecast are exceeded, he said. The National Weather Service also said it expects Friday will easily be the hottest day of the year.
So what is an “urban heat island”? And how does this meteorological phenomenon apply to Chicago? Here’s what you need to know about urban heat islands as Chicago braces for a heat wave.
What do you mean, an island?
The “island” part of urban heat island just suggests one area, usually the core of a major city, is cut off from the surrounding areas, said Charles Mott, a meteorologist with the National Weather Service’s Chicago office. A heat island occurs when a metropolitan area is significantly warmer than the surrounding countryside, according to a city of Chicago glossary of emergency weather terms.
Different materials absorb and store heat from the sun in different ways, and an urban heat island can occur in a city heavy on materials that best absorb heat from sunlight.
For example, the steel, concrete and blacktop so prevalent in downtown Chicago absorb more heat from the sun than do materials found in nature, such as trees, soil and water, Edwards said.
“Those materials absorb the heat in a more efficient manner than other materials found in nature, and as a result, those objects will be able to warm up in the sun at a more efficient rate,” he said.
Chicago can become an urban heat island in hot weather, but because it has sprawling suburbs with materials that also retain heat, the temperature difference is less pronounced, say 3 or 4 degrees, than a city such as Indianapolis, where one can drive 20 miles in any direction and be met with cornfields, Edwards said.
“As it is with any large urban area, when you get a really hot, sunny day, you can always expect the urban center to run a few degrees hotter,” he said.
A force field that can split a storm system
Mott, from the weather service, said an urban heat island not only makes a city hub significantly warmer than the surrounding area, it also creates a force field of sorts.
“Especially Friday, downtown is gonna be hot. There’s no question about it,” Mott said. “On days like that, say a storm is moving in — I’m not saying that’s happening, this is just an example — storms will split. A line will go north and another line will go south because it can’t break through the heat dome that a city can create.”
Mott used to live in Fort Worth, Texas, and he said he often noticed storms would dissipate before reaching the city because of its urban heat dome.
“There’s so much heat that builds up, and storms can’t break through, or they have a tough time breaking through and they’d just die, or sometimes split and go around the core,” he said.
Lake Michigan as air conditioner
Chicagoans know about the lake effect in winter that results in bands of snow near Lake Michigan, but in hot weather, there’s a different meaning to lake effect, Edwards said. The proximity of Lake Michigan to downtown also means Chicago doesn’t typically get the maximum fluctuation in temperatures that other urban heat islands do, he said.
“It takes a lot of solar energy to warm up such a large body of water, such as Lake Michigan. That lake water is still going to be significantly less warm than the city center, and that makes the air near it cooler too,” Edwards said.
“General proximity to a cooler body of water will diminish that (urban heat island) effect, if even by a few degrees,” he said. “It takes more solar energy to warm up water than the ground itself.”
No nighttime cooling in the urban heat island
Just as water takes longer to heat, some materials used to build up the city center keep people in cities from seeing the benefits of cooling that naturally happen at night, because steel and asphalt trap heat.
So not only does the city heat up faster, it stays hot even after the sun goes down, meteorologists said.
“That’s why at night, downtown Chicago around here is warmer than anyplace else because it keeps the heat it absorbed in the daytime,” Mott said.
“You look at how much heat energy it would need to absorb in order to increase the temperature. And it doesn’t take much energy to warm up that pavement,” he said.
Edwards also reminds Chicagoans that while it’s important to check on loved ones, especially the elderly and those without air conditioning, living in an urban heat island is not going to have an enormous effect on their lives.
“The temperature difference isn’t the most noticeable thing in the world,” he said. “For most people, this is not a shocking difference in temperature from downtown to further out in the suburbs.”
Record temperatures stoked by the island?
With the help of the heat island effect, the temperature in Chicago could break some long-standing records, most likely Thursday and Friday, Edwards said.
The high temperature Thursday is forecast to be 92 to 97 degrees but could trend higher, and the hottest July 18 on record is 100 degrees in 1946, according to the weather service. The high Friday is forecast to be 95 to 100 degrees, with the July 19 record of 101 degrees set in 1930, according to the weather service.
“We’ve had winds generally out of the south and southwest being pulled up from the Gulf of Mexico and the mountainous West. On top of that we have a really strong ridge building across the area, and that means weather more conducive to extreme temperatures and high humidity,” Edwards said.
Saturday’s high of 98 degrees may not contend with the record of 101 set in 1980, and by Sunday, the heat wave is expected to end.
kdouglas@chicagotribune.com
