Sven Sundgaard: It’s not just the heat, it’s the urban heat islands

We’re fond of saying "it’s not the heat, it’s the humidity." Or in winter, it’s not the temperature but the wind chill that can make conditions brutal. Extreme heat is deadly in many places, but we’ve created unnecessary heat ‘death traps’ in urban heat islands.

Urban heat islands create extra warmth because of how we’ve developed and structured our cities. We use heat-absorbing materials to pave roads, highways and bridges and have larger buildings in greater density. There’s also more energy used in a smaller area. To counter the excess heat, nature helps. The more green spaces (parks, etc.) and water, the better.

In the Twin Cities, Climate Central did an analysis based on census groups within Minneapolis of our own urban heat island. Pockets of our urban core see up to 9 degrees F warmer temperatures. In fact, they estimated that 44,000 people live within those hot spots.

Note also the coolest spots of the city are near the lakes and parks where there is more water, trees and grass.

For reference, compared to a much larger city, below is a look at New York City. Six million people in New York City live in areas where the urban heat island makes turns the temperature up to 9 degrees warmer.

Climate Central’s study broke down the extra heat specifically in formerly ‘red-lined’ neighborhoods in America’s cities:

An in-depth study in the American Meteorological Society’s Journal of applied Meteorology and Climatology , we can see a further breakdown of the Twin Cities’ urban heat island and the magnitude of temperature disparities by region.

The big takeaway is that the largest magnitude in temperature difference is in the summer at night. This is why heat waves are particularly deadly in cities. Heat during the day is one thing, and the magnitude is less, but at night cities cool off much less and not as fast. The poor and elderly who may be less apt to use or afford air-conditioning suffer particularly on those nights.

A breakdown of the magnitude of the urban heat island by hour shows this nighttime trend, which has a peak impact between 9 p.m. and 3 a.m.

Some of our recent weather shows the Urban Heat Island:

The recent couple of days with very light winds at the surface and aloft have helped to allow the urban heat island to be more visible than normal. That’s because on a normal day there’s surface wind to mix up the atmosphere between urban, suburban and rural. There’s also winds aloft that are almost always much faster to mix things further.

Wednesday and Thursday this week saw surface winds and winds in the middle of the atmosphere at just 5-10 mph or less, allowing a magnified, uneven heating of our urban areas. Take a look at this satellite loop from Thursday afternoon. There was a thicker field of cumulus clouds, which develop due to surface heating and instability over the Twin Cities, compared to surrounding areas.

Note for example, many fewer clouds over Lake Minnetonka (a cooler surface) and Lake Mille Lacs in central Minnesota. Temperatures over both bodies of water are at least several degrees cooler than inland areas.

A temperature map from 5 p.m. Thursday shows the temperature difference clearly as the result of the urban core. Take a look...

So, while on a normal day, urban heat islands result in later first frosts, and just a few degrees in temperature difference, they can become deadly in heat waves, especially in our largest cities.