Urban heat island in full force this evening

The graphic below shows metro temperatures shortly after 8pm tonight and one thing stands out: it's hotter in the city than in the suburbs and outlying areas (by as much as 10-14 degrees)!  This is known as the "urban heat island effect" and is typical in major cities. Heat buildup during the day dissipates much less quickly from cities than rural areas as buildings, asphalt/concrete, and "urbanization" in general releases heat more slowly than more rural areas where there tends to be more grass, trees, and other vegetation.

Not only does it retain the heat longer, but the mere presence of man-made objects and surfaces will often keep those areas warmer throughout the night, thus having warmer low temperatures than the surrounding rural areas.  Often, you'll see MemphisWeather.net provide a certain low temperature in our forecast, then also include a cooler low for "outlying areas" or the "suburbs."  Now you know why! We understand the effects of the urban heat island, and now you do too!

The urban heat island is easy to spot in these 8:15pm temps around the Memphis metro.
While the airport and midtown and still in the 80s, temps are as much as 10-14 degrees cooler in outlying areas.

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Urban heat island effect

The Memphis urban heat island effect was in full force the past couple of mornings due to ideal conditions for radiational cooling in the suburbs and outlying areas, while the warmth of the city kept temperatures higher in the urban area. Radiational cooling occurs when heat escapes from the surface into the atmosphere, causing surface temperatures to fall.

The heat island effect occurs when temperatures in a city are warmer than the surrounding areas. The effects are most pronounced at night, but can occur at any time. Nighttime conditions promote the heat island effect as cool high pressure and calm wind allows the outlying areas to cool more rapidly than the city. The lack of wind keeps the warmer air from the urban area from mixing with the surrounding cooler air over the outlying areas. The city cools more slowly than outlying areas due to the preponderance of buildings, concrete, and vehicles, while more vegetative ground cover and fewer buildings and less concrete in the suburban and outlying areas cool more quickly.

The map above shows the temperatures around the metro area at about 6:20am this morning. Notice the upper 30s to lower 40s in the suburbs and mid to upper 40s in the city and at the airport (south side of the city) - a "dome" of heat over the city. The WXLIVE station operated by MemphisWeather.net in northern Bartlett is highlighted (see more data from WXLIVE).

Photo above copyright 2010 and courtesy of Chuck Lajeunesse of Collierville Photography.

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