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.