Scientists have known for 200 years that the temperature in a city can be higher than that in its environs — something they learned when an amateur weather watcher detected a 1.58F temperature difference between London and its suburbs. Modern cities, with their cars and heat-trapping buildings, can create an even bigger temperature gap, sometimes as much as 10F.
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Islands of urban heat can do funny things with weather. Hot city air, like hot air anywhere else, rises — even more so because of the turbulence caused by tall buildings. When that air is damp enough and collides with colder layers above it, water can condense out as a sudden burst of rain, especially if there are few frontal systems to disrupt the layers, as in summer. In a spot storm above a city or just downwind of it, it's likely that nature alone isn't behind the downpour.
NASA and the University of Arkansas have been using satellite mapping and ground-based temperature readings to determine how widespread this phenomenon is. This spring researchers got a surprise when they turned their attention to Houston. Because it's near a coast and sea breezes tend to cool and disperse hot air, Houston was thought to be comparatively safe from homemade rain. Now it appears that the opposite may be true. "The sea breeze may exacerbate the rainfall," says research meteorologist Marshall Shepherd of NASA's Goddard Space Flight Center. The warm air and sea air collide, he explains, and "move straight up like the front ends of two cars that hit head on, providing a pump of moist air that helps thunderstorms develop."
Hot, waterlogged cities can be cooled off in the usual ways — by limiting auto exhaust, for example. Using light-colored roofing and paving materials in place of black, heat-absorbing tar will also help. As a bonus, the cooler roof will reduce the need for air conditioning.