Urban heat island effect

The urban heat island effect, first noticed in the mid-1800s in the US, is the retention of thermal radiation (generally from the sun's infrared rays) absorbed during the day from sunlight. Buildings, especially tall ones, tend to heat up during the day. Although they radiate heat at night, they don't cool down completely. Although the gradient is a small one, over the years an urban area tends to become 1 or 2 degrees warmer than the surrounding countryside.

To what degree this effect impacts the global climate is a matter of controversy. Some researchers believe that even thermometers in rural areas are compromised by the urban heat island effect and want satellite and weather balloon data to be taken into account. (See [Global Warming Controversy]?.)