NASA’s ClimateBits has an excellent short video on what an urban heat island is. You can watch it below from YouTube. The urban heat island can be seen in cities as small as Athens GA (population about 115,000) and I think you could even see effects in smaller cities with distinct urban centers. In Athens alone based on research done my my son for a science fair project he showed as much as a ten degree difference between the center of the city and the rural areas outside. Bigger cities like Atlanta and Charlotte have even bigger effects, which makes forecasts for frost difficult for television meteorologists in these cities, since what is happening temperature-wise in the suburbs is quite different than what is happening in town. You can see a lot more of NASA’s resources for teachers and 4-H agents at http://nasawavelength.org/.