Cities can get hotter than rural areas, especially at night, because of the heat trapped in buildings and roads. Scientists worked together to make a guide on how to measure, model, and watch this kind of city heat island. They want to help city planners and weather experts understand and deal with this extra heat. The guide explains what a city heat island is, why it happens, and how to figure out how intense it is. It's helpful to people interested in weather and city planning to know more about stopping cities from getting too hot.