The urban heat island has traditionally been studied by showing its magnitude at a particular point in time, normally at night, when it reaches its maximum. The following paper adopts the strategy of Oke and Maxwell (1975) using heating and cooling rates to study the heat island. This method has the advantage of showing how the heat island develops and decays throughout the day. As the results reveal the most important changes occur at sunrise and sunset, the relative importance of the generating processes are considered with reference to the radiation budget of the urban surface. Results are reinforced by using urban and rural thermograph data.