Cities have variations in their natural environment and setting that influence the weather and climate - any urban climate or warming effect will be overlaid across this natural domain. Indeed, it could be claimed that because of the extent of human activity across the South East, few places now have a truly rural climate here today. There is far more to an urban climate than a heat-island. An urban area changes the energy and radiation balance, hydrology and airflow characteristics of the local weather and climate. The heat-island itself arises not just from the heating of buildings and the human activity within the city.
Urban areas reflect a lower proportion of incoming solar radiation.
This reflectivity is called the albedo of a surface. C during daytime. This is based on the temperature difference between the rural hinterland and the warmest area within the city. There is also likely to be a displacement effect caused by the prevailing SW wind. Of course, the intensity of the heat-island is also dependent upon city size, often judged in terms of population.