The Effect of Clouds and Wind on the Difference in Nocturnal Cooling Rates between Urban and Rural Areas

The urban wanning effect is interesting in its own right and is important for understanding global warming. The aim of this study is to determine how the urban warming effect changes with cloud conditions and with wind speed. Studies of the urban warming effect have mostly concentrated on the urban–rural difference in daily maximum or minimum temperatures. The problem was approached using a new technique. Instead of comparing a city, represented by a first-order weather station, with the surrounding rural area, represented by data collected by cooperative observers; pairs of cities, each with a first-order weather station, were studied. One city was large. The other city was small enough to have a minimal warming effect and was close enough to the larger city to approximately represent the rural area. In this way, hourly temperatures, cloud cover, and wind data could be studied rather than only the differences between the daily maxima or minima. Results show that wind disrupts the normal nocturnal cooling pattern in which the smaller city, with lower thermal inertia, cools more quickly than the larger city. Clouds also disrupt this pattern, at least to the extent that one must be careful about extrapolating either magnitudes or patterns of urban–rural temperature difference observed by satellites under clear sky conditions to partly cloudy or cloudy conditions.