The difference in long-wave radiation balance between rural and urban areas is one of the main causes of the urban heat island. However, causes other than the areal difference of radiative cooling are considered to generate urban heat islands when radiative cooling does not develop. We analyzed development of heat islands in Tokyo on both cloudless and cloudy nights when the surface. receives enough solar radiation in the daytime. First, comparison is made for the diurnal variation of the horizontal distributions of temperature under the two conditions. Second, we compare the diurnal variation of heat island intensity on a cloudless night with that on a cloudy night. We select the observational data for autumn because radiative cooling occurs more frequently in that season. The results are as follows: 1. On cloudless nights, the heat island intensity increases from sunset, becomes maximum at midnight, and continues until early morning. 2. On cloudy nights, the heat island intensity increases from sunset, becomes maximum at midnight, and declines towards early morning. 3. On cloudless nights, the cooling rate in rural areas is faster than in urban areas from sunset to midnight. It is at the same level after midnight towards early morning. 4. On cloudy nights, the cooling rate in rural areas is faster than in urban areas from sunset to midnight. After midnight, however, it becomes faster in urban areas than in rural areas towards early morning.