Climate Changes in Rainfall in due to Urbanization

Observational studies show that the climate of a city or an urbanized area is significantly different from that of the surrounding rural areas. The differences are due to four factors1. First, urbanization changes the thermal properties of the earth surface. Areas of concrete, asphalt and other construction materials replace vegetation. The result of these changes is a higher surface temperature of the urbanized area. Second, buildings and other structures in the urban area are obstacles to the wind. The obstacles alter the prevailing flow pattern by generating turbulence and upward motions. Third, urbanization changes the moisture conditions at the earth surface. The relatively moist vegetated surface of the original rural area is replaced by a dry surface. Hence, very little evaporation occurs from the surface that results in a drier and a warmer surface air over the city. Lastly, the urbanized area is a source of anthropogenic heat and pollutants to the atmosphere.

The changes in surface temperature and roughness associated with urbanization produce
climatological effects on rainfall. This is due to the fact that these two changes enhance upward air motion and cloud formation over the urban area. The enhancement of rainfall has been detected in many previous studies. The most intensive study of rainfall effects has been done over the St. Louis City (Missouri Area)2. The results of the study show that the maximum urban effect occurs northeast of the city, about 30 km from the downtown area and outside of the main urbanized area.

The paper describes a combined observational and theoretical of the effects of
urbanization on rainfall. The objective of the observational part is to determine the effects of urbanization by using raingauge data from PAG-ASA stations. The objective of the theoretical part is to formulate a model to explain the observed effects.

[Presented at the Samahang Pisika ng Pilipinas (SPP) Congress, 2000.]