Shanghai urban influences on humidity and precipitation distribution

Abstract

By comparing simultaneously observed data of humidity and precipitation in Shanghai Urban area with those in its suburbs, we first deal with the urban influences on humidity. We find that both the average absolute humidity and relative humidity (diurnal, monthly and annual) are lower in the city than those in the surrounding 10 counties but night-time absolute humidity excess in the city exhibits a moisture “island” similar to that of temperature. The example is shown from a summer night at 5 h on Aug. 10, 1959.

When 20-year's records of isohyets of annual average precipitation, average precipitation of rainy seasons (May–September), and that of other seasons (October–April) are analyzed, the urban influences on the distribution of annual precipitation and the rainy season precipitation become obvious, but it seems that the other seasons are not affected.

Some rain events that illustrate the main contributing causes are also discussed, namely, heat island, obstacle effects and pollution products, for the modification and augmentation of precipitation in urban area and in the lee of the city.