Published on: 12/22/2020 2:23:39 AM

Concentrations of pollutants in the air above a city create an urban aerosol, which attenuates insolation, especially when the sun angle is low as is the case at high latitudes and in winter. The aerosol is best developed (that is, at its worst) during conditions of stable air and calms or light winds. The urban aerosol is somewhat selective, for it reduces the proportion of ultraviolet radiation more than the longer wavelengths. It reduces the number of bright sunshine hours as well as the horizontal visibility. In spite of the diminished insolation, the centre of the typical metropolis constitutes a "heat island" that has a shape and size related to urban morphology, buildings, and industries and that results largely from urban heat generation and storage.

Temperatures normally are highest near the city centre and decline gradually toward the suburbs, beyond which there is a steep downward temperature gradient at the rural margin. The differences are greater at night latitude city and suburb. Although heat islands tend to be larger and more intense over large urban areas, the relation is not direct. Spacing of buildings and both kind and amount of activity influence heat island development.

Owing to the blanketing effect of pollutants on the radiation budget, diurnal ranges of temperature are less in urban areas than over the countryside. In view of the importance of vertical temperature lapse rates to atmospheric stability it is significant that night-time inversions tend to be weaker over cities, where the heat island generates modest convection. To the lee of cities, an

urban heat plume at several meters above the surface may intensify rural inversions. The roughness of the city surface increases frictional drag and turbulence. Gustiness and erratic flow of wind through the maze of urban canyons are well known to the city dweller, although gusts are more likely to reach their maximum speeds in the open countryside. Except under conditions of low regional wind speeds the mean wind speed within the city is lower than in the surrounding rural environment. When night-time winds are light, the speeds in the central city tend to be higher than in the country. Under nocturnal inversions the stable rural air inhibits surface flow, and calms are more frequent, whereas the relative instability of city air promotes turbulence, and stronger winds from above reach the surface more often.

A strong heat island generates its own circulation system. The inflow of cooler rural air toward the rising air over the city is generally weaker than might be expected, however, and is best developed on relatively calm, clear nights. Smoke plumes of residential areas have been observed to point toward a city centre. Instrumental observations from towers and balloons have confirmed upward movement of air above urban heat islands.

The tendency of air to rise above the heat island is a possible explanation of greater cloudiness over cities and may account in part for greater precipitation, for cities are good sources of condensation and ice nuclei One of the problems in urban precipitation studies is the difficulty of finding unpolluted rural areas with which to make experimental comparisons. A small number of ice nuclei injected into supercooled clouds might enhance rainfall, but a massive addition of condensation nuclei to air before it reaches its saturation temperature and produces warm clouds should lead to formation of many small droplets and inhibit rainfall.

The net effect of the urban aerosol on precipitation over the city and to the leeward is not clearly understood. It depends on the kind and number of nuclei emitted by urban sources, natural nuclei, relative humidity, thermallapse rates, and mechanical turbulence. Although climatological evidence is sparse, it may be that the incidence of hail is relatively greater over cities owing to convective activity. The proportion of precipitation in the form of snow appears to be less over urban centres, presumably because of higher temperatures associated with the heat island, and when it falls it effect on the surface albedo is rapidly modified by snow removal operations and urban dust.

The potential effects of the urban aerosol and heat island suggest processes leading to increased precipitation downwind from a city. Although conclusive results for a large number of cities under different conditions are lacking, specific studies appear to support a modification hypothesis. The mean relative humidity in city air is usually a few percent lower than in the surrounding country, especially at night and in summer when the heat island is well developed. Rural-urban differences in specific humidity are much more complex. Urban surfaces promote rapid runoff of precipitation, whereas vegetation and soil in the country retain moisture for evaporation over a longer period. On the other hand, the difference in availability of water vapor is offset to some extent by the many combustion sources in the city. During periods of light winds, tall buildings may inhibit the flow of air at ground level and thus reduce the upward diffusion of moist air. The net effect of all factors in a given city is to produce specific humidity values that reflect influences of urban form and function.