Thus, changes in the
energy and water balances in the urban surface will occur, resulting in an increase (decrease) in
the absorbed (reflected) solar short-wave radiation during the daytime, a decrease in the emitted
ground long-wave radiation into the sky, and an increase (decrease) of sensible (latent) heat flux.
This, in mid- to high latitudes, in turn causes a higher surface air temperature, in particular during
the nighttime, a smaller diurnal temperature range (DTR), a smaller near-surface wind speed, and
a generally lower (higher) relative humidity in urban areas of humid (arid) regions.
The tall buildings in the urban areas of mega cities increase the surface aerodynamic
roughness, strengthening the air convection and resulting in stagnant weather systems. These
effects, in combination with the thermal effect from UHI, increase the precipitation amount and/or
intensity in central urban areas and/or leeside areas of prevailing airflows, increasing the
frequency and intensity of short-duration, intense precipitation events.
To a certain extent, anthropogenic air pollution within cities may weaken (strengthen) urban
warming rates in the daytime (nighttime), leading to a further decline in DTR. The greater
concentration of aerosols in the middle to lower troposphere will also affect precipitation in urban
areas by modifying the micro-physical processes of clouds (van den Heever and Cotton, 2007;
Rosenfeld et al., 2008).