ancient cities

oundary layer) and in the urban surface temperatures.
The urban heat island may be detected by
networks of thermometers or, in the case of the
surface heat island, by remote sensing from aircraft
or satellites. The urban heat island arises
because cities warm and cool at different rates
than their surroundings. The differences in the
warming and cooling rates are related to the
changes in surface and atmospheric characteristics
of urban areas described earlier: Cities, consisting
of rough, dry surfaces, are good absorbers and
storers of heat, and they release this heat more
slowly than their rural surroundings at night. The
urban heat island (in air temperatures) is maximized
at night under clear and calm conditions,
when differences of up to 12° C may occur on an
hourly basis (Table 1). During the daytime, atmospheric
urban heat islands are typically weaker or
sometimes even cooler than their surroundings.
Surface temperatures in urban areas are usually
warmer than their rural surroundings. A complete
Table 1 Urban Climate Effects
Climate Variable Change in Urban Environment
Urban Climate
Air temperature Warmer (1–3° C on an annual basis, up to 12° C on an hourly basis)
Surface temperature Warmer: large variability depending on surface conditions, but may be 20° C or
more for individual surface types
Wind speed Reduced (5–30 percent at 10 meters) except for weather associated with calm or
very light winds when a slight increase in urban airflow may occur
Wind direction Turning (anti-clockwise in northern hemisphere, clockwise in southern
hemisphere) of 1–10 percent
Turbulence Increased (10–50 percent)
Humidity Drier (summer daytime)
More moist (summer nighttime, winter all day)
Evaporation Less
Soil moisture Less
Solar radiation Less
Visibility Reduced
Pollutant concentrations Increased
Clouds More, downwind of city
Precipitation Less snow
Rainfall may be increased downwind of the city
Thunderstorms More (but fewer tornadoes)
Source: Adapted from Oke, T. R. 1997. “Urban Climates and Global Environmental Change.” Table 21.2, p. 275 in Applied
Climatology, edited by R. D. Thompson and A. Perry. London: Routledge. Reprinted with permission of Taylor and Francis
Books UK.
851
explanation of the urban heat island requires
application of the surface radiation and energy
balance frameworks but some generalizations can
be made (Table 2).
Impacts of Urban Climates
Urban climates impact the nature of the urban
atmosphere through the climate effects described
in Table 1. These alterations arise through direct
modification of atmospheric properties or through
surface–atmosphere interactions. Other implications
for urban activities and functions also arise
from these changes as outlined in Table 3.
Intentional Modification
of Urban Climates
Understanding the physical basis of urban climates
and the impacts arising from the unintentional
modifications to urban atmospheres