ancient cities

852 Urban Climate
Table 2 Factors That Lead to the Formation and/or Influence of the Magnitude of Urban Heat Islands
Heat Island Influence Effect on Urban Heat Island (UHI) Intensity
Surface geometry UHI intensity increases as building height to street width ratio increases and view of
the nighttime sky is obstructed.
Surface thermal properties UHI intensity increases as urban thermal admittance increases relative to rural
thermal admittance (city being a better storer of heat).
Anthropogenic heat input UHI intensity increases as anthropogenic heat increases. There can be large seasonal
variations in this heat input as well as intraurban spatial variability related to the
density of development and intensity of energy use.
City size UHI intensity tends to increase as city size increases.
Wind speed UHI intensity decreases rapidly as wind speed increases.
Cloud cover UHI intensity decreases as cloud cover increases.
Season UHI intensity is typically largest in the warm season in mid-latitudes. In high
latitudes, the UHI is largest in the winter due to anthropogenic heat input. In
tropical cities with distinct wet and dry seasons, the UHI is typically largest in the
dry season.
Time of day UHI intensity is largest at night (air temperatures).
Source: Oke, 1997a.
Table 3 Impacts of Urban Climates
Impacts of Urban Climate Description
Altered urban water balance Faster runoff, higher peak flows, and potentially higher concentrations of
contaminants due to the increased amount of impervious surfaces.
Increased urban air pollution Increased rates of air pollution formation (e.g., ground-level ozone) and
precursor emissions due to warmer urban atmospheres).
Human health Increased summer urban heat stress.
Improved cold season thermal comfort.
Safety and comfort of pedestrians in local wind climates influenced by tall
buildings.
Changes in energy demand Increased demand for summer cooling and reduced demand for winter heating.
Feedbacks in demands on pollutant and greenhouse gas emissions arising from
fossil fuel combustion used to supply energy needs, especially for cooling.
Increased water demand Demand for urban irrigation needs.
Urban biological activity Increased rates of urban biological activity due to longer growing seasons and
warmer temperatures.
Source: Oke, 1997b.
provides a basis for intentional efforts to alter
urban climates. Intentional modifications to
urban climate usually are focused on treatments
to the surface, because of the importance of the
urban surface in controlling the exchange of
energy between it and the overlying atmosphere.
The surface urban heat island intensity is larger during the day (clear, sunny
conditions).
Strategies to intentionally modify urban climates
are generally related to modifications to the built
surface, especially roofs and other impervious
surfaces, the use of trees and vegetation within
urban areas, and design strategies for urban
areas.