Urban impacts on climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 216

Table 7.2 A summary of urban properties compared to rural properties, and urban climate effects
(after Oke 1997 ) a

Property Urban-rural comparison

Roughness length Rural: 0.01-0.5; Suburban: 0.6-1.0; Urban: 1.5-2.5

Albedo Rural: 0.12-0.20; Suburban: 0.15; Urban: 0.14

Emissivity Rural: 0.92-0.98; Urban: 0.94-0.96

Thermal admittance (Jm 2 s ½ K)

Rural: 600-2000; Suburban: 800-1700; Urban: 1200-2100

Anthropogenic heat (Wm 2 ) Rural: Nil; Suburban: 15-50; Urban: 50-100 (winter up to 250)

Condensation nuclei (cm 3 )
Aitken

Rural:10 2 -10 3 ; Urban:10 4 -10 6

Rural: 2500; Urban:10 3 -10 4 Cloud

Variable Urban change compared to rural

Turbulence 10-50% greater

Wind speed 5-30% less in strong wind; increased in weak winds with UHI

Wind direction 1-10 8 variation, more in canyons

UV radiation 25-90% less

Solar radiation 1-25% less

Infrared radiation 5-40% greater

Evaporation 50% less

Convective heat flux 50% greater

Heat storage 200% greater

Air temperature 1-3 8 C greater annual average; up to 12 8 C hourly average greater

Humidity Less in summer daytime; higher in summer night, winter day

Cloud More, especially downwind of city

Fog More or less depending on aerosol numbers; local environment

Total precipitation Greater, especially downwind of city

Thunderstorms More

Snow Less, turns to rain

Tornadoes Fewer

a Assumes mid-latitude city with about 1 million population; summer season except where indicated.

In 1954, T. J. Chandler produced the first major description of the London
heat island, including spatial distribution maps. During the early 1960s, the
highly controversial La Porte (Indiana, USA) precipitation anomaly was identi-
fied, and blamed on urban effects downwind of Chicago. In 1964, D. B. Turner
produced the first major air pollution dispersion model for urban areas, based on
Gaussian principles. In 1968, the World Meteorological Organisation (WMO)
hosted the first international symposium on urban climates (and building clima-
tology). By 1969, there was enough information available for J. Peterson to
publish a US government survey of known literature on urban climates.