Geoscience Reference
In-Depth Information
smog and inhibits both incoming and outgoing radiation. Pollution domes and plumes are produced
around cities under appropriate conditions of vertical temperature structure and wind velocity. H
and G dominate the urban heat budget, except in city parks, and as much as 70-80 percent of
incoming radiation may become sensible heat, which is very variably distributed between the
complex urban built forms. Urban influences combine to give generally higher temperatures than
in the surrounding countryside, not least due to the growing importance of heat production by
human activities. These factors lead to the urban heat island, which may be 6-8
°
C warmer than
surrounding areas in the early hours of calm, clear nights, when heat stored by urban surfaces is
being released. The urban-rural temperature difference under calm conditions is statistically related
to the city population size; the urban canyon geometry and sky view are major controlling factors.
The heat island may be a few hundred meters deep, depending on the building configuration. In
some cases, summer daytime cold islands are observed. Urban wind speeds are generally lower
than in rural areas by day, but the wind flow is complex, depending on the geometry of city
structures. Cities tend to be less humid than rural areas, but their topography, roughness and
thermal qualities can intensify summer convective activity over and downwind of the urban area,
giving more thunderstorms and heavier storm rainfall. Tropical cities have heat islands, but the
diurnal phase tends to be delayed relative to mid-latitude ones. The temperature amplitude is largest
during dry season conditions.
•
In what ways do vegetated surfaces modify the surface climate compared with unvegetated ones
and what processes are involved?
•
What are the major effects of urban environments on atmospheric composition?
(Data from air sampling sites in North America and Europe are available on the Web.)
•
Look for evidence from local weather station reports and/or vegetation types for topoclimatic
differences in locations where you live/travel and consider whether these arise from differences
in solar radiation, day/night temperatures, moisture balance, wind speed or combinations of
these factors.
•
Look for evidence of urban-rural climatic differences in cities near you using weather reports of
day/night temperatures, visibility, snowfall events and so on.
concepts and processes, the climatic regimes of
six different natural surfaces, as well as
agricultural and urban surfaces]
Brimblecombe, P. (1986) Air: Composition and
Chemistry, Cambridge University Press,
Cambridge, 224pp. [Suitable introduction to
atmospheric composition, gas phase chemistry,
aerosols, air pollution sources and effects, and
stratospheric ozone for environmental science
students]
REFERENCES AND FURTHER
READING
Books
Bailey, W. G., Oke, T. R. and Rouse, W. R. (eds) (1997)
The Surface Climates of Canada, McGill-
Queen's University Press, Montreal and
Kingston, 369pp. [Sections on surface climate
