Geoscience Reference
In-Depth Information
appear to have greater evapotranspiration losses than
those with a grass cover. Forest microclimates have lower
temperatures and smaller diurnal ranges than their
surroundings.
Urban climates are dominated by the geometry and
composition of built-up surfaces and by the effects of
human urban activities. The composition of the urban
atmosphere is modified by the addition of aerosols,
producing smoke pollution and fogs, by industrial gases
such as sulphur dioxide, and by a chain of chemical
reactions, initiated by automobile exhaust fumes, which
causes 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 domi-
nate the urban heat budget, except in city parks, and as
much as 70 to 80 per cent of incoming radiation may
become sensible heat, which is distributed very variably
between the complex urban built forms. Urban influences
combine to give generally higher temperatures than in the
surrounding countryside, not least because of the growing
importance of heat production by human activities. These
factors lead to the urban heat island, which may be 6 to
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 related statistically to the city
population size; the urban canyon geometry and sky view
factors are major controlling factors. The heat island may
be a few hundred metres deep, depending on the building
configuration. 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 temper-
ature amplitude is largest during dry season conditions.
DISCUSSION TOPICS
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, 369 pp.
[Sections on surface climate 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.]
Chandler, T. J. (1965)
The Climate of London
, Hutchinson,
London, 292pp. [Classic account of the urban effects
of the city of London in the 1950 to 1960s.]
Cotton, W. R. and Pielke, R. A. (1995)
Human Impacts on
Weather and Climate
, Cambridge University Press,
Cambridge, 288pp. [Treats intentional and accidental
weather and climate modification on regional and
global scales.]
Garratt, J. R. (1992)
The Atmospheric Boundary Layer
,
Cambridge University Press, Cambridge, 316 pp).
[Advanced level text on the atmospheric boundary layer
and its modelling.]
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 differ-
ences in cities near you using weather reports of
day/night temperatures, visibility, snowfall events
and so on.
