Geography Reference
In-Depth Information
warmest place because wind will assist the release of water from
plants as water vapour and result in cooling. The temperature and
wind effects are amplified in forests and it can be much cooler in
forests during the day than outside of the forest.
Urban climate
Urban areas often have a different climate from that found imme-
diately outside the urban area. This is because of the nature of the
building and road surfaces. It was noted above when we discussed
mountain environments how surface friction can slow down wind.
Urban areas tend to have much more 'rough' and variable terrain
than surrounding rural areas and this creates friction and reduces
wind speed, which can also mean that air pollution (e.g. from cars)
within urban areas is not dispersed very quickly. However, in
strong winds tall buildings create powerful gusts as the wind is
squeezed through the gaps between them, sometimes making it
difficult to walk or open doors.
Urban building materials tend to be good absorbers of the Sun's
energy and do not reflect as much as non-urban land surfaces. Fur-
thermore, because humans heat the local environment (within
houses, offices, shops, industrial premises etc.) then energy can be
released into the urban environment, especially in the winter in
mid- and high latitudes. Air pollution in urban areas can also add
to increased temperatures by increasing the concentration of green-
house gases. There also tends to be less vegetation cover or open
soils within urban areas and so evaporation is reduced. Evaporation
normally causes cooling since energy is used up to transform water
into water vapour. Hence if evaporation is reduced there will be
less of this cooling effect compared to surrounding rural areas. The
above factors combine to create the 'urban heat island' effect where
the urban area acts as an island of warmer air surrounded by cooler
rural air. The heat island is more apparent at night or when wind
speeds are low. The heat island effect is related to the city size and
types of building in the city. The largest heat islands are found in
cities such as Beijing and New York and a heat island effect as large
as 12°C has been found in Montreal during winter.
Because of atmospheric pollution in urban areas there can be
periods when the weather conditions within urban areas are a
