URBAN CLIMATE
By Sam C M Hui, Ph.D., Department of Architecture.
Urban areas have particular climatic conditions with a higher
temperature than exposed country-side, weak winds and an amount of
sunshine that varies according to the degree of pollution, the
urban density, the orientation of the streets and the shade
provided by other buildings.
Urban Microclimates
Urban microclimates are complex because of the number and diversity
of factors which come into play. Solar radiation, temperature and
wind conditions can vary significantly according to topography and
local surroundings. In addition, layout density can provide further
constraints: the precise plot division, the need for access and
privacy, and the noise and impact of atmospheric pollution must all
be taken into account.
In winter, most urban microclimates are more moderate than those
found in suburban or rural areas. They are characterised by
slightly higher temperatures and, away from tall buildings, weaker
winds. During the day, wide streets, squares and non-planted areas
are the warmest parts of a town. At night, the narrow streets have
higher temperatures than the rest of the city. In summer, green
spaces are particularly useful in modifying the environment during
the late afternoon, when the buildings are very hot inside.
Strong local winds can modifiy the temperature distribution
described above. Usually winds in towns are moderate because of the
number and range of obstacles they face. However, a few
configurations such as long straight avenues or multi-storey
buildings can cause significant air circulation. Tall buildings
rising above low-rise building can create strong turbulent wind
conditions on the ground as the air is brought down from high
levels. Strong winds can flow through gaps at the base of tall
buildings. To protect pedestrians from this, the turbulent flow has
to be prevented from descending to street level, for example by
modifying the building form or by using wide protective canopies.
In semi-open areas, adjacent buildings can be used as protective
screens against some winds.
Urban Heat Island
Visit any city on a hot summer day, and you will feel waves of
blistering heat emanating from roads and dark buildings. Stay in
the city past nightfall, and you will notice that the streets are
still radiating heat, while surrounding rural areas are rapidly
cooling.
Almost every city in the world today is hotter - usually between 1
to 4 deg C hotter - than its surrounding area. This difference
between urban and rural temperatures is called the
"urban-heat-island" effect", and it has been intensifying
throughout this century. During hot months a heat island creates
considerable discomfort and stress and also increases
air-conditioning loads and the incidence of urban smog (do you
notice this in Hong Kong!). Research shows that for every degree of
increased heat, electricity generation rises by 2% to 4 %, and smog
production increases by 4% to 10%. People also believe there is a
direct link between global warming and urban heat islands. First,
the greenhouse effect could aggravate rising urban temperature
significantly. Second, heat islands may contribute to the
greenhouse effect.
In general, there are three main factors causing the urban heat
islands:
Surface - The characteristics of the surfaces in urban and rural
areas are different and their thermal properties also differ a lot.
As compared with rural areas, urban districts have high absorption
(of the heat of the sun and atmosphere), low reflection, low
evaporative heat loss and fast transmission of heat.
Heat emission - "Artificial heat" emitted in urban districts is
much higher than that in rural areas.
Air quality - Air pollution in urban areas is high and the
particulates will form a shield for trapping heat.
