Climatic Design of Buildings - An Overview

6. URBAN CLIMATE

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.

6.1 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.

6.2 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.

Figure 10 - Urban heat island

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:


 
< Back
Home
Foreward >