Urban development, including land use changes, dense building developments, heat emissions, human activities, etc., has a great impact upon the local climate of a city. One of the best-known effects of urbanization is the urban heat island (UHI) effect, which develops when urban cooling rates are slower than rural ones. Some of the main factors that may bring about the difference in temperatures between urban and rural areas include :
- the high heat capacity of the buildings in the urban area compared to surrounding rural areas, resulting in more of the sun's energy being absorbed and stored in urban;
- the high-density buildings in urban areas block the view of the sky and reduce the heat release back to space;
- Man-made heat emissions by buildings, air conditioning, transportation and industries in urban areas; and
- dense development in urban areas, which reduces wind speeds and inhibits cooling by convection.
Urbanization effects on the heat energy balance in the urban area
Urbanization effects on the low level wind flow over the urban area
In Hong Kong, the UHI effect is primarily a nighttime phenomenon. The UHI effect (temperature difference between urban and rural areas) is more significant in winter and is usually best displayed on individual days with stable atmosphere, light winds and a clear sky.
Variations of the average temperatures from urban to rural areas of Hong Kong at 5:00 a.m. in 2007
An example on 23 and 24 December 2001 showing the large difference in the temperature falling rate at night between the Hong Kong Observatory Headquarters (urban station) and the Ta Kwu Ling (rural station) due to urbanization effect.