Urban heat island
Urban heat island (UHI) is a term referring to an urban area that is significantly warmer than the surrounding areas. The temperature difference is usually greater at night and is most noticeable when the winds are light. The UHI is particularly sensitive in summer and winter. The average annual temperature in a city of 1 million people or more may be 1.0-3.0°C higher than its surroundings.
There are several reasons for the emergence of an urban heat island:
- Dark surfaces absorb significantly more solar radiation. This is the reason why the UHI heats up more during the day than in suburban and rural areas due to the high density of roads and buildings.
- Concrete and asphalt, which are commonly used in urban areas for pavements and roofs, have significant heat capacity and thermal conductivity, as well as higher surface radiation properties. This leads to their heating and to higher temperatures compared to suburban areas.
- Lack of vegetation is another reason for higher temperatures in the UHI, as heavily built-up urban areas cannot benefit from the shade and evaporative cooling effect of trees.
- Tall buildings provide a much larger surface that absorbs and reflects solar heat. Scientists call this the "urban canyon effect".
- Another effect of the presence of densely located tall buildings is the blocking of the wind, which in turn reduces the natural ventilation and cooling by convection, prevents the dispersion of pollutants.
- The heat released during the operation of cars, air conditioners, industry and others also contributes to the formation of urban heat islands.
Not all cities have a separate urban heat island. Reducing the effect of UHIs can be achieved by building green roofs on buildings and using brighter surfaces in urban areas to reflect more sunlight and absorb less heat.
