Figure 8 – Urban Heat Island Profile

There are three main factors responsible for this:

♤ The direct production of heat in city from fires, industry, home

♤ Heat conserving properties of the bricks and fabric of the city

♤ Blanketing effect by atmospheric pollution on outgoing radiation

Heat trapped in concrete buildings, pavements during day time is released very slowly in the form of long wave radiations, making cooling a slow process. With a decreased amount of vegetation, cities also lose the shade and cooling effect of trees, the low albedo of their leaves, and the removal of carbon dioxide. The tall buildings within many urban areas provide multiple surfaces for the reflection and absorption of sunlight, increasing the efficiency with which urban areas are heated. Tall buildings also inhibit cooling by convection and pollution from dissipating. Chemicals emitted by cars, industries affect sunshine in different ways, often trapping it and creating more heat. All these factors cause a change in the energy balance of the urban area.

As a population center grows, it tends to expand its area and increase its average temperature. For instance, Los Angeles has been very much affected by its urban heat island. The city has seen its average temperature rise approximately 0.5 °C every decade since the beginning of its super-urban growth since the World War II era. Other cities have seen increases of 0.1°-0.4°C each decade.

Each city's urban heat island varies based on the city structure and thus the range of temperatures within the island varies as well (figure 8). Parks and greenbelts reduce temperatures while the Central Business District (CBD), commercial areas, and even suburban housing tracts are areas of warmer temperatures. Every house, building, and road changes the microclimate around it, contributing to the urban heat islands of our cities.