Urban Heat Island

Heat Balance

• Since incoming radiation heats the atmosphere and the earth through absorption, these two regions should continue to get hotter and hotter. The reason for their not getting hotter is due to a complex system of re-radiation from the earth and clouds. Over a long period, the heat loss from the earth and atmosphere is equal to the heat gain.
• However, temporary imbalances exist at some points on earth. The system is complex because the water vapour in the atmosphere can absorb long-wave (terrestrial) radiation.
• Thus, the water vapour absorbs and re-radiates long-wave radiation to and from the earth. Because it absorbs long-wave radiation more effectively than sort waves, a green-house effect is created.

Urban Heat Island

• Urban heat island (UHI) is a urban area which is significantly warmer than its surrounding rural areas.
• The temperature difference usually is larger at night than during the day, and is most apparent when winds are weak.
• UHI is seen during both summer and winter. Due to their high heat capacities, urban surfaces act as a giant reservoir of heat energy. It has been shown that concrete can hold roughly 2,000 times as much heat as an equivalent volume of air.
• Latest studies show that UHI is one among the main reasons for Global warming.
• The rising columns of warm air in urban areas are commonly called as “Urban Thermal Plume“.