What are Urban Heat Islands?

Heat islands are parts of the urban landscape where there is a steep temperature variation between that area and cooler surrounding areas. There can be a temperature variation as much as ten degrees.

We all know that the population of the planet Earth is increasing exponentially. Along with that population increase will be a rapid development of technology, too. Population pressure causes direct and indirect effects on the environment.One such unknown, usually neglected effect is the uncontrollable expansion of "urban heat islands." The formation of these heat islands are mainly due to the construction materials used for the buildings. The fast paced expansion of cities also contribute to these heat islands. What are these heat islands, and how are they formed?

What are Heat Islands?
An island is a piece of land surrounded on all sides by water. So what are heat islands? These are a particular part of land where there is a steep temperature variation between this part and its surrounding land. Usually heat islands in cities have a mean temperature that is 8 to 10 degrees more than the surrounding rural areas. The temperature difference is greater at night than in the day and larger in winter than in summer. The reason for maximum temperature difference at nights are for the reason that the rural areas gets cooled faster than the urban areas. Thus the temperature difference is maximum at 5 to 6 hours after sunset.

Urban heat island (UHI) refers to relative warmness of the urban areas compared to nearby rural areas. Heat islands occur especially during clear and calm evenings and nights. They can prevail despite the season and time of the day. Occasionally, however, temperature difference between urban and rural areas can be reversed and urban cold island (UCI) occurs. UCI is most common in daytime in summer, and the UCI intensity is often substantially lower than UHI intensity, which can be even 10˚C. The size of the city, building density, geographic location and layout affect the intensity of UHI. In general, the largest cities usually have also the strongest UHI intensities.

As shown in the picture above, the slightly higher mean temperature forms a bubble or dome that is warmer than the surrounding areas. This forms a heat island.

Factors contributing to the formation of UHI
Anthropogenic heat release from (heated or cooled) buildings, traffic and industry urban building materials such as concrete and asphalt store solar heat during daytime and release it during night. Urban materials have a high heat capacity, which enables heat to be stored in a thick layer.

Evapotranspiration is lower in urban areas due to plenty of imperveous surfaces. The availability of water for evaporation is lower in urban areas where rainwater is gauged effectively away by sewers. Evapotranspiration cools down the surfaces and air, because the change of water from liquid state to vapor state requires energy. In other words, energy is transferred to latent heat instead of sensible heat. In cities where less evapotranspiration takes place, less energy is used in evaporation and more energy is available in the form of sensible heat, thus raising surface and air temperatures.

Buildings trap air between them and reduce wind speed, especially at street level. Buildings also trap long-wave radiation in the street canyons and part of the radiation is reflected from building walls back to the street level.

The main cause of the UHI effect is from the modification of land surfaces, which use materials that effectively store short- wave radiation. Waste heat generated by energy usage is a secondary contributor. change in the energy balance.

Causes Of Heat Islands
Many buildings found in urban areas have dark surfaces. Dark surfaces absorb more light energy, and heat, making the entire building warmer. Buildings with brick walls or roofs, if coated with dark coatings, also get heated up very quickly. In both cases, this heat is transferred both inside the structure and radiated into the surrounding air.

Buildings with dark surfaces heat up more rapidly and require more cooling from air conditioning, which requires more energy from power plants, which causes more pollution. Also air conditioners exchange heat with atmospheric air, causing further local heating. Thus there is a cascade effect that contributes to the expansion of urban heat islands.

Asphalt and concrete, needed for the expansion of cities, absorb huge amounts of heat, increasing the mean surface temperatures of urban areas. Tall buildings, and often, accompanying narrow streets, hinder the circulation of air, reduce the wind speed, and thus reduce any natural cooling effects.
Expanded transportation systems and the unimpeded use of fossil fuels also add warmth to urban areas. These pictures show smog formation in two cities. The formation of smog and increasing air pollution contributes to the problems of global warming. In my next article, we'll discuss different green construction materials and construction tips.

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