Thursday, 9 February 2012

Heat Island

What Is a Heat Island?

Heat islands are characterized by urban air and surface temperatures that are higher than nearby rural areas. Many U.S. cities and suburbs have air temperatures up to 10˚ F (5.6˚ C) warmer than surrounding natural land cover. The heat island sketch below shows a city's heat island profile. It demonstrates how temperatures typically rise from the urban-rural border, and that the warmest temperatures are in dense downtown areas. On the other words heat island is the presence of any area warmer than its surrounding landscape. They can be developed on urban or rural areas. As it would be expected, there is a relatively minor knowledge about non urban heat islands, since they usually do not represent a risk for the human being or the environment. Meanwhile, urban heat islands have been profusely addressed during decades in urban areas with a wide range of climates and landscapes.

Heat islands are often largest over dense development but may be brokenup by vegetated sections within an urban area.

What Causes Heat Islands?

Heat islands form as cities replace natural land cover with pavement, buildings, and other infrastructure. These changes contribute to higher urban temperatures in the following ways:

•Displacing trees and vegetation minimizes the natural cooling effects of shading and evaporation of water from soil and leaves (evapotranspiration).

•Tall buildings and narrow streets can heat air that is trapped between them and reduce wind flow.

•Waste heat from vehicles, factories, and air conditioners may add warmth to the air, further increasing temperatures.

Heat islands are also influenced by a city’s geography and prevailing weather conditions. For example, strong winds and rain can flush out hot, stagnant air from city centers, while sunny, windless conditions can exacerbate heat islands.

When Do Heat Islands Form?

Heat islands can occur year-round during the day or night. Urban-rural temperature differences are often largest during calm, clear evenings. This is because rural areas cool off faster at night than cities, which retain much of the heat stored in roads, buildings, and other structures.

Effects of Heat Island

The well-known phenomenon allusive to the atmospheric temperature rise experienced by any urbanized area. The heat island phenomenon has been commonly associated to cities, because their surfaces are characterized by low albedo, high impermeability and favorable thermal properties for the energy storage and heat release. Besides, many cities present narrow urban canyons with reduced sky view factors that tend to absorb and reemit the radiated energy from their surfaces. These factors contribute to urbanized areas increasing their temperatures in relation to their rural peripheries that are usually more vegetated, and therefore moderate the temperatures mainly through the evapotranspiration process, shades production and solar radiation interception.

How Do Heat Islands Affect Us?

Increased urban temperatures can affect public health, the environment, and the amount of energy that consumers use for summertime cooling.

Public Health: Heat islands can amplify extreme hot weather events, which can cause heat stroke and may lead to physiological disruption, organ damage, and even death – especially in vulnerable populations such as the elderly.

The Environment: Summertime heat islands increase energy demand for air conditioning, raising power plant emissions of harmful pollutants. Higher temperatures also accelerate the chemical reaction that produces ground-level ozone, or smog. This threatens public health, the environment, and, for some communities, may have implications for federal air quality goals.

Energy Use: Because homes and buildings absorb the sun’s energy, heat islands can increase the demand for summertime cooling, raising energy expenditures. For every 1°F (0.6°C) increase in summertime temperature, peak utility loads in medium and large cities increase by an estimated 1.5 – 2.0 percent. Cities in cold climates may actually benefit from the wintertime warming effect of heat islands. Warmer temperatures can reduce heating energy needs and may help melt ice and snow on roads. In the summertime, however, the same city may experience the negative effects of heat islands.

A brief definition of the main Heat Island types

Surface urban heat island: The remotely sensed urban heat island. It is observed by using thermal infrared data that allow to retrieve land surface temperatures. Usually, close relationships between the near surface air temperatures and land surface temperatures have been found. Therefore, the surface urban heat island is a reliable indicator of the atmospheric urban heat island.

Micro urban heat islands: They refer to urban hot spots as poorly vegetated parking lots, non-reflective roofs and asphalt roads. Micro urban heat islands are strongly affected by micro climate factors, therefore remotely sensed data are more suitable than atmospheric data for identifying heat spots.

Urban heat sink: Also called negative heat island. It is the expression of a city colder than their countrysides. There are few references about this phenomenon. Heat sinks have been observed in cities with temperate, tropical, semi-arid and arid climates, and mainly during the mornings.

United State Environmental Protection Agency
Urban Heat Islands.

1 comment:

  1. I say that energy is really important and to our lives and that we should use our energy wisely.
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