Urban heat island effect
Urban heat island effect (UHI) describes the built up areas that are hotter than nearby rural areas. The annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings. In the evening, the difference can be as high as 22°F (12°C). Heat islands can affect communities by increasing summertime peak energy demand, air conditioning costs, air pollution and greenhouse gas emissions, heat-related illness and mortality, and water quality. The phenomenon was first investigated and described by Luke Howard in the 1810s.
Urbanization negatively impacts the environment mainly by the production of pollution, the modification of the physical and chemical properties of the atmosphere, and the covering of the soil surface.
Contents |
[edit] Causes
The continuing replacement of natural surfaces by built-surfaces, through urbanization, and infrastructure constitutes the main cause of urban heat island formation. Natural surfaces are often composed of vegetation, and moisture-trapping soils. Therefore, they utilize a relatively large proportion of the absorbed radiation in the evapotranspiration process and release water vapour that contributes to cool the air in their vicinity. In contrast, built surfaces are composed of a high percentage of non-reflective and water-resistant construction materials. As consequence, they tend to absorb a significant proportion of the incident radiation, which is released as heat.
Other causes of a UHI are due to geometric effects. 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. This is called the "urban canyon effect". Another effect of buildings is the blocking of wind, which also inhibits cooling by convection. Waste heat from automobiles, air conditioning, industry, and other sources also contributes to the UHI. High levels of pollution in urban areas can also increase the UHI, as many forms of pollution change the radiative properties of the atmosphere.
[edit] Impact
[edit] Health
A direct relationship has been found between UHI intensity peaks and heat-related illness and fatalities, due to the incidence of thermal discomfort on the human cardiovascular and respiratory systems. Heatstroke, heat exhaustion, heat syncope,and heat cramps, are some of the main stress events, while a wide number of diseases may become worse, particularly in the elderly and children. In a similar way, respiratory and lung diseases have shown to be related to high ozone levels induced by heat events.
[edit] Weather
The anomalous warm of the city creates relatively low air pressures that cause cooler, rural air to converge on the urban center, thus forcing warm air to ascend, which at higher altitudes condensates and precipitates. The anomalous warming have shown in initiating precipitation and thunderstorm around the affected area.
[edit] Power Consumption
Elevated summertime temperatures in cities increase energy demand for cooling. Research shows that electricity demand for cooling increases 1.5–2.0% for every 1°F (0.6°C) increase in air temperatures, starting from 68 to 77°F (20 to 25°C), suggesting that 5–10% of community-wide demand for electricity is used to compensate for the heat island effect. Urban heat islands increase overall electricity demand, as well as peak demand, which generally occurs on hot summer weekday afternoons, when offices and homes are running cooling systems, lights, and appliances. During extreme heat events, which are exacerbated by urban heat islands, the resulting demand for cooling can overload systems and require a utility to institute controlled, rolling brownouts or blackouts to avoid power outages.
[edit] Aquatic Life
High pavement and rooftop surface temperatures can heat stormwater runoff. Tests have shown that pavements that are 100ºF (38°C) can elevate initial rainwater temperature from roughly 70ºF (21ºC) to over 95ºF (35ºC).4 This heated stormwater generally becomes runoff, which drains into storm sewers and raises water temperatures as it is released into streams, rivers, ponds, and lakes. Water temperature affects all aspects of aquatic life, especially the metabolism and reproduction of many aquatic species. Rapid temperature changes in aquatic ecosystems resulting from warm stormwater runoff can be particularly stressful, even fatal to aquatic life.
[edit] Mitigation
There are two main UHI reduction strategies: first, to increase surface reflectivity, in order to reduce radiation absorption of urban surfaces, and second, to increase vegetation cover, mainly in the form of urban forests and parks, in order to maximize the multiple vegetation benefits in controlling the temperature rises.
Reflective surfaces simply results from light colored or white paint on the surface of a given construction material or from cover the construction material surface with a white membrane. Both techniques have been mainly applied on roofs and pavements. Cool roofs are specially important in commercial and residential buildings, where significant energy demand for cooling can be saved by reducing heat gain to the building. Cool pavements have mainly based on the use of whitened asphalt roads, a very warm material.
Increasing vegetation cover is mainly focused on planting trees at nearby houses, and residential and commercial buildings. A particular emphasis has been placed on vegetation planting on the roofs of buildings, in order to achieve the same aim as lighter-colored roofs. Strategically placing trees in front of windows and on the sunniest sides of a house maximizes energy savings. Trees placed on the east and west sides of a structure are most effective because they block the morning sun as well as the afternoon sun. Larger trees also tend to be more effective, as they provide a greater canopy cover and shade area.
Roof sprinkling is another evaporative cooling solution. Sprinklers on the roof wet the surface so that the air around it cools through evaporation. Urban planners also set up traditional parking lots along lots where trees and vegetation grow. Tall trees not only contribute to evaporative cooling but also provide much-needed shade.
