what is urban heat island effect

URBAN CLIMATE
Urban areas have particular climatic conditions with a higher temperature than exposed country-side, weak winds and an amount of sunshine that varies according to the degree of pollution, the urban density, the orientation of the streets and the shade provided by other buildings.

Urban Microclimates
Urban microclimates are complex because of the number and diversity of factors which come into play. Solar radiation, temperature and wind conditions can vary significantly according to topography and local surroundings. In addition, layout density can provide further constraints: the precise plot division, the need for access and privacy, and the noise and impact of atmospheric pollution must all be taken into account.
In winter, most urban microclimates are more moderate than those found in suburban or rural areas. They are characterised by slightly higher temperatures and, away from tall buildings, weaker winds. During the day, wide streets, squares and non-planted areas are the warmest parts of a town. At night, the narrow streets have higher temperatures than the rest of the city. In summer, green spaces are particularly useful in modifying the environment during the late afternoon, when the buildings are very hot inside.

Strong local winds can modifiy the temperature distribution described above. Usually winds in towns are moderate because of the number and range of obstacles they face. However, a few configurations such as long straight avenues or multi-storey buildings can cause significant air circulation. Tall buildings rising above low-rise building can create strong turbulent wind conditions on the ground as the air is brought down from high levels. Strong winds can flow through gaps at the base of tall buildings. To protect pedestrians from this, the turbulent flow has to be prevented from descending to street level, for example by modifying the building form or by using wide protective canopies. In semi-open areas, adjacent buildings can be used as protective screens against some winds.

Urban Heat Island
Visit any city on a hot summer day, and you will feel waves of blistering heat emanating from roads and dark buildings. Stay in the city past nightfall, and you will notice that the streets are still radiating heat, while surrounding rural areas are rapidly cooling.

Almost every city in the world today is hotter - usually between 1 to 4 deg C hotter - than its surrounding area. ****This difference between urban and rural temperatures is called the "urban-heat-island" effect"****, and it has been intensifying throughout this century. During hot months a heat island creates considerable discomfort and stress and also increases air-conditioning loads and the incidence of urban smog (do you notice this in Hong Kong!). Research shows that for every degree of increased heat, electricity generation rises by 2% to 4 %, and smog production increases by 4% to 10%. People also believe there is a direct link between global warming and urban heat islands. First, the greenhouse effect could aggravate rising urban temperature significantly. Second, heat islands may contribute to the greenhouse effect.

In general, there are three main factors causing the urban heat islands:

Surface - The characteristics of the surfaces in urban and rural areas are different and their thermal properties also differ a lot. As compared with rural areas, urban districts have high absorption (of the heat of the sun and atmosphere), low reflection, low evaporative heat loss and fast transmission of heat.
Heat emission - "Artificial heat" emitted in urban districts is much higher than that in rural areas.
Air quality - Air pollution in urban areas is high and the particulates will form a shield for trapping heat.

Many big cities have this effect. Like New York, Tokyo, Singapore, etc.

An urban heat island (UHI) is a metropolitan area which is significantly warmer than its

surroundings. As population centers grow in size from village to town to city, they tend

to have a corresponding increase in average temperature, which is more often welcome

in winter months than in summertime. The EPA says: "On hot summer days, urban air

can be 2-10°F [2-6°C] hotter than the surrounding countryside. Not to be confused

with global warming, scientists call this phenomenon the 'urban heat island effect.

There is no controversy about cities generally tending to be warmer than their

surroundings. What is controversial about these heat islands is whether, and if so how

much, this additional warmth affects trends in (global) temperature record. The current state of the science is that the effect on the global temperature trend is small to negligible—see below.

Scientists compiling the historical temperature record are aware of the UHI effect, but they vary as to how significant they think it is. Some scientists (see Peterson, below) have published peer reviewed papers indicating that the effect of the UHI has been overestimated, and that it does not affect the record at all. Other scientists have used various methods to compensate for it. Some advocates charge that temperature data from heat islands has been mistakenly used as evidence for global warming.

As a result of the urban heat island effect, monthly rainfall is about 28% greater between 20-40 miles downwind of cities, compared with upwind. [2]

There are several causes of a UHI, as outlined in Oke (1982). The principal reason for

the night-time warming is (comparatively warm) buildings blocking the view to the

(relatively cold) night sky. (See Thermal radiation) Two other reasons that UHIs occur

are changes in the thermal properties of surface materials and lack of evapotranspiration in urban areas. Materials commonly used in urban areas, such as concrete and asphalt,

have significantly different thermal bulk properties (including heat capacity and thermal conductivity) and surface radiative properties (albedo and emissivity) than the
surrounding rural areas. This initiates a change in the energy balance of the urban area,
often causing it to reach higher temperatures (measured both on the surface and in the air) than its surroundings. The energy balance is also affected by the lack of vegetation and standing water in urban areas, which inhibits cooling by evapotranspiration.

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, increas

heat island effect is a phenomenon describes urban and suburban temperatures that are 2 to 10°F (1 to 6°C) hotter than nearby rural areas. Elevated temperatures can impact communities by increasing peak energy demand, air conditioning costs, air pollution levels, and heat-related illness and mortality.

How Do Heat Islands Form?
Heat islands form as cities replace natural land cover with pavement, buildings, and other infrastructure. These changes contribute to higher urban temperatures in a number of 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 trapped between them and reduce air flow.
Waste heat from vehicles, factories, and air conditioners may add warmth to their surroundings, further exacerbating the heat island effect.
In addition to these factors, heat island intensities depend on an area's weather and climate, proximity to water bodies, and topography. Measuring heat islands can help determine how these factors influence the heat island effect.

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. As a result, the largest urban-rural temperature difference, or maximum heat island effect, is often three to five hours after sunset.