Urban Heat Effect

Urban areas are usually warmer than their rural surroundings, a phenomenon known as the “heat island effect.” As cities develop, more vegetation is lost and more surfaces are paved or covered with build ings.

The change in ground cover results in less shade and moisture to keep urban areas cool. Built-up areas also evaporate less water, which contributes to elevated surface and air temperatures. Properties of urban materials, in particular solar reflectance, thermal emissivity, and heat capacity, also influence the development of urban heat islands, as they determine how the sun’s energy is reflected, emitted, and absorbed.

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.

Evapotranspiration.

Trees and vegetation absorb water through their roots and emit it through their leaves—this movement of water is called “transpiration.”
A large oak tree, for example, can transpire 40,000 gallons of water per year; an acre of corn can transpire 3,000 to 4,000 gallons a day.

Evaporation, the conversion of water from a liquid to a gas, also occurs from the soil around vegetation and from trees
and vegetation as they intercept rainfall on leaves and other surfaces. Together, these processes are referred to as
evapotranspiration.

Evapotranspiration cools the air by using heat from the air to evaporate water.
Evapotranspiration, alone or in combination with shading, can help reduce peak summer air temperatures.

-Peak air temperatures in tree groves that are 9ºF (5ºC) cooler than over open terrain.

-Air temperatures over irrigated agricultural fields that are 6ºF (3ºC) cooler than air over bare ground.

-Suburban areas with mature trees that are 4 to 6ºF (2 to 3ºC) cooler than new suburbs without trees.

-Temperatures over grass sports fields that are 2 to 4ºF (1 to 2ºC) cooler than over bordering areas.

Trees and other large vegetation can also serve as windbreaks or wind shields to reduce the wind speed in the vicinity of
buildings. In the summertime, the impacts can be positive and negative. In the wintertime, reducing wind speeds, particularly
cold north winds, can provide substantial energy benefits.

Further information ; http://www.epa.gov/heatisland/resources/compendium.htm