Heat Island Effects
Effects of Street Trees on Urban Heat Islands
What is and Where to Find a Heat Island?
The heat island effect is a term used to describe higher air and structure temperatures in an urban setting as opposed to the lower temperatures found in more rural areas. With fewer trees in cities and large communities solar energy is more readily absorbed into manmade structures, causing higher urban area temperatures. Trees help to reduce the urban heat island effect, which contributes to increased levels of such things as higher ambient temperatures, air pollution, higher demands on cooling systems, and health problems related to heat and pollution.
Trees are a very simple, attainable means of reducing the effects.They act as nature's air conditioners. They help to cool the surrounding air in two ways: (a) trees provide shade, thereby keeping street and building surfaces cooler; and (b) trees use evapotranspiration to cool themselves and the surrounding air. Evapotranspiration is the process by which trees "transpire", or perspire, so to speak, from both the leaves and the root systems. The result is, as the water evaporates it dissipates the heat in and around the tree which leads to cooler air in the area encompassing the tree.
Higher temperatures and air pollution can result in the formation of smog in our larger cities, and can cause very real health problems. Smog is the result of pollutants in the air acted upon by the sun's light, this is known as a photochemical reaction. These pollutants are such things as: nitrogen oxide, hydrocarbons and other particulates in the air.
Leaves help reduce air pollution by "capturing" airborne particles, such as Nitrogen dioxide, NO2, Nitrogen oxide, NO, and Sulfur dioxide, SO2, while at the same time they are releasing Oxygen, O2. Trees intercept and absorb rain through their leaves and roots, thus reducing the amount of water falling on the pavement and hard surfaces and subsequently removed via storm water drainage systems.
Trees, their leaves, and the soil around them act as natural filters for water purification. Leaves collect the dust that blows around the city on their leaves. This helps to reduce some of the air pollution. The dust, for the most part, remains on the leaves until it rains where upon it washes to the ground.
Minerals, some heavy metals, and impurities are pulled into the roots where they are stored until the trees utilize them. Some are stored in its shoots, stems, leaves or fruit.