Heat islands are caused by urbanization, when buildings, roads and paved surfaces store the heat during the day and then release it slowly during the evening keeping urban lands hoter than surrounding areas. Artificial heat released by combustive processes from vehicles and industrial activities, and heat escaping from commercial and domestic air conditioning, also contribute to higher air and surface temperatures within the city.

Heat islands affect urban climate, energy use and habitability of cities. Higher temperatures affect cooling energy use and accelerate urban smog formation.

Urban trees can offset or even reverse the heat island effect by transpiring water and shading surfaces! According to previous studies (Akbari et al. 1992; Akbari & Taha, 1992; McPherson & Rowntree, 1993), large number of trees and urban parks reduce local air temperature by 0.5-5 degrees Celsius (1-9 degrees Fahrenheit). Each 1 degree C drop in daily maximum temperature up to a threshold of 15 to 20 degrees, lower the peak electric demand by 2-4%. Cooling energy savings and smog reduction are other potential benefits.

We looked at urban heat island phenomenon and its implications in Rhode Island, with our initial focuse being on Providence.

By analyzing thermal data obtained with ETM+ onboard Landsat 7 on July 2001 and the land use/land cover classification from Rhode Island Geographic Information System we identified spatial distribution of land uses, vegetative cover, and the daytime land surface temperature in Providence.

The color-coded map of Providence displays land surface themperature patterns, with the hotest areas in the central and south-eastern parts of the city.

The land surface temperature map was overlaid with the vegetative cover to assess the approximate warmest signature of tree cover; a threshold of 29 degrees C was applied to determine the location of heat islands in Providence.

In Providence, densely populated residential districts, commercial and industrial areas represent urban heat islands, with the surface temperature as high as 43 degrees Celsius in summer days. On average, the summertime land surface temperature in Providence heat islands is 20° C higher than in the surrounding suburban areas and 13 degrees higher than in treed neighborhoods in Providence.

Higher level of mean land surface temperature (30-32 degrees C) is observed in commercial, manufacturing and mixed industrial areas.

The maximum mean land surface temperature in high density residential areas ammounts to 28 degrees C. Vegetation in these areas is lacking or insufficient to reduce mitigate urban heat island effect.