Cities are well known hot spots – literally. The urban heat island effect has long been observed to raise the temperature of big cities by 1 to 3°C, a rise that is due to the presence of asphalt, concrete, buildings, and other so-called impervious surfaces disrupting the natural cooling effect provided by vegetation. According to a new NASA study that makes the first assessment of urbanization impacts for the entire continental United States, the presence of vegetation is an essential factor in limiting urban heating.

Impervious surfaces’ biggest effect is causing a difference in surface temperature between an urban area and surrounding vegetation. The researchers, who used multiple satellites’ observations of urban areas and their surroundings combined into a model, found that averaged over the continental United States, areas covered in part by impervious surfaces, be they downtowns, suburbs, or interstate roads, had a summer temperature 1.9°C higher than surrounding rural areas. In winter, the temperature difference was 1.5 °C higher in urban areas…

The study, published this month in Environmental Research Letters, also quantifies how plants within existing urban areas, along roads, in parks and in wooded neighborhoods, for example, regulate the urban heat effect…

The northeast I-95 corridor, Baltimore-Washington, Atlanta and the I-85 corridor in the southeast, and the major cities and roads of the Midwest and West Coast show the highest urban temperatures relative to their surrounding rural areas. Smaller cities have less pronounced increases in temperature compared to the surrounding areas. In cities like Phoenix built in the desert, the urban area actually has a cooling effect because of irrigated lawns and trees that wouldn’t be there without the city…

At the human level, a rise of 1°C can raise energy demands for air conditioning in the summer from 5 to 20 percent in the United States, according the Environmental Protection Agency. So even though 0.3°C may seem like a small difference, it still may have impact on energy use, said Bounoua, especially when urban heat island effects are exacerbated by global temperature rises due to climate change.

I grew up in a southern New England factory town. What changed it from being as much of an environmental disaster as most – was the struggles for Keynesian solutions like the CCC during the Great Depression included the transformation of vacant brownscapes into parks. Sufficiently done that the city began calling itself a park city. Every neighborhood acquired a little more life, more human friendliness, with the addition of a greenscape of grass and trees.