"The idea of the heat island -- that densely built-up urban areas are considerably hotter than the rural and semi-rural landscapes that surround them -- has been extensively studied and is widely accepted by academics and the public.
But a new study by a Concordia researcher takes a closer look at the phenomenon and what can be done to mitigate it. According to Carly Ziter, an assistant professor of biology in the Faculty of Arts and Science, extensive tree canopy cover in an urban area can dramatically reduce the temperatures of their immediate environs -- enough to make a significant difference even within a few city blocks.
In a new paper published in the journal Proceedings of the National Academy of Sciences of the United States of America, Ziter argues that there is a non-linear relationship between canopy cover and temperature reduction: when canopy cover reaches a certain threshold, temperatures will begin to drop far more dramatically than they do below that point.
"We found that to get the most cooling, you have to have about 40 per cent canopy cover, and this was strongest around the scale of a city block," she says. "So if your neighbourhood has less than 40 per cent canopy cover, you'll get a little bit of cooling, but not very much. Once you tip over that threshold, you really see large increases in how much you can cool areas off."
She adds that the difference between areas with heavy canopy cover and those that are treeless can be as high as four or five degrees Celsius, even within just a few hundred metres of each other.
The effects of shading contribute to that decrease but are not the only factor.
"Trees transpire," she explains. "They give off water vapour, almost like a little air conditioner."
This transpiration occurs mainly during the day. Her research shows that during nighttime there is a much smaller difference in temperature between areas with significant canopy cover and those without".City trees can offset neighborhood heat islands, Concordia researcher says | EurekAlert! Science News
Concordia University associate professor of biology Carly Ziter writes that dense neighborhood tree canopy coverage can lower local temperatures significantly even compared to uncovered neighborhoods nearby.www.eurekalert.org