Hamilton's expanses of asphalt contribute to the heat island effect that puts our citizens at risk during heat waves. Why do we continue to permit them?
By John Neary
Published July 21, 2011
It's unclear whether global warming has anything to do with the current heat wave in Southern Ontario. But it's crystal-clear that urban heat islands are making the problem significantly worse than it needs to be.
Surface parking is not just bad for urban vitality; it also contributes to the heat island effect
The urban heat island effect has been known for two hundred years. Its major causes are succinctly summarized in a 1972 paper in Environmental Research by John F. Clarke:
The brick and concrete that make up a high percentage of the urban surface have a higher heat conductivity and storage capacity than the soil and grass that generally comprise the rural surface: the specific heat per unit volume of concrete is about twice that for dry clay or dry sand, and the thermal conductivity of concrete is about 10 times that of dry clay or dry sand.
During daylight hours, much of the solar radiation striking the rural surface is reflected back into space or is used in evaporative processes or to heat the atmosphere by the processes of conduction and convection from the heated surface. Little of the solar radiation striking the urban surface is used in evaporative processes; because of the high thermal admittance of the urban surface, much of the solar radiation is effectively stored in the urban structure as heat.
After sunset both urban and rural surfaces cool by the radiative process. Because of the large reservoir of heat and counter radiation between buildings, the urban structure cools at a slower rate resulting in an urban temperature excess. Another factor contributing to the urban temperature excess is heat produced within the city by man's activities.
While the above factors contribute to increased ambient temperatures in cities, Clarke also draws attention to lower wind speeds and increased exposure to radiant heat as factors that lead to increased heat stress on human beings.
This discussion would be purely academic if the heat island effect had no tangible consequences for city-dwellers. Yet even in 1972, the relationship between the heat island effect and heat wave mortality was well-documented, as Clarke describes:
The results of several studies suggest that a large percentage of heat-related deaths may be due to climate modification brought about by urbanization: Shattuck and Hilferty (1932, 1933) found that the urban heat-related death rate is higher than the rural heat-related death rate. The heat-related death rate tends to increase markedly with increase in size of the city.
Henschel et al. (1968) found that, of the 246 deaths attributed primarily to heat in the St. Louis metropolitan area during July 1966, 85% occurred within the city limits (which contained about 32% of the population); the death rate due to heat within the city limits was 5 times greater than in the adjacent suburbs. Buechley et al. (1972), in a study of the July 4, 1966, heat wave in the New York metropolitan area, found a high correlation between the spatial distributions of excess deaths and of minimum temperature.
The medical literature even contains reports of people who suffered severe burns from contact with paved surfaces. In 1996, James Berens published three such cases from Phoenix in the Journal of the American Medical Association. Here's the first:
A 22-year-old woman of approximately normal weight was struck by a car while walking across a city street in the early afternoon on June 12, 1968. She was rendered unconscious, and reportedly lay on a hot asphalt pavement for an estimated ten minutes. Her injuries included contusion of the brain with coma, second and third-degree burns present on all four extremities, and second-degree burns on the left side of her face.
Third-degree burns after ten minutes of lying on asphalt. Berens observes that an air temperature of 25 degrees in Phoenix corresponded to an asphalt temperature of 52 degrees - enough to damage skin after 60 seconds of exposure. At 31 degrees air temperature, the resulting asphalt temperature of 62 degrees would damage skin after only five seconds.
As Berens notes:
Most of us have had the experience of accelerating our gait to shorten the time that our bare feet are in contact with hot pavement, and have been witness to expressions such as "You could fry an egg on that pavement." We have not, however, been sufficiently alerted to the occurrence of contact burns in this situation. Indeed, it is possible in hot weather to fry an egg on our streets and highways; furthermore, it is possible to "fry" people.
D. A. Vardy et al reported a similar case from Israel in 1989 in the straightforwardly-named journal Burns:
A 70-year-old woman was admitted to hospital with heat-stroke. She had also sustained deep partial and full skin thickness burns covering 20 per cent of her total body surface area (TBSA) from contact with an asphalt pavement. She had undertaken a 4 h bus ride during a heat-wave. She arrived in the early afternoon and had to walk several hundred metres in the mid-day sun when the air temperature was 43°C (109°F). While walking she became exhausted, stopped to rest and went into a coma. She lay unattended on the asphalt pavement for about half an hour until she was found and rushed to hospital.
How long will we allow our city to remain covered with expanses of asphalt like these? And how long will we continue with transportation planning that demands them?
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