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Concrete, a lack of trees and dark colours: These are the key factors that helped turn Montreal’s urban core into a heat island this week, contributing to the deaths of 28 people. On Friday, officials in Quebec said the heat wave has been linked to at least 50 deaths across the province.

Urban heat islands refer to the fact that cities tend to be a few degrees warmer than rural areas on hot days. The reasons why are fairly simple, but the impacts can be severe.

One of the main reasons cities get hotter is they way they’re built. Concrete and paved roads tend to be made of dark colours, which absorb sunlight, whereas green grass, trees and brown soil can better deflect the sun’s rays, said David Wachsmuth, Canada Research Chair in urban governance at McGill University.

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Absence of green space is also a major contributor to the urban heat island phenomenon. Trees can provide shade and the sun’s rays cause water to evaporate from their leaves, which provides a cooling effect.

But there are other factors at play. For instance, large, tall buildings interrupt air flow and can allow heat to build up, Mr. Wachsmuth said. And pollution can exacerbate the situation.

The urban heat island effect is at its strongest at night. That’s because as the sun shines all day, being absorbed by roads and buildings, the energy accumulates. In rural areas, nighttime typically offers a reprieve from the heat thanks to the presence of trees and better airflow; the opposite is true in cities.

“It takes them hours and hours to radiate that heat out,” Mr. Wachsmuth said.

urban heat island effect

Urban centres tend to be a few degrees hotter

than outlying areas, which experts say is

attributable to several causes. Concrete roads

and buildings absorb sunlight, allowing heat to

accumulate throughout the day. Cities also don’t

have as many trees or green spaces, which

means they don’t experience the cooling effect

these provide. A lack of airflow from tall

buildings, pollution and other factors also make

cities hotter than the country.

Emissions from vehicles

and industry exacerbate

the situation

(trap heat and

decrease

air quality)

Solar radiation

Warm

moist

air

Rising heat

1

2

Prevailing

winds

The height of a

concrete apartment, as

well as poor ventilation

and bad insulation can make

them very hot.

These factors are usually

more prevalent in low-

income buildings without AC

Precipitation

downwind

past the city

THE TWO BIGGEST FACTORS

Too much concrete

Buildings, roadways,

sidewalks,

parking lots etc.

absorb heat

1

Not enough green spaces

decreases heat

by providing shade

and through

evaporation

2

JOHN SOPINSKI and carrie cockburn/

THE GLOBE AND MAIL,

SOURCES: brighthubengineering.com;

The Climate Change Action Partnership

McGill University School of Urban Planning

urban heat island effect

Urban centres tend to be a few degrees hotter than

outlying areas, which experts say is attributable to several

causes. Concrete roads and buildings absorb sunlight,

allowing heat to accumulate throughout the day. Cities

also don’t have as many trees or green spaces, which

means they don’t experience the cooling effect these

provide. A lack of airflow from tall buildings, pollution and

other factors also make cities hotter than the country.

Too much concrete

Buildings, roadways,

sidewalks, parking

lots etc. absorb heat

Emissions from vehicles

and industry exacerbate

the situation

(trap heat and decrease

air quality)

Solar radiation

Warm

moist air

Rising heat

Prevailing

winds

The height of a

concrete apartment, as

well as poor ventilation

and bad insulation can make

them very hot.

These factors are usually

more prevalent in low-

income buildings without AC

Precipitation

downwind

past the city

Not enough green spaces

decreases heat by

providing shade and

through evaporation

JOHN SOPINSKI and carrie cockburn/THE GLOBE AND MAIL,

SOURCES: brighthubengineering.com; The Climate

Change Action Partnership McGill University School

of Urban Planning

urban heat island effect

Urban centres tend to be a few degrees hotter than outlying areas, which experts say is

attributable to several causes. Concrete roads and buildings absorb sunlight, allowing heat

to accumulate throughout the day. Cities also don’t have as many trees or green spaces,

which means they don’t experience the cooling effect these provide. A lack of airflow

from tall buildings, pollution and other factors also make cities hotter than the country.

Solar radiation

Emissions from

vehicles

and industry

exacerbate

the situation

(trap heat and

decrease

air quality)

Warm

moist air

Too much concrete

Buildings, roadways,

sidewalks, parking

lots etc. absorb heat

Rising heat

Prevailing winds

The height of a

concrete apartment, as

well as poor ventilation

and bad insulation can make

them very hot.

These factors are usually

more prevalent in low-

income buildings without AC

Precipitation

downwind

past the city

Not enough green spaces

decreases heat by

providing shade

and through evaporation

JOHN SOPINSKI and carrie cockburn/THE GLOBE AND MAIL,

SOURCES: brighthubengineering.com; The Climate Change Action

Partnership McGill University School of Urban Planning

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