Why night-time heat can be deadly

It’s not just daytime heat in the summer that can be deadly — a rising trend in night-time temperatures is also raising health concerns.

Over the past five decades, the average minimum temperature in July has gone up from 13.7 C to 16.4 C, according to Environment Canada.

Those figures don’t appear deadly on the surface. But they show a dramatic trend that the night-time temperatures in Toronto are definitely getting warmer during the summer, says Dave Phillips, senior climatologist with Environment Canada.

 “There is more and more evidence that climate change is about night-time warming rather than day-to-day heat,” said Phillips.

And that may bring big trouble. If you overheat and have no way of cooling down you can suffer from heat exhaustion. In more severe cases, it can lead to heat stroke, which if not treated immediately, can be fatal.

Usually, the body is able to cool down through evaporation of sweat on the skin. But in cases of extreme heat or high humidity the body can’t do this.

The phenomenon of higher evening temperatures most likely occurs because “heat absorbed throughout the day by dark structures and surfaces in the city is released overnight, preventing the city from cooling off,” according to a recent Toronto Public Health report.

 “Having the opportunity to cool off, even for a few hours, is critical in preventing heat-related illness and death.”

The report, presented to the Toronto Board of Health earlier this week, adds that “the daily minimum temperatures are increasing faster than the average or maximum temperatures.”

That trend is something Toronto Public Health is concerned about. “Higher night temperatures can have an effect on the ability to cope with the heat,” said Elaine Pacheco, a manager with Toronto Public Health.

 “In the evening, if temperatures cool down significantly, especially in an extended heat wave, people have an opportunity to get relief from the day time high,” she said.

In Europe in 2005 and Chicago in 1995 hundreds of people died during heat waves. The problem then was the night time temperatures didn’t fall, said Phillips. Even those in good health succumbed to the high temperatures. There simply wasn’t any relief.

There will be more of the same in the years to come says Phillips. In fact, the difference between daytime highs and night-time lows in the summer is slowly shrinking.

In addition, the dew point – a measurement of humidity – is increasingly elevated at night, which means your body is not able to perform as its engineered to do – get rid of natural heat and cool off, said Phillips.

Some nights earlier this summer the dew point was 23 C or 24 C during the day and even when the temperature dipped to 25 or 14, the dew point remained the same. “That’s unbelievable,” said Phillips.

“It’s like being in Savannah, Georgia rather than Phoenix, Arizona.”

Traditionally, the difference between a daytime high and night-time low in Toronto was quite wide with a high of about 28 C and a low of 15 or 16 C.

But during the latest heat wave, the daytime high reached about 35 C and the night time low only dipped to 25 C. That just doesn’t offer any relief — especially for people who don’t have air conditioning.

Phillips ran some data for the Star to illustrate the shrinking range. The difference in the 1960s was about 12.7 degrees; in 2000, it was 10.6 degrees. “This is clearly how the thermal climate in Toronto in the summer is changing.”

According to the Environment Canada data from Pearson Airport, in the 1960s there were 168 days in July where the temperature was over 30 C. Between 2000 and 2009, there were 189. 

But the next set of numbers is alarming, he said. In the 1960s there were only 21 nights where the minimum temperature in July was over 20 C. But from 2000 to 2009, that number rose to 109.

And that phenomenon is not just occurring in Toronto, but across many parts of the world, he said.

“Over the past 130 years night-time minimum temperatures are increasing,” said Pacheco. The rise in night-time temperatures is not the only factor that affects people in a heat wave, she said, “but it’s more significant when there is an extended heat event.”

Her advice for those that don’t have air conditioning is to take cool showers, baths, mist themselves with cold water, drink lots of water, minimize the actual time outdoors, close blinds and drapes to block out the sun, avoid using the oven, visit a place with air conditioning so you can get some relief.

--Debra Black, Staff Reporter

Comments

[mhmmm]

Less paving, cut the size of parking lots, plant more trees and grass and this will lead to less absorbed heat.

[Paul B]

California TV shows tell us how cold we are 'up north in Canada', but GTA residents are on the edge of ZONE A, and heat causes far more discomfort and misery than cold. We should do more to block solar heat gain from windows.

[Kelsa]

Personally, I've been loving the heat. I must disagree with Paul B. The slightest cold causes my bones to ache. A/C is not permitted anywhere near me :) I feel no discomfort in the heat we've had. It's been a wonderful summer thus far. It's been medicine for my particular health condition. But the city cement does hold the heat. Black road surfacing. Maybe scientists should work on lighter colour roads so as not to hold the heat?

[matt]

I've really come to hate Summer.

[Liberal conservative]

I'd much rather have a -60C winter than a +30C summer...the past few years you could count on two hands the number of days that AC was needed. This year I think you'd need two sets of hands to count the same number of AC days.

[Randal Oulton]

So climate change is now also responsible for all the dense, ugly construction in Toronto?

[snooty fox]

I lived in T.O. for a good # of years and truly loved the heat......but yes, pavement does hold the heat
and reflexion o glass pains off tall windows, making
it much retain the heat longer.....as has been said
more green is needed ie trees gras open spaces....

[Climate Realist]

It would be interesting to see the night-time trend for purely rural temperatures. Data I've seen from other countries seem to indicate that these increased night-time temperatures are primarily an artifact of increased urbanization!

[gunner21]

We have had very hot days in Elliot Lake, but for us the many Lakes and dense forest seems to cooll things down at night. Great for sleeping, it was a comfortable 15 or 16 at 6:00 AM this morning.

[Ginny]

I meant: Try finding ANY business in Toronto that has ANY a/c. and those who do, it is on so low it can barely be felt.
That is because many buildings in Toronto are so old and everything is broken. And because business owners are too cheap to pay for hydro so the a/c is not turned on....YET they have spot lights...all over the place...hot lamps everywhere that put out a lot of heat! How many lights do we need in one place????

As a contrast, Texas has super cold a/c in most if not all places of business, and theirs actually works.