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CINDY DAY: Mercury rising while you are sleeping

A warm night sets in as the moon comes up!  Lew Turner captured the glow in the sky over the Bedford Basin in Halifax, N.S.
A warm night sets in as the moon comes up! Lew Turner captured the glow in the sky over the Bedford Basin in Halifax, N.S. - Contributed

Summer is beginning to wind down. It’s been a hot one, not only here in Atlantic Canada, but in many parts of the world.

In Halifax  between July 1 and Aug. 21, there were 38 days with a temperature of 25 degrees C or more: more than double the average number of days the temperature is over 25 for that time period.  Charlottetown, P.E.I., marked its 17th consecutive day with a humidex reading of 30 or higher.  St. John's, N.L., had two consecutive days with a maximum temperature of 28 degrees twice this month! 

While it’s natural to react and comment on how warm it is during the day, scientists have been remarking on the nighttime anomalies for several years. 

In 2015, NOAA – the National Oceanic and Atmospheric Administration – noted: “As the world warms, overnight temperatures are slightly outpacing daytime temperatures in the rate of warming.”

The following year, 2016 ranked as the third warmest year ever across North America when looking at average temperatures.  But, when overnight minimums were examined, 2016's nights were the warmest ever.  When the cooler part of the day tends to warm up more than the warmer part of the day, the result is a smaller daily temperature range.  Without those temperature swings, it gets warmer at a faster rate. 

When temperatures fail to drop at night, you start the day from an already elevated position.   Daytime heating continues and that heat can become deadly. 

In recent decades, the average overnight lows have gone up a few degrees, and there are significantly more days each year above 110 degrees Fahrenheit in the U.S.  We don’t have to go south of the border to validate these finding. Since mid-July, the number of days with temperatures above 25 degrees Celsius was record setting across Atlantic Canada.  In many cases, our overnight lows failed to drop below 20.  The normal overnight low for that period is in the 16-degree range. 

Cooler nighttime temperatures allow our bodies to “reset” and our houses to cool, but when outside temperatures consistently climb above 25 and sit near 20 at night, our internal body temperatures don't have a chance to cool down.  

Cindy Day is the chief meteorologist for SaltWire Network.

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