Nights Can Be More Deadly Than the Daytime During a Heat Wave

Heat waves kill more people in the United States than any other natural disaster. While much attention is paid to extremely high temperatures during the daytime, it's the high minimum temperatures combined with high humidity that is more worrisome.

As the day ends and temperatures remain high — over 80 degrees — as well as humidity, people's bodies do not have an opportunity to recover from the heat and are more likely to suffer from heat-related illness, according to Grist.

Among the most vulnerable are those without air conditioning, children under 4 years of age and the elderly.

Paris Heat Wave

In 2003, an estimated 70,000 people died across Europe during a heat wave that lasted for three weeks and brought temperatures to heights that hadn't been seen in hundreds of years, according to Live Science. In France alone, nearly 15,000 succumbed to the heat, USA Today reported, and many of those deaths occurred at night when temperatures failed to fall below 80 degrees in apartments lacking air conditioning. 

(MORE: Excessive Heat Grips a Large Portion of the U.S.)

A contributing factor to the death toll was the lack of air conditioning in Parisian apartments. Few are equipped with air conditioners because the average summer temperature hovers right around 70 degrees and the French tend to disdain its use.

With temperatures exceeding 100 degrees in Paris that August, Bernard Mazeyrie of Paris' General Funeral Services told the Chicago Tribune that "particularly elderly people, people living in hotels and alone, were the victims, often of cardiac arrest." Four of every five elderly victims, he estimated, "died because of the heat."

City-Dwellers At Higher Risk

Concrete and asphalt heats up rapidly and releases heat more slowly at night, which can lead to a difference in temperatures in urban areas up to 22 degrees hotter than nearby rural areas.

Couple the urban heat island effect with people's fear of opening windows at night because of safety reasons, and you have homes that can become like deadly ovens. 

"Overnight temperatures which don't drop below 80 degrees are dangerous because those without air conditioning can't simply open their windows for relief at night. So, essentially, there's no break in the heat at night," said weather.com meteorologist Jonathan Erdman.

A Dangerous Combination

When high temperatures combined with high dew points remain elevated during the night, our bodies perspire to cool off. However, when the perspiration does not evaporate because of the high humidity, body temperature can rise quickly, leading to illnesses like heat exhaustion or heat stroke, according to National Geographic.

In addition, up to two liters of fluids can be lost at night through perspiration when temperatures remain above 85 degrees. The elderly are particularly sensitive to severe dehydration because they may be unaware that they have become dehydrated. They may also be taking medications that can exacerbate the problem. Children, too, can dehydrate rapidly.

High heat, high humidity and a lack of fluids can very rapidly lead to organ failure — sometimes within an hour. 

MORE ON WEATHER.COM: Deadly India Heat Wave