The deadly combination of heat and humidity

Desdemona Despair

By Robert Kopp, Jonathan Buzan, and Matthew Huber
6, June 2015

(The New York Times) – The most deadly weather-related disasters aren’t necessarily caused by floods, droughts or hurricanes. They can be caused by heat waves, like the sweltering blanket that’s taken over 2,500 lives in India in recent weeks.

Temperatures broke 118 degrees in parts of the country. The death toll is still being tallied, and many heat-related deaths will be recognized only after the fact. Yet it’s already the deadliest heat wave to hit India since at least 1998 and, by some accounts, the fourth- or fifth-deadliest worldwide since 1900.

These heat waves will only become more common as the planet continues to warm.

They don’t just affect tropical, developing countries; they’re a threat throughout the world. The July 1995 heat wave in the Midwest caused over 700 deaths in Chicago. The August 2003 heat wave in western Europe led to about 45,000 deaths. The July-August 2010 heat wave in western Russia killed about 54,000 people.

But as anyone who’s spent a summer in the eastern United States knows, it’s not just the heat; it’s also the humidity. Together, they can be lethal, even if the heat doesn’t seem quite so extreme.

Scientists measure the combination using a metric known as wet-bulb temperature. It’s called that because it can be measured with a thermometer wrapped in a wet cloth, distinguishing it from the commonly reported dry-bulb temperature, measured in open air. Wet-bulb temperature can also be calculated from relative humidity, surface pressure and air temperature.

It’s essentially a measure of how well you can cool your skin by sweating, which is how humans stay alive in the worst heat. But high humidity can defeat that cooling system; it makes the heat that much more dangerous.

The wet-bulb temperature is not typically reported. While dangerous levels depend on a person’s activity level and clothing, wet-bulb temperature offers a stark measure of risk in a warming world that will experience more extreme combinations of both heat and humidity.

Temperature and wet-bulb temperature are not in a one-to-one relationship; both higher temperatures and higher humidities increase wet-bulb temperature. For instance, during the Chicago heat wave, on July 13, 1995, the maximum wet-bulb temperature of 85 degrees occurred at noon when the temperature was 99 degrees. But when it hit 106 degrees at 5 p.m., the wet-bulb temperature was 83 degrees. The former was more dangerous.

A human’s core temperature is about 98.6 degrees, but the skin temperature of the trunk is about 4 to 9 degrees colder, depending on how warm it is and how active a person is. But sweating, which helps keep the core body temperature constant, becomes increasingly ineffective in increasingly humid air, and it can never cool the skin to below the wet-bulb temperature.

A person who is physically active at a wet-bulb temperature of 80 degrees will have trouble maintaining a constant core temperature and risks overheating. A sedentary person who is naked and in the shade will run into the same problem at a wet-bulb temperature of 92 degrees. A wet-bulb temperature of 95 degrees is lethal after about six hours.

In the Indian state of Andhra Pradesh, the highest wet-bulb temperatures of the latest heat wave have peaked around 86 degrees — levels approaching the worst of the 1995 Midwest heat wave, which set records in the United States for humid heat.

Heat waves are the natural disasters easiest to tie to climate change. Statistical analyses and climate modeling indicate that the 2010 Russian heat wave was about five times more likely to have occurred in 2010 than it would have been in the cooler 1960s. An analysis conducted after the 2003 European heat wave concluded that it was twice as likely as it would have been before the Industrial Revolution. A recent study in the journal Nature Climate Change found that the 1.5 degrees of global warming since the start of the Industrial Revolution had quadrupled the probability of moderate heat extremes. [more]

The Deadly Combination of Heat and Humidity