It’s not the heat; it’s the humidity: Moisture increases heat risk in urban climates

A day with an air temperature of 95 degrees and a relative humidity of 50% will “feel" like 107 degrees. The feel-like temperature is how hot or cold the air feels to the human body when factoring in the variables.

Residents of major cities know that when summer rolls around, the concrete and lack of vegetation can lead to temperatures well above normal, but researchers believe there is growing evidence that humidity plays a larger role in the increasingly stressful environment.

A new study by experts at the Yale School examined the urban heat effect and its correlation with humidity. The study determined that some efforts to reduce the heat, either through trees or vegetation, were essentially erased due to the air’s moisture causing feel-like temperatures to soar.

"Green vegetation can lower air temperature via water evaporation, but it can also increase heat burden because of air humidity. The question then is to what extent this humidifying effect erases the cooling benefit arising from temperature reduction," Xuhui Lee, a professor of meteorology and director of the study, said in a statement.

The effects of urban heat impact the majority of the world’s population, with what the school estimated to be some 4.3 billion people who live in an urban setting.


Researchers found the enhanced stress caused by humidity more dominant over equatorial regions and points southward than in more temperate climates.

"In dry, temperate, and boreal climates, urban residents are actually less heat-stressed than rural residents. But in the humid Global South, the urban heat island is dominant over the urban dry island, resulting in two to six extra dangerous heat stress days per summer," Lee stated.

The World Economic Forum estimated that by 2050, 80 percent of the world’s population will live in an urban setting and be at risk for increased impacts brought on by heat.

Heat waves are the greatest weather-related killer in the U.S., and many countries, with hundreds of fatalities reported every year.

According to the Centers for Disease Control and Prevention, extended exposure to warm air can cause heat exhaustion and even a stroke.

Humans and some animals perspire to keep cool, but when the air’s moisture content is high, the FOX Forecast Center says the rate of evaporation from the skin decreases due to higher humidity levels.

A day with an air temperature of 95 degrees and a relative humidity of 50% feels more like 107 degrees to the body due to the reduction of natural cooling.


In addition to warm air potentially being unhealthy, the summer heat can lead to irrational behavior and even violence.

Psychologists and the Department of Justice have found significant seasonal patterns with violent crimes increasing over June, July and August.

"We get uncomfortable before physically feeling the impacts. So we’re hot or dehydrated, and the combination of the distress is a function of the weather," Dr. Josh Klapow, a psychologist and professor at the University of Alabama, told FOX Weather.

Researchers hope further studies will serve as a guide to cities on how to mitigate stress and combat a warming climate.