Humid Heat: Hidden but Hazardous

KEY CONCEPTS

• Heat is dangerous. When extreme heat is combined with high humidity, the health risks multiply.

• Climate Central analysis explores changes in equivalent temperature—a key metric of humid heat in our warming climate.

• Since 1950, summer equivalent temperature has increased three times more than summer air temperature on average across the contiguous U.S.

• Humid heat metrics—and equivalent temperature in particular—give us a more comprehensive understanding of the changing health risks and weather extremes we face with climate change.

Dangerous heat: missing humidity misses the point

When we talk about climate change, we most often talk about changes in air temperature—and with good reason. The rapid rise in global and U.S. average air temperatures is a direct result of climate change. Likewise for recent increases in extreme heat—the deadliest form of weather. 

But climate change is shifting every aspect of the planet’s energy balance—of which air temperature is just one (very important) part. In many parts of the country and the globe, dangerous heat often occurs along with high humidity—and the pair of conditions multiplies the health risks.

The combination of high heat and humidity can compromise the human body’s main cooling mechanism: sweating. The evaporation of sweat from skin cools our bodies, but higher humidity levels limit evaporative cooling. As a result, we can suffer heat stress and illness, and the consequences can even be fatal. We’ve seen these dangerous effects in rates of heat-related illness across the country this summer. 

When it comes to heat, focusing on air temperature alone misses the changes in humidity—and underestimates how our warming planet affects our health and weather extremes. This week, we explore changes in a key metric of humid heat across the U.S.

What is humid heat?

Humid heat—the combination of high temperatures and humidity—is deceptively simple. But scientists and meteorologists use several different metrics for measuring both humidity and humid heat.

Humidity is simply a measure of how much water vapor there is in the air. There are two primary definitions of humidity:

• Specific humidity is a measure of moisture content—how much water vapor there is relative to the total moist air mass.

• Relative humidity is a measure of how saturated the air is—how much water vapor the air contains compared to the maximum it could contain at a given temperature. 

Humid heat metrics vary in several ways, including which humidity metric they use. Common humid heat metrics like the heat index and wet bulb globe temperature are both based on relative humidity.

But there are compelling reasons to consider another humid heat metric—called equivalent temperature—that uses specific humidity instead.

What is equivalent temperature?

Equivalent temperature is a humid heat metric based on air temperature and specific humidity (the amount of water vapor in the air). Representing humid heat using a metric that scales with specific humidity (like equivalent temperature) more closely reflects the heat-related health risks we face on our warming planet. This has to do with the pace of warming and its impacts on humidity.

Global temperatures are rising rapidly. And warmer air can hold more moisture. These two facts partly explain why: 

• the amount of water vapor in the air (the specific humidity) has increased globally since the 1970s, 

• while the air saturation levels (the relative humidity) have decreased—outpaced by rapidly warming air’s increasing capacity to hold moisture.  

The higher sensitivity of specific humidity to recent warming is partly why recent studies  (Song et al., 2022; Matthews et al., 2022) encourage the use of equivalent temperature. Compared to other humid heat metrics, equivalent temperature is a more sensitive indicator of human heat stress—especially at higher air temperatures that are likely to occur more often with climate change.  

Increasing U.S. equivalent temperatures

New analysis from Climate Central shows how summer equivalent temperatures have changed across the contiguous U.S. since 1950.

During the summer, when we experience the highest air temperatures across the U.S., much of the country has also  experienced increasing humid heat, as represented by equivalent temperature. Since 1950, summer equivalent temperature has increased three times more than summer air temperature on average across the contiguous U.S. 

Humid heat trends reveal hotspots for warming-related health risks that could go unnoticed if we were to look only at changes in air temperature.

For example, the North Central and Midwest regions have seen relatively modest increases in summer air temperatures since 1950 compared with other U.S. regions. But based on equivalent temperature, those regions have experienced an exceptional rise in humid heat and its associated risks. 

Parts of South Dakota, Nebraska, Kansas, Missouri, Iowa, and Oklahoma have seen an additional 5-6°F of summer equivalent temperature increase since 1950, compared to the change in summer air temperature. That’s additional heat stress that our bodies feel, but that isn’t reflected in air temperature alone.

Metrics that matter

These increases in equivalent temperature mean greater heat-related health risks for vulnerable populations including children, older adults, athletes, outdoor workers, and communities of color.

Recent trends in equivalent temperature are also strongly correlated with the changing frequency and intensity of weather extremes including extreme heat waves and heavy rainfall events (especially in the tropics).

These are among the reasons that humid heat metrics—and equivalent temperature in particular—gives us a more comprehensive understanding of how our warming planet affects health risks and changing weather extremes.

POTENTIAL LOCAL STORY ANGLES

How are heat and humidity changing in your area?
The national weather service forecasts wet bulb globe temperature values by region. For long-term projections, check out this interactive heat index map from the Union of Concerned Scientists.

How is humid heat affecting health and who is most vulnerable?
The 2018 National Climate Assessment details how the rise of extreme heat is already impacting health. In addition the CDC maps heat-related illnesses in its heat and health tracker. To identify the most vulnerable counties check out this extreme heat vulnerability mapping tool that combines NOAA projected heat events and CDC's Social Vulnerability Index (SVI). A 2019 report from Climate Central explores the impacts of humid heat extremes on outdoor sports. 

How to prepare and respond to humid heat?
The National Integrated Heat Health Information System advises on how to plan & prepare for extreme heat. These CDC infographics detail how to respond (both in English and Spanish).

LOCAL EXPERTS 

The SciLine service, 500 Women Scientists or the press offices of local universities may be able to connect you with local scientists who have expertise on humid heat and climate change. The American Association of State Climatologists is a professional scientific organization composed of all state climatologists. 

NATIONAL EXPERTS 

Radley Horton, PhD
Lamont Research Professor
Columbia University Earth Institute
Contact: Radley.m.horton@gmail.com
Related expertise: Climate change and extreme weather; resilience to humid heat

Colin Raymond, PhD
Assistant Research Scientist
University of California Los Angeles
Contact: csraymond@ucla.edu
Related expertise: Extreme heat, compound extreme events

Gredia Huerta-Montañez, MD, FAAP
Senior Principal Research Scientist, Northeastern University
President, Puerto Rico Chapter of American Academy of Pediatrics
Contact: grediamd@gmail.com
Related expertise: Climate change impacts on pediatric health
*Available for interviews in Spanish and English

Perry E Sheffield, MD, MPH
Associate Professor
Icahn School of Medicine at Mount Sinai
Contact: perry.sheffield@mssm.edu
Related expertise: Health effects of climate change; children's heat vulnerability

Amruta Nori-Sarma, PhD
Assistant Professor
Boston University School of Public Health
Contact: anorisar@bu.edu
Related expertise: Health impacts of climate change

METHODOLOGY

Average annual summer (June, July, and August) dry-bulb and equivalent temperatures were calculated from 1950-2021 using monthly data from the ERA5 reanalysis (Hersbach et al., 2020). Equivalent temperatures were calculated using equations from Raymond et al. 2021.