Temperatures are soaring across the eastern United States for the third day in a row, and officials in cities from Boston to Washington, D.C., are trying to keep residents cool during outdoor Fourth of July celebrations. The combination of heat and humidity has caused the heat index to soar, with dozens of records being set each day.
But “How hot is it?” It’s not as simple a question as it might seem. And since heat is the number one cause of weather-related death in the United States, understanding different heat indicators can save lives.
“The problem with temperature alone is that there are other factors that cause heat stress,” said Andrew Grunstein, a researcher at the University of Georgia who specializes in climate and health. Relative humidity, dew point, sun angle, and wind speed all play a role.
Over the years, researchers have devised various ways to quantify what the temperature is and how extreme heat affects the human body. Each has advantages and disadvantages, and some are becoming more important as the climate warms.
🌡️ Get your local temperature from the CNN Weather app
The first (and probably the most familiar to people) is the heat index. The heat index is calculated by combining air temperature and relative humidity, and approximates the “feel-like temperature.” That is, the higher the heat index, the more oppressive it feels outside, and the more dangerous the heat is for human health.
As with any heat index, calculating the heat index requires making some assumptions about people and their surroundings.
“The heat index is based on a physiological model of human thermoregulation, which assumes that healthy adults can walk in the shade and keep their skin moist with sweat,” said David Romps, a climate scientist at the University of California, Berkeley, who has studied the heat index in detail.
“For people whose ability to regulate body temperature is impaired for some reason, who walk in direct sunlight, or who exercise more vigorously, the underlying model will be less representative,” he said.
Another issue with the heat index concerns how it is calculated. The system implemented by the National Weather Service is mathematically unstable when both temperature and humidity are particularly high. In such situations, heat indexes can underestimate actual conditions by up to 20 degrees Fahrenheit, Romps said.
Heat indexes in an ongoing heat wave across the eastern United States reached 110 degrees or higher in some large cities in the Mid-Atlantic region and parts of the Midwest earlier this week. However, at the same time, these may be underestimates because actual temperatures in these same locations were in the low triple digits and humidity was high.
The other two indicators focus more on the physiological effects of extreme heat on the human body. First, there is wet bulb temperature. This is the lowest temperature at which air is cooled by water evaporation. This temperature changes as the air gets hotter and the humidity changes. As humidity increases, wet bulb temperature increases, but as humidity increases, wet bulb temperature decreases.
Wet bulb temperature measures how effectively your body cools itself through sweat. The higher the wet bulb temperature, the harder it is to be comfortable. Wet bulb temperature is usually lower than the actual air temperature because it is a measure of evaporative cooling. There is a theoretical upper limit to the wet bulb temperature at which humans can survive, fixed at approximately 95 degrees.
However, recent research has shown that the human body does not cool down as easily as the theoretical maximum would assume. In fact, people become more susceptible to heat-related illnesses starting at about 86 degrees. Some experts place the survival limit at closer to 91 degrees, depending on the exact circumstances of heat exposure.
⛅ Get CNN Weather in your inbox
The forecast is just beginning. We bring you expert coverage and the stories behind the weather. So you always know more than just the numbers. Sign up for our newsletter
Finally, there is the wet bulb Earth temperature. It incorporates far more factors than other metrics. This is particularly useful for assessing occupational accidents for outdoor workers and safety for athletes participating in outdoor sports.
Wet bulb bulb temperature measures heat stress in direct sunlight, taking into account temperature, humidity, wind speed, and solar radiation. Some experts believe this is the most accurate measure of heat stress. However, this indicator is rarely used by mainstream weather maps or television meteorologists. They prefer the heat index, which has long reigned supreme and is easily understood by the general public.
This is thought to be because even if the temperature is high, the temperature of the wet bulb is often lower than the air temperature due to factors such as humidity, cloud cover, and wind speed. This can be confusing to general viewers, especially those who are used to watching the Heat Index instead.
According to Grundstein, the transmission of thermal measurements is almost as important as the measurement itself. “They all have pluses and minuses,” he said. “And ultimately what we’re trying to get out of these heat metrics is to understand how much heat stress people are experiencing and whether there are any dangerous conditions.”
He noted that there is no perfect heat index, or one that he prefers. Instead, he said, “I think the right audience needs the right metrics. I think that’s what’s really important.”
Climate scientists tend to prefer to use wet-bulb temperature and wet-bulb temperature when studying when and where heat stress may have exceeded, or may soon exceed, human survival limits at certain times of the year.
Research shows that this is already happening in parts of South Asia and the Middle East, where continued fossil fuel emissions will greatly expand the areas where people will have a hard time surviving if they spend hours outside during the day.
“Temperatures are irreversibly rising due to the burning of fossil fuels, and we are not far from global temperatures where we are seeing unsustainable conditions on Earth,” Romps said.
