Explaining the Heat Index: How Forecasters Determine How Hot You Feel

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Each summer, weather forecasters start tell their audiences how hot it is according to the thermometer and how much hotter they feel, based on the heat index.

Society has measured temperature using Fahrenheit and Celsius since at least the 18th Century. The heat index has been around just a fraction of that time.

How the National Weather Service Calculates the Heat Index

Some question how objective a measurement of how you feel can really be.

But at the National Weather Service, they’ve got a formula to measure just that – and it's a very complicated one.

One of the meteorologists there, Eli Jacks, said the heat index concept was developed in the late 1970s, and it is actually an attempt to measure the rate at which human sweat evaporates into the air. That’s because when sweat evaporates more slowly, you feel warmer.

And there are plenty of values taken into account besides the air temperature and humidity. “There is actually an assumption that a person is wearing long trousers, and a short sleeved shirt, and that they’re walking at 3.1 miles per hour, which causes a certain wind,” Jacks said.

The actual equation is pretty complex. It’s long, and involves various combinations of temperature, humidity and those other variables. With a couple of multiplications, additions and squaring, you end up with the heat index.

For example, on July 22, the warmest day of the year so far, the temperature hit 104 degrees in Central Park, while the heat index rose to around 115.


This chart shows how the National Weather Forecast determines heat alerts, based on the heat index.



Another Measure Aimed at Predicting How You'll Feel: The AccuWeather RealFeel

But the heat index isn’t the only game in town.

The commercial service AccuWeather has a measurement called the RealFeel, which is akin to the heat index in the summer and the wind chill factor in the winter, but uses a different formula.

AccuWeather Forecaster Brett Anderson said it’s also calculated on a daily basis.

“Some of those factors we look at: the sun angle during the day and also during the season. That’s important. Rate of evaporation. We also look at the wind speed throughout the day. The temperature of course, and the humidity. And then if there’s a cloud cover or not. Obviously if there are more clouds, the AccuWeather RealFeel temperature will certainly be less than if it's a sunnier day," he said.

If you think that’s complex, an independent meteorologist, Steven DiMartino, who runs the Web site NY NJ PA Weather, said factors that have changed since the 1970s could also influence attempts to calculate how hot someone feels.

DiMartino said those factors could include the average person’s weight, amount of exercise and even how much sodium he or she ingests.

“I think that it does need to be updated to deal with today’s physical standards, as far as the average human being, or American, depending on where you want to use it in the world," he said. But he added that it's "still an excellent way" to inform the public, and let people know about the dangers of excessive heat.

How Accurate is the Heat Index? And Why is it Used?

Meteorologists admit that not everybody will feel exactly the same under the same weather conditions. AccuWeather's Anderson said whether a person has light or dark skin, is heavy or thin and what weather they’re generally used to, can also have an affect.

But forecasters still say the heat index or RealFeel can be very important tools.

The heat index, for example, helps triggers weather alerts from the National Weather Service, including advisories, watches and warnings.

There is actually a chart that places high temperatures, high humidity and the heat index into certain ranges that line up with precautions to take under extreme weather conditions. Those alerts mean that unless people take extra care, they can suffer from very real physical conditions -- from minor fatigue and sunburn, to heat exhaustion and even heat stroke.

“The key point is that, within a good approximation, the heat index provides for a given temperature and humidity in the air a very good sense about how much more the body is perceiving it to be, based on its ability to cool itself,” Jacks said.

So if hearing about a heat index of 115 degrees sounds a bit alarmist, well, that’s sort of the point. It’s meant to get people’s attention, and influence them to take extra precautions.