Heat index warnings can save lives on dangerously hot days – if people understand what they mean

You’ve probably heard people say, “It’s not the heat, it’s the humidity.” There’s a lot of truth to that phrase, and it’s important to understand as summer temperatures rise.

Humidity doesn’t just make you feel sticky and uncomfortable, it also creates additional dangerous conditions on hot days. Together, too much heat and humidity can make you sick. And in severe cases, it can cause the body to shut down.

Meteorologists talk about the risk of heat and humidity using the heat index, but it can be confusing.

I am a risk communication researcher. Here’s what you need to know about the heat index and some better ways meteorologists can talk about the risks of extreme heat.

What is heat index and how is it measured?

The heat index is the combination of the actual air temperature and the relative humidity:

• Air temperature is how hot or cold the air is, which depends on factors such as the time of day, season of the year, and local weather conditions. It’s what your thermometer reads in degrees Celsius or Fahrenheit.
• Relative humidity compares how much water vapor is in the air to how much water vapor the air could hold at that temperature. It is expressed as a percentage.

The heat index tells you how it “feels” outside when you factor in the humidity. For example, if it’s 98 degrees Fahrenheit (36.7 Celsius) with 55% relative humidity, it might feel more like a 117 F (47.2 C) burn.

But there’s a catch: the heat index is measured in shaded conditions to prevent the sun’s angle from affecting its calculation. This means that if you are in direct sunlight, it will feel even hotter.

Apparent temperature, alerts and wet bulb

“Apparent temperature” is another term you may hear this summer.

The apparent temperature is the “felt” temperature. Not only temperature and humidity are taken into account, but also wind speed. This means it can tell us both the heat index and the wind chill – or the combination of temperature and wind speed. When conditions are humid, it feels warmer, and when it’s windy, it feels cooler.

We have found that apparent temperature is even less understood than heat index, possibly because the word apparent has various interpretations.

There are a few other ways you might hear meteorologists talk about heat.

The wet bulb temperature takes temperature, humidity, wind and sunlight into account. It is especially useful for those who spend time outdoors, such as workers and athletes, because it reflects conditions in direct sunlight.

HeatRisk is a new tool developed by the National Weather Service that uses colors and numbers to indicate heat risks for different groups. However, more research is needed to know whether this type of information helps people make decisions.

In many places, the National Weather Service is also issuing alerts such as excessive heat watches, advisories and warnings.

The risk is to get lost in translation

It is important to know about heat and humidity, but my colleagues and I have found that the term heat index is not well understood.

We recently held 16 focus groups across the United States, including hot dry areas like Phoenix and wetter areas like Houston. Many of those involved did not know what the heat index was. Some have confused it with the actual air temperature. Also, most did not understand what the alerts meant, how serious they were, or when they should protect themselves.

In our discussions with these groups, we found that meteorologists could understand risk more clearly if, instead of using terms like heat index, they focused on explaining what it feels like outside and why those conditions are dangerous.

Watches, warnings and advisories could be improved by telling people what temperatures to expect, when, and steps they can take to stay safe.

Climate change is exacerbating heat risks by making extreme heat more common, intense and long-lasting. This means clear communication is needed to help people understand their risks and how they can protect themselves.

What you can do to protect yourself

In both hot and humid conditions, extra precautions are needed to protect your health. When you’re hot, you sweat. When sweat evaporates, it helps cool the body. But the humidity prevents sweat from evaporating. If sweat cannot evaporate, the body has trouble lowering or regulating its temperature.

Although everyone is at risk of health problems in the heat, people over 65, pregnant women, infants and young children may have difficulty cooling their bodies or be at greater risk of dehydration. Certain health conditions or medications can also increase a person’s risk of heat illness, so it’s important to talk to your doctor about your risk.

Heat illnesses such as heat exhaustion and heat stroke can be prevented if you take the right precautions. The US Centers for Disease Control and Prevention focuses on staying cool, hydrated and informed.

• Stay calm: Use air conditioning in your home or spend time in air-conditioned spaces such as a shopping mall or public library. Limit or reschedule exercise and other outdoor plans that occur in the middle of the day when it’s hottest.
• Stay hydrated: Drink more water than you might otherwise, even if you’re not thirsty, so your body can regulate its temperature through sweating. But avoid sugary drinks, caffeine or alcoholic drinks, because they can cause you to become dehydrated.
• Stay informed: Know the signs of heat illness and the symptoms that may occur, such as dizziness, weakness, thirst, profuse sweating and nausea. Learn what to do and when to get help, because heat illness can be deadly.

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Mickey Olson is a Senior Research Fellow in Emergency and Risk Communication, University at Albany, State University of New York