What is Heat Index chart and how to use it for your outdoor activities

What is Heat Index chart and how to use it for your outdoor activities


For sports and outdoor activities, we usually consider the Heat Index in order to dress correctly for the different weather. In this article, we’ll analyze why the temperature on the thermometer often differs from your feelings and how to read the Heat Index chart in order to stay comfortable outdoors in any weather and avoid heatstroke — for example, during a long bike ride, mountain hike or a boat fishing trip in the sea.

What is the Heat Index?

The Heat Index is an index that shows how humidity affects the feels like temperature in hot weather.

The higher the humidity, the higher the perceived temperature appears than the actual temperature. And the more difficult it is to do outdoor activities.

It applies when the feels like temperature is higher than the actual air temperature, that is, in the warm season.

The Heat Index is always higher than the actual air temperature.

Heat Index is a kind of the opposite of Wind Chill chart, which is used when the temperature is affected by the air humidity and wind speed.

How Heat Index is calculated? Is there any formula?

There is no single way to calculate the Heat Index, it can be counted differently in different countries and meteorological services.

The formulas for calculating this indicator are quite complicated. For example, the US National Weather Service of the NOAA uses this formula:

Photo: Heat Index formula / NOAA

How to read the Heat Index chart?

There are visual tables (charts) that can be used to calculate the Heat Index value.

The Heat Index chart displays air temperature and relative humidity along the axes. At their intersection, there will be a "feel of temperature" or the Heat Index value.

The background of the table is covered in different colors from yellow to red — they represent the risk of heat disorder. Yellow means to be careful, and red means the extreme danger of being in such conditions.

Heat index values were created for shady and light wind conditions. But remember, full sunshine can increase heat index values by up to 15 °F.

Heat Index Chart by the US National Weather Service (US NWS). Valerya Milovanova / Windy.app

The following are the perceived temperature ranges at which physical activity can cause problems:

  • Caution: 80...89 °F (27...32 °С) — fatigue is possible;
  • Extreme caution: 89...102 °F (32...39 °С) — heat fatigue, convulsions, and heat stroke are possible;
  • Danger: 102...123 °F (39...51 °С) — heat fatigue, convulsions and heat stroke are likely;
  • Extreme danger: 123+ °F (51+ °C) — most likely there will be a heat stroke.

For example, with an air temperature of 80 degrees Fahrenheit and relative humidity of 40%, the Heat Index is 80 degrees too and your sensations correspond to the real temperature.

But with an air temperature of 88 degrees Fahrenheit and relative humidity of 75%, you will feel the temperature as 103 degrees. And a long stay at such a temperature, especially for an unprepared organism, is very harmful.

Why is it important to know the Heat Index for outdoor activities?

High air temperatures are not a reason to stop exercising outdoors. Especially if you are healthy and in good physical shape. In fact, exercising in the heat can increase endurance. But when doing outdoor activities, it’s important to consider the Heat Index to properly plan your workout for the weather and not get heatstroke.

For example, marathon are some of the most grueling disciplines in sports and outdoor activities. According to research, the optimal temperature for marathons is between 4 °C and 10 °C. When it rises, the risk of health problems for athletes increases, as well as their productivity decreases.

  • Therefore, back in December 2019, the International Olympic Committee planned to transfer the competitions in these sports at the 2020 Olympics (which didn’t take place last year but will in 2021) from Tokyo to Sapporo or to hold them at night due to the predicted abnormal heat.
  • The Ironman Triathlon Championship — a long-distance race, consisting of a 2.4-mile swim, a 112-mile bicycle ride, and a marathon 26.22-mile run — was held in Hawaii in 2016 with an average temperature of 29 °C and relative humidity of 80–90%. These weather conditions make the competition really tough and exhausting.
  • In 2013, at the Tel Aviv Marathon, Israel, due to the intense heat, the organizers shortened the marathon distance and postponed the start earlier in the morning so that the last runner could cross the finish line by 10 a.m.

Photo: Christian-chrome / Unsplash

How to avoid heatstroke using Heat Index Chart?

Acclimate to the heat

If the heat is sudden or you are traveling to another region for biking, hiking, or trail running competitions, you should acclimate to the heat — this usually takes 7—10 days. At this time, it’s better to reduce the weekly volume of training in half — in the short term, this won’t affect your results, but it’ll help you gradually get used to the conditions or wait out a hot period without heat stroke.

Set up workout time

If you do sports like hiking, trail running, cycling, or any kinds of bike riding, and many others, try doing sports not for intensity, but for the duration. For example, if you need to run 10 kilometers, take the time in which you usually cope with such a distance, and exercise during this period, without monitoring the speed and distance — most likely, you will run less, but the body will not be depleted.

Wear proper clothes

When going on a hike, wear light-colored clothes that are well ventilated and don’t restrict air access to your skin. Also, minimize the weight of the backpack or take frequent breaks on the route.

Drink water

In particular, dehydration of the body is one of the main health problems (and even mortality) factors with a high Heat Index.

Drink well ahead of time, because thirst occurs when the body is already dehydrated. Therefore, drink a glass of water before training, and while hiking, running, and cycling, take a couple of sips every 20—30 minutes. If you exercise for more than two hours and consume more liters per hour, replace the water with sports drinks to avoid sodium deficiency.

Where to get a Heat Index chart?

Windy.app has its own feels like temperature formula too, using both indicators depending on the weather conditions.

We have a combination of Heat Index and Wind Chill Factor: at temp <50 °F (10 °C) we use Wind Chill Factor, and at temp >80 °F (26.6 °C) — Heat Index. And for the average values, we consider wind, also.

To get feels like temperature in Windy.app:

1. Open your favorite or the nearest sport to your current location.

2. Choose Lite Weather profile by an icon of the umbrella right to the weather forecast models on the spot screen.

3. Get feels like temperature for today:



To change °F for °C or vice-versa, go to Home screen > Main Menu Bar > Setting > Units > Temperature.


Text: Natalia Kirasheva.

Cover photo: Jonny-kennaugh / Unsplash

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