Use the Heat Index Chart for Hot Weather Exercise

Hot Weather Walking
Hot Weather Walking. © Dirima /

Before you exercise in hot weather, check the heat index. The thermometer doesn't tell the whole story when it comes to deciding whether it is too hot to exercise. The heat index is based on both relative humidity and the air temperature. It a better indicator of the apparent temperature that your body feels and your risks of heat illness in hot weather.

You can view the heat index chart at the National Weather Service. You will also see the heat index listed in many weather apps and weather reports.

The heat index is calculated for shade. If you will be exercising in direct sunlight, without shade, the heat index may be as much as 15 degrees higher, according to the Occupational Safety and Health Administration guidelines for outdoor work.

When to Cancel Outdoor Exercise Based on the Heat Index

A high heat index can warn you to restrict activities to prevent heat sickness. The American College of Sports Medicine (ACSM) says that the risk of exertional heat sickness is raised when the heat index is above 82 F (28 C) for people exercising for over an hour.

  • At a heat index of 82 F, they recommend canceling competitions and continuous activity.
  • At the 86 to 90 F range, the ACSM recommends canceling athletic activities for less-fit and non-acclimated people and limiting the duration and intensity for fit and acclimated people.
  • A value over 90 F should prompt cancellation of athletic activities for everyone, and is an indicator you should plan a different workout rather than a long walk or run outdoors.

If you are trying to decide whether to walk or run outdoors, check the heat index to see whether it will be below 90 at the time you will be doing it. If your route doesn't have shade, factor in another 15 degrees to the heat index.

Physical Risk Factors for Heat Sickness

The ACSM notes that these factors increase the risk of heat sickness. If they apply to you, take extra precautions when the heat index is high.

  • Obesity
  • Low physical fitness
  • Lack of acclimatization to heat
  • Dehydration
  • Previous history of exertional heat sickness
  • Sleep deprivation
  • Sunburn
  • Diarrhea, illness, certain medications

You can acclimatize yourself to hotter conditions by 10 to 14 days of exposure to warmer conditions for 60 to 90 minutes per day. By doing so, your body adapts and begins to sweat and try to cool itself sooner during an exercise session.

Environmental Heat Factors

There are several factors that affect how hot you feel when exercising outdoors. These all affect the way your body is heated and how it can get rid of excess heat.

  • Humidity and dew point are measures of the amount of moisture in the air. You perspire in order to get the benefits of the cooling that comes when the sweat evaporates. This is the primary way your body keeps from overheating when it is hot outside or with exercise (or both). If the air is already saturated with moisture, your perspiration can't evaporate as fast to cool you. If the humidity is low, sweating works better to cool you.
  • Heating from the sun: Sunlight warms you with radiation. Its electromagnetic waves directly heat your body and other surfaces without actually touching them.
  • Heating from the wind: Air can carry heat with it from one object to another, and you will feel the heat transfer when the air temperature is at 72 F or above. Below that temperature, it feels like a cool breeze, while above that temperature the wind contributes to heating. This is known as convection.
  • Heating from the pavement: When you touch something hot, the heat is transferred directly to your skin. This is known as conduction and it happens with hot pavement or asphalt heating your feet through your shoes.
  • Temperature gradient: The temperature difference between your body and the outdoor factors will determine how much and how fast you heat up or cool down.

You are more at risk of heat effects with high temperatures, high humidity, no wind, and being exposed to the rays of the sun.

A Word From Verywell

No workout is worth risking heat sickness. Your body won't be able to benefit from the exercise. A hot day is a good day to take your exercise indoors where there are shade and air conditioning. You can put in the hard outdoor workout on a better day.

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