What is Extreme Heat?

Extreme heat is generally defined as a prolonged period of excessively hot weather, which may be accompanied by high humidity. In Massachusetts, a “heat wave” is usually defined as a period of three or more consecutive days above 90 °F.

Why Prepare?

Extreme heat can be dangerous and even life-threatening if proper precautions are not taken. In extreme heat and high humidity, the body must work extra hard to maintain a normal temperature.

Most heat-related illnesses occur because the victim has been over-exposed to heat or has over-exercised for his or her age and physical condition. Although anyone can suffer from a heat-related illness, some people are at greater risk than others such as older adults, young children and those who are sick or overweight. To reduce the risks of extreme heat conditions, take the proper safety precautions to protect yourself and your family.


The National Weather Service issues heat watches and warnings to alert the public of potential extreme temperatures. Learn the difference between watch and warning so you know what to do to stay safe:

Excessive Heat Watch

Conditions are favorable for an excessive heat warning in the next 24-72 hours.

Heat Advisory

Daytime heat indices of 100ºF–104ºF for two or more hours. The heat index is a measure of how hot it feels when relative humidity is factored in with air temperature.

Excessive Heat Warning

Daytime heat indices of greater than or equal to 105°F for two or more hours.

Learn more about the Heat Index and how it is calculated.

Before Extreme Heat

  • Be Informed by receiving alerts, warnings, and public safety information before, during, and after emergencies. Download the Massachusetts Alerts app.
  • Create and review your family emergency plan.
  • Assemble an emergency kit.
  • Prepare your home for possible emergencies.
    • Install air conditioners snugly, insulating if necessary.
    • Cover windows that receive morning or afternoon sun. Outdoor awnings or louvers can reduce the heat that enters a home by up to 80 percent.

During Extreme Heat

  • Never leave children or pets alone in a closed vehicle. Even with the windows cracked open, interior temperatures can rise almost 20°F within 10 minutes.
  • Slow down and avoid strenuous activity.
  • Wear lightweight, loose-fitting, light-colored clothing. Light colors reflect heat and sunlight, and help maintain normal body temperature.
  • Drink plenty of water — even if you are not thirsty. Avoid alcoholic beverages and liquids high in sugar or caffeine. If your doctor generally limits the amount of fluid you drink, ask how much you should drink during hot weather.
  • Eat well-balanced, light, regular meals.
  • Stay indoors as much as possible and limit exposure to the sun. Do not leave pets outside for extended periods of time.
  • If you must be outdoors, limit your outdoor activity to the morning and evening hours. Try to rest often in shady areas so your body temperature will have a chance to recover. Use sunscreen with a high SPF and wear a wide-brimmed hat.
  • If you do not have air conditioning, stay on your lowest floor, out of the sun. Use fans to stay cool and avoid using your stove and oven. Consider spending time in air-conditioned public spaces, such as schools, libraries, theaters, and other community facilities.
  • Check with your local authorities or Call 2-1-1 to find locations of cooling centers or shelters near you.
  • If there are power outages during warm weather, you may need to take additional precautions or go to a cooling center or emergency shelter to stay cool.
  • Know the symptoms of and watch out for heat-related illnesses. Call 9-1-1 to report emergencies.
  • Be a good neighbor. Check on family, friends, and neighbors, especially the elderly, those who live alone, those with medical conditions, those who may need additional assistance, and those who may not have air conditioning.