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What do all the heat warnings, advisories, and watches mean?

The calculation of the heat index triggers all the warnings, watches and advisories. It's a measure of air temperature and relative humidity.

Media outlets around the state want you to know... IT'S HOT!!

They have several different ways of telling you it's hot.

There are heat advisories, warnings and watches, heat indices, and ozone action days.

But what do all these terms really mean?

Here's a breakdown of the terms you might be reading or hearing about.

The following terms are issued from NOAA's National Weather Service:

  • "Excessive Heat Watch" (hot weather could be coming)

The NWS issues an " excessive heat watch" when conditions are right for hot weather in the next 12 to 48 hours.

A Watch is used when the risk of a heat wave has increased, but its occurrence and timing is still uncertain. A Watch provides enough lead time so those who need to prepare can do so, such as cities who have excessive heat event mitigation plans.

  • "Excessive Heat Advisory" (hot weather is here or will be here soon, but it's not too serious)

The NWS says an "excessive heat advisory" when hot weather occurring, is imminent, or has a very high probability of occurring. An 'advisory' is for less serious conditions that cause significant discomfort or inconvenience and, if caution is not taken, could lead to a threat to life and/or property.

  • "Excessive Heat Warning" (hot weather is here or will be here soon, and it could be dangerous)

The NWS sends out "excessive heat warnings" when an excessive heat event is occurring, is imminent, or has a very high probability of occurring. The warning is used for conditions posing a threat to life or property.

  • Heat Index

The NWS "heat index" is how forecasters decide whether or not to issue watches, advisories, or warnings (see the chart above).
The heat index is how hot it really feels when taking humidity into consideration. So, the actual air temperature and relative humidity make up what's known as the heat index.

When it's 92 degrees F, and the humidity is at 60%, it really feels like it's 105 degrees F. The NWS might issue a warning under these conditions.

Ozone Action Days

Ozone Action Days are declared by meteorologists from the Michigan Department of Environmental Quality. They determine where conditions are right for the development of ground level ozone.

Ground-level ozone pollution is formed on hot, sunny days when emissions from cars and trucks, industrial smokestacks, and vapors from gas and other chemicals are cooked by the sun into "bad" ozone.

The EPA says ground-level ozone can cause a variety of health and environmental problems:

"... including chest pain, coughing, throat irritation, and congestion. It can worsen bronchitis, emphysema, and asthma. Ground-level ozone also can reduce lung function and inflame the linings of the lungs. Repeated exposure may permanently scar lung tissue. Ground-level ozone also damages vegetation and ecosystems. In the United States alone, ozone is responsible for an estimated $500 million in reduced crop production each year.

The general advice on hot days is to stay indoors in a cool place, drink plenty of fluids, and limit physical activity.

One is more affected by heat the older you are. From the National Weather Service:

Studies indicate that, other things being equal, the severity of heat disorders tend to increase with age. Conditions that cause heat cramps in a 17-year-old may result in heat exhaustion in someone 40, and heat stroke in a person over 60.

If you hear another term bandied about that isn't on this list, let me know in the comment section below!

Mark Brush was Michigan Radio’s Digital Media Director. He succumbed to a year-long battle with glioblastoma, an aggressive brain cancer, in March 2018. He was 49 years old.
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