When the sun is bright in the sky, it's like a magnet drawing us outdoors. But those inviting rays can come with some dangerous temperatures. Sure, it's supposed to be hot in summer, but extreme heat and heat waves aren't just uncomfortable; they can be life-threatening.

Before you head outside when the temperatures start to soar, here's a look at summer weather and how to stay safe when the mercury skyrockets.

Defining heat

You may hear meteorologists talking about "extreme heat." Although the term is used loosely to refer to high temperatures, in most of the United States, extreme heat is at least two to three days of high heat and humidity with temperatures above 90 degrees, according to Ready.gov. The University of Washington defines extreme heat slightly differently — as a period when temperatures hover 10 degrees or more above the average high for the region and stay that way for several weeks.

There's also no agreed-upon definition of a heat wave. The World Meteorological Organization suggests a heat wave is when the daily maximum temperature for more than five consecutive days exceeds the average maximum temperature by 9 degrees F (5 degrees C). The American Meteorological Society defines a heat wave as a period of abnormally and uncomfortably hot and humid weather. The period should last at least one day, but could last several days, or several weeks.

When discussing weather, people often refer to the "heat index." That's what the temperature feels like to the human body when you combine the outside air temperature with relative humidity. The National Weather Service issues a heat advisory when the heat index is expected to reach 105 to 109 F (40 to 42 C) (east of the Blue Ridge) or 100 to 104 degrees (37 to 40 C) (west of the Blue Ridge) in the next 12 to 24 hours.

Dangerous weather

Each year, more than 600 people in the U.S. are killed by extreme heat, according to the Centers for Disease Control and Prevention. High temperatures kill more people in the U.S. than hurricanes, lightning, tornadoes, earthquakes and floods combined.

If you're exposed to extreme heat for too long, your body slowly starts to shut down. You can lose your natural cooling system as you lose your ability to sweat.

You may even experience lower cognitive function. A new study shows that exposure to high temperatures can result in slower response times. A team followed a group of college students in Boston — one set who lives in air-conditioned dorms and the other set lives in dorms without air conditioning. The students performed several tests, and researchers found those who didn't have air-conditioning in their living quarters had slower response times to answering questions and also missed more questions. The largest gap in cognitive function was when the students were outside and then went inside to "cool down."

Beyond the mental ramifications, long-term exposure to extreme heat can also lead to severe physical side effects.

In the early stages of heat exhaustion, you may experience nausea, lightheadedness and you may be tired and weak, reports WebMD. Left untreated, this can turn into heat stroke, which can be life-threatening. Symptoms include confusion, agitation, warm, dry skin and uncontrolled body temperature.

How to stay heat smart

woman in shade wearing hat and with a glass of water Hot weather help includes shade, lots of water and a hat. (Photo: Gajus/Shutterstock)

Just because it's hot doesn't mean you have to stay inside, but you should take precautions to stay safe when temperatures are high. Know the symptoms of heat-related illness and take these steps:

Hydrate. Drink plenty of fluids, even if you're not thirsty. Avoid drinks with caffeine, alcohol or lots of sugar.

Dress. Wear loose-fitting, lightweight, light-colored clothing. Consider wearing cotton, which absorbs extra moisture and helps your body cool down.

Rest. Limit outdoor activity to morning and evening hours when it's cooler. Rest often in shady areas. Don't overexert yourself. Your body will tell you when it's time to take a break, so listen.

Slather. Wear sunscreen, sunglasses and a loose-fitting hat. Sunburn can impact your body's ability to cool off and can contribute to dehydration.

Eat light. Eat small, light meals and eat more often. Heavy meals add more heat as your body works harder to digest them.

Friendship. Use the buddy system when working or exercising in the heat. Don't leave pets outside or in cars. Check on people you know who are sick or elderly; they are most likely to have problems from the heat.

Get wet. If you know you're going to be outside for a while, soak your shirt, hat or a towel in cold water and use it to keep cool outside. This works whether you're gardening or hiking. Just use the hose or a nearby creek to keep wet.

And when the temperatures are really high, try to stay indoors and enjoy some air-conditioning, especially from 10 a.m. to 4 p.m. when the temperatures are hottest. Electric fans can provide some relief, but when temperatures hit the high 90s, they won't prevent heat-related issues, according to the CDC. Take cool showers or baths to cool off instead.

Editor's note: This article has been updated since it was originally published in July 2018.

Mary Jo DiLonardo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.