5 Things You Should Know To Stay Safe During A Heat Wave

Heat waves may not make for the flashiest weather headlines but they cause widespread, life-threatening impacts to the U.S. each year.

So, what exactly is a heat wave?

A heat wave is a period of excessively hot weather, generally lasting two or more days. But the key is that the temperatures are being compared to historical averages, which can mean different conditions depending on the region. For example, what may be considered a heat wave in Boston would not be considered one in Death Valley.

But, regardless of where you live, heat can be incredibly impactful.

Here are five things you should know to help keep you and your family safe and informed during a heat wave:

1. Heat Is The Number One Weather-Related Killer

While your guess may have been hurricanes, tornadoes or even flooding — more people die from heat, on average, than any other weather hazard in the U.S., according to NOAA.

Extreme heat — especially for several days, as is the case with heat waves — can put stress on the human body, which can lead to various heat-related illnesses. Everyone is sensitive to the heat, but the elderly and children are particularly vulnerable.

The first signs of heat-related illnesses are increased sweating and muscle cramps. Other signs include confusion, dizziness, rash, nausea and fatigue.

Over time, heat exhaustion can set in, and if not treated, heat stroke can develop which can be fatal.

2. Knowing The Heat Index Is Key

Temperature alone doesn't tell the whole story.

We all know how a hot, muggy day feels far more oppressive than a hot, dry one. The reason: humidity.

The added moisture in the air makes it harder for sweat to evaporate off your skin — the body's natural cooling mechanism — therefore, making you feel hotter. Thus, the heat index was born.

The heat index, or the "feels like temperature," factors in both the temperature and the relative humidity to give a more accurate reading of what it feels like outside to the human body.

(MORE: What Is The Heat Index?)

As shown in the chart below, when the heat index reaches 103 degrees or higher, conditions become dangerous.

During a heat wave, several days of high temperatures accompanied by high humidity can become even more problematic and result in illness or even death.

3. Overnight Temperatures Matter

Warm nighttime temperatures, especially 80 degrees or warmer, don't allow people to recover from daytime heat. When warm low temperatures are combined with high humidity, the impacts can be exacerbated.

Without relief from the heat at night, heat stress can continue to build and increase the risk of heat illnesses and death. In fact, nights can be more deadly than the daytime during a heat wave.

(MORE: The Hidden Danger Of Heat Waves)

4. Heat Waves Tend To Be Worse In Cities

The effects of heat waves can be exacerbated in cities because of what is known as the urban heat island effect, which is when a city experiences much warmer temperatures than nearby rural areas.

Some neighborhoods in large cities can be up to 20 degrees hotter than surrounding areas with more trees and grass, according to NOAA.

This temperature difference is because in cities, vegetation is replaced by pavement and dark buildings. The asphalt and concrete absorb the sun's heat, causing surface temperatures to rise. The taller buildings and narrow streets also reduce air flow and trap heat.

(MORE: What Are Urban Heat Islands?)

And this added heat isn't just a daytime thing.

This temperature difference is typically larger after sunset. At night, heat is retained in cities, while it can escape easier in rural areas. 

5. Heat Safety Tips

The most important thing to know regarding heat waves is how to stay safe during the persistent, extreme heat.

Even just small changes in your routine can help. For example, move any strenuous outdoor work or recreational activities to the coolest time of the day, typically the early morning or evening.

Here are some more heat safety tips to consider:

Be sure to check on family, friends and neighbors, especially the elderly, those with health concerns and those without air conditioning.

And always pay attention to the latest weather forecast to stay on top of any upcoming heat waves.

Caitlin Kaiser graduated from the Georgia Institute of Technology with both an undergraduate and graduate degree in Earth and Atmospheric Sciences before starting her career as a digital meteorologist with weather.com.