6 Things You Should Know About Heat Waves

A heat wave is a prolonged period of excessively hot weather, which can mean different conditions depending on the region, as it is compared to average temperatures. For example, in many areas of the northern U.S., a heat wave is defined as at least three days with high temperatures 90 degrees or higher.

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Below are six important things you need to know about heat waves.

1. Heat Waves Can Be Deadly

Extreme heat – especially for several days, as is the case with heat waves – can put stress on the human body; heat illnesses and death could result. 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.

(MORE: Four Things Extreme Heat Does To Your Body)

Over time, heat exhaustion can set in, and if not treated, heat stroke can develop. The elderly, children under 4 years old, those living in homes without air conditioners and people with chronic disease are at the highest risk for heat stroke, which can damage the brain, heart, kidneys and other muscles.

More people die from heat, on average, than any other weather hazard, based on the 30-year average from 1987 through 2016, according to NOAA. Over this 30-year period, there were an average of 131 fatalities each year, many of which occurred during heat waves.

The Southwest experienced an intense heat wave in June, which resulted in up to 12 deaths. One notorious heat wave occurred in the Midwest in July 1995, when the combination of extreme heat and humidity resulted in nearly 750 deaths.

2. Overnight Temperatures Matter

When blistering temperatures persist over several days, overnight temperatures may not cool as they normally would.

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

(MAPS: 10-Day Forecast)

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

3. Dew Points and Humidity Are Also Important

Dew points and relative humidity provide an idea of how much moisture is in the atmosphere. The dew point temperature measures how much the air temperature needs to be cooled to in order to become saturated.

This means once the dew point rises above 60 degrees, conditions generally begin to feel humid. When the dew point reaches 70 degrees, it becomes uncomfortable, and most would consider anything above 75 degrees to be oppressive.

Relative humidity is calculated using the air temperature and dew point. Once the air temperature and relative humidity are known, the heat index, which describes how hot it really feels, can be determined.

As shown in the chart above, when the heat index, or feels-like temperature, reaches 105 degrees or higher, conditions become dangerous.

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

4. They Tend To Be Worse In Cities

The effects of heat waves can be even worse in cities due to the urban heat island effect, which refers to a metropolitan area that is warmer than the surrounding locations.

Temperatures in a city with one million people or more can be 1.8 to 5.4 degrees warmer than surrounding areas, according to the EPA. This temperature difference can be as high as 22 degrees after sunset since rural areas cool down faster.

This temperature difference is because vegetation is replaced by pavement and dark buildings in cities. 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?)

At night, heat is retained in cities, while it can escape easier in rural areas. This increase in temperature, both during the day and at night, can make a heat wave even worse in urban areas.

5. Lack Of Air Conditioning In Some Areas Can Add Concern

One of the best ways to escape and limit the impacts of heat waves is air conditioning. However, many people in the northern U.S. don't have air conditioners because temperatures usually don't warrant them most of the time.

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An additional concern is with residents who live on the upper floors of buildings that lack air conditioning. Heat tends to rise, so higher levels tend to be warmer – especially in locations without cooling devices.

6. Heat Safety Tips

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

A change in work or recreation may be in order during a heat wave. If possible, eliminate or reschedule strenuous work and recreational activities until the coolest time of the day, typically the early morning or evening.

The Centers for Disease Control and Prevention (CDC) offers a few tips to stay safe during heat waves: stay in air conditioning; take frequent breaks; drink more water than usual; avoid using the stove or oven; take cool showers or baths and wear lightweight and light-colored clothing.

(MORE: Heat Is The Deadliest Kind Of Weather)

Also, be sure to check on neighbors and friends, especially the elderly and those with health concerns, and always pay attention to the latest weather forecast to stay on top of any upcoming weather hazards.

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