Heat Waves: 6 Things You Should Know

Heat waves and excessive heat impact the United States every summer and can have life-threatening consequences.

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.

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 in the U.S., based on the 30-year average data from NOAA. Over the most recent 30-year period, there were an average of 168 fatalities each year, many of which occurred during heat waves.

In July 1995, the combination of extreme heat and humidity during a Midwest heat wave claimed over 700 lives over a five-day period.

2. Overnight Temperatures Matter

When blistering heat persists 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.

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.

(MAPS: 10-Day Forecast)

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.

(MORE: Heat Index Explained)

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

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.

S​ome neighborhoods in a large city can be up to 20 degrees hotter than surrounding areas with more trees and grass, according to NOAA.

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?)

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. 

This increase in temperature, both during the day and at night, can make a heat wave even worse in urban areas.

5. A 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, some in the northern U.S. don't have air conditioners because temperatures usually don't warrant them most of the time.

(MORE: The Triple-Digit Club)

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 your routine 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 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)

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 heat waves.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.