What is a heat wave? Causes, impacts and how to stay safe

Heat waves are prolonged periods of extreme heat and humidity that can put people, pets and communities at risk. They form when a strong high-pressure system traps hot air close to the ground, creating a “heat dome” that blocks cooler air and cloud formation. In this guide, you’ll learn what a heat wave is, what causes it, how it affects the environment and the power grid and the steps you can take to stay safe and reduce energy use during extreme heat.

Definition of a heat wave

The Department of Homeland Security measures extreme heat starting at temperatures above 90° F. This level of heat can manifest as atmospheric high pressure that stays over a certain area for an extended period of time. When pressure keeps air from rising, conditions on the ground stay hot. Outdoor conditions like this can be dangerous for people.

More about what causes a heat wave

The high-pressure air that brings about a heat wave settles in at 10,000-25,000 ft. and doesn't move. This causes warm air to sink. When these conditions persist for several days the result is a heat dome -- like hot air with a lid over it -- trapping the heat near the ground and preventing cooling convection currents from entering.

Heat waves turning into heat domes have persisted as an annual occurrence, with the frequency of high heat days and the peak temperature on those days both increasing across the United States.

How hot can a heat wave get?

The temperature of a heat wave is well over 90° F, with high humidity making it feel a great deal hotter. Weather forecasters use a heat index to determine how much hotter the effect of the humidity is making the day feel. For example, if it is 84.5° F with humidity levels of 84%, the day feels more like 98° F. 

How to know when to expect a heat wave

The best way to know when to expect a heat wave is by tuning to the National Weather Service and understanding what their warnings mean. 

• Extreme Heat Warning: An Extreme Heat Warning is issued when dangerous heat is happening or about to happen. Avoid heavy activity and direct sunlight. Stay hydrated, find a cool indoor place and check on children, elderly and pets. Take action.
• Extreme Heat Watch: An Extreme Heat Watch is issued when dangerous heat is possible but its timing isn't certain. Reschedule outdoor activities in the coming days. Make sure that children, the elderly and pets have a place to cool off during the heat. Be prepared.     
• Heat Advisory: A Heat Advisory is issued for dangerous heat conditions that are not expected to reach warning criteria. Consider postponing or rescheduling outdoor activities, especially during the heat of the day. If you must be outside, be sure to drink plenty of water and take frequent breaks in the shade. Stay in a cool place, especially during the heat of the day and evening. Take action.

Heat alert criteria vary by region and so does the issuing of these alerts. Florida, for example, where high temperatures are common, usually requires different warnings about hot weather than Alaska, where high temperatures are more unusual. The local National Weather Service office handles these public announcements.

What do I do during a heat wave if I don’t have air conditioning?

During a heat wave it’s important that you try and keep yourself cool. Fortunately, it's possible to stay cool even without air conditioning.

You can seek out options outside your home that have AC. Possibilities include urban cooling stations or the local mall, or even a movie showing for a couple hours of cooler temperatures.

Keep your home as cool as possible by opening your windows at night to let in cool air. In the morning, close windows to try and shut out the day's heat for as long as possible. Only open them when it gets too stuffy. You can also place fans strategically in your home to create cross breezes.

Can heat waves impact the power grid?

When temperatures rise, increased air conditioning use means more electricity demand, which can put extra stress on the electric grid. High temperatures can also have a physical impact on transmission lines. Working together to conserve electricity, especially during a heat wave, means a better chance for us to stay cool and comfortable. Our Reduce Your Use program makes it easy for customers to support grid stability and earn rewards simply by using a little less energy during peak times.

How do heat waves affect animals?

Animals, just like humans, are affected by the high temperatures and high levels of humidity. But unlike humans, they may not have access to an air-conditioned building. You may see fewer animals out around you – as they will probably seek out shaded areas to try and escape the heat.

How do heat waves affect the environment?

Heat wave effects go beyond people and can have serious environmental impacts. Extended extreme heat can dry out soil and vegetation, increasing the risk of drought and creating conditions where wildfires can spread faster and burn more intensely. These events can damage forests, strain water supplies and disrupt entire ecosystems. Wildlife may struggle to find shade or reliable water, while plants that depend on cooler temperatures or steady moisture can experience long-term stress or die off.

How do heat waves affect the economy?

Potentially higher prices – Even short droughts caused by the excessive heat in a heat wave can decrease plant production and result in higher prices in the market.

More energy usage – Keeping cool during a heat wave increases energy consumption due to running air conditioners. 

Is a heat wave a natural disaster?

While heat waves may not have the excessive winds or rain that you think of when considering natural disasters, they are in fact a natural disaster. Heat waves are classified as natural disasters because they can cause widespread health risks and environmental damage, as well as straining critical infrastructure.

Sources

Department of Homeland Security

Columbia University

"The Economic Realities of Heat Waves" Forbes