A heat wave is defined as a period of abnormally hot weather lasting more than two days. They have the potential to cover a large area, exposing a large number of people to hazardous heat. Let's go over heat wave facts, basic meteorology, and safety tips.

Heat Wave Facts

• Heat waves can occur with or without high humidity.

Learn more - Dew Point vs. Humidity, Explained

• Extreme heat is the number one weather-related killer in the U.S. 
• The minimum temperature during a heat wave is as important as the maximum temperature.
• If the nights are unusually hot, higher temperatures will be reached earlier in the day and last longer.

Heat waves impact human health, public safety, and the environment. Heat waves can lead to worsened air pollution and limit the functioning of key infrastructure.  

• High heat can deteriorate and buckle pavement, warp or buckle railway tracks, and exceed certain types of aircraft operational limits.
• Electricity usage increases, and transmission capacity across electric lines is reduced during high temperatures, straining the electrical grid.
• Water resources are strained as conventional power plants require large quantities of water for cooling, and crops may need increased water consumption.
• People increase water consumption to stay hydrated and cool.
• Heat can have lasting impacts as crops may be damaged, reducing production.

The National Weather Service and the World Meteorological Organization provided these facts.

Meteorology Behind a Heat Wave

The meteorology behind a heat wave is driven by a high-pressure system and an associated upper-level ridge. Under this weather system, the air sinks toward the surface where it warms and dries, increasing the temperature and often inhibiting clouds, leading to warmer temperatures.

The strength of the high-pressure system and upper-level ridge will affect the strength and duration of the heat wave. Generally, stronger systems will lead to more severe and longer-lasting heat waves. The weather pattern surrounding the heat wave has an important effect. If there is a lack of weather systems surrounding a heat wave, the intensity and duration of the heat wave can be more severe and last longer.

An additional factor to consider is whether moisture transport is favorable during a heat wave. If there is favorable moisture transport, this can lead to high dew points and high humidity. This combination with high temperatures leads to high heat indices ('feels like' temperature), which can make the heat wave more severe.

• This is especially true in areas of the Eastern U.S., where heat waves become more severe when there is higher humidity.

The National Weather Service (NWS) will issue heat-related warnings and advisories, which help warn of the specific heat wave threats for the defined county areas, including how hot it will get and how long the heat wave will last.

Can it rain during a heat wave?

Yes, it can rain during a heatwave. Depending on the strength and orientation of the high-pressure system, moisture from surrounding areas can lead to atmospheric instability and cause thunderstorms.

Thunderstorms are not common during heat waves but can occur, and are often referred to as single-cell or 'pop-up' thunderstorms. Pop-up thunderstorms will form and dissipate quickly due to the surrounding high-pressure system associated with the heat wave. They are often less severe than other types of thunderstorms and rarely have hail or tornadoes due to unfavorable atmospheric conditions.

Do heat waves cause wildfires?

Heat waves do not directly cause wildfires, but can lead to heightened and more severe conditions. During a heat wave, hot temperatures will dry fuel loads, which can lead to more severe and widespread wildfires.

In the Western U.S., heat waves are often associated with low dew points and low humidity, which increases the threat of wildfires. Furthermore, during heat waves in the West, dry thunderstorms can occur, which increases the ignition source for wildfires.

Learn more - Understanding Fire Danger & Fire Weather Advisories in the U.S. and How Spring Weather Affects Wildfire Season

The forecast for the Summer of 2025 across the U.S. favors above-normal temperatures, which is associated with an above-normal wildfire potential.

Climate Prediction Center's (CPC) outlook of temperatures for July, August, and September 2025.

National Interagency Fire Center’s Significant Wildland Fire Potential map July 2025.

How to Stay Safe

The best way to stay safe during a heat wave is to have access to cool air-conditioned spaces and plenty of cool water.

• Reduce or eliminate strenuous activities until the coolest time of the day.
• Dress for the heat with lightweight, loose-fitting, and lightly colored clothing.
• Cities and towns will often open access to public cooling centers.

See this article from the National Weather Service about Heat Illness.

Finally, it is important to check in on others during a heat wave. Some people are more sensitive to heat waves and might not have access to key safety measures.

Stay posted on more News articles for notable weather events, educational articles, and OpenSnow updates!

Zach Butler