What Causes Heat Waves? Formation, Impact, and Climate Analysis

Heat waves, also known as excessive or extreme heat events, are abnormally hot stretches of weather that usually last for two or more days. Wondering how hot heat-wave hot is? The answer varies from place to place since what's considered to be a normal summer day for one location (Las Vegas, Nevada, for example) might not be so normal for others (such as Bangor, Maine).  

There's one thing about heat that doesn't vary across the United States: It has claimed more U.S. lives since 1991 than any other weather hazard.

The more familiar you are with the signs of an impending heatwave in your weather forecast, the more prepared you'll be to respond to potentially deadly temperatures, especially as these become more commonplace due to climate change.

How Heat Waves Form

One of the key ingredients needed for heat waves to form is, of course, high temperatures. Another is a persistent region of high pressure in the upper atmosphere.

High-pressure systems are associated with clearing conditions, but also stable, sinking air. So whenever a high-pressure area moves over a region, the air in the nearby atmosphere sinks toward the surface. This sinking action acts as a dome cover, sealing off the air underneath the high pressure from the surrounding atmosphere.

This "cap" that forms over the affected area traps heat that would otherwise rise into the air and cool before circulating back to the surface. The inability to rise not only reduces the chance for precipitation but also allows the continual buildup of heat, which we on Earth's surface experience as a heat wave.

Summer weather patterns, including summertime high-pressure systems, move slower than those in winter, says the National Oceanic and Atmospheric Administration (NOAA). So when one arrives, it can be days or weeks before it moves again.

During the Summer 2012 North American heat wave, for example, high pressure lingered over the U.S. Plains from late June to mid-July. Its presence triggered one of the most intense heat events in U.S. history, leading to more than 8,000 warm temperature records being broken or tied, nationwide.

Whenever the blocking pattern responsible for keeping the high stationary dissolves, the dome of high pressure will become unstuck, and will once again push onward. When this happens, the heat wave will break.

Heat Waves and Urban Heat Islands

As if heat waves aren't oppressively hot enough by themselves, environmental conditions such as urban heat islands can exacerbate them. According to one study, the high population density and developed land (concrete sidewalks, asphalt roadways and parking lots, and so on) found in cities have made the frequency of hot days grow by 48% and hot nights by 63% in urban areas from 1973-2012.

Measuring Heat Wave Intensity

During bouts of extreme heat, you'll likely hear the term "heat index" pop up. This is a fictitious temperature based on air's actual temperature and humidity that expresses how hot the human body perceives the air to be. Meteorologists use it to gauge when a heat threat will reach dangerous levels, thereby impacting human health.

In Orlando, Florida, a heat advisory is issued when heat indices or feels-like temperatures reach at least 108 degrees F (42 degrees C) are forecast. Similarly, local heat watches and warnings are issued when heat indices of 113 degrees F (45 degrees C) are expected soon or are already occurring.

How Climate Change Affects Heat Waves

Heat waves in major cities across the United States have gone from occurring around twice per year during the 1960s to occurring more than six times per year during the 2010s, according to the U.S. Global Change Research Program. What's more, the average heat wave season has lengthened by nearly 50 days.

Without the influence of climate change, heat events like the 2021 Western North America heat wave wouldn't have occurred, scientists say.

These changes aren't limited to the United States alone. According to the Intergovernmental Panel on Climate Change's (IPCC's) recently released sixth assessment report, extremely hot days have become more frequent and more intense across most land regions since the 1950s. The report also finds that hot extremes (including heat waves) that used to occur once every 10 years are now nearly three times more likely to occur, and they are 2.2 degrees F (1.2 degrees C) hotter than they were before humans heavily influenced the climate.

Unfortunately, this trend is expected to continue. And once global mean temperatures have risen 3.6 degrees F (2 degrees C), hot extremes are projected to be nearly six times as likely and over 5 degrees F (3 degrees C) hotter.

As greenhouse gases such as carbon dioxide trap extra heat in Earth's atmosphere, temperatures all over the world rise. Not only is this warmer air able to “hold” more water vapor, but it's also able to evaporate more liquid water from soils, plants, oceans, and waterways, transferring this moisture from ground levels up into the atmosphere aloft. So, global warming essentially makes high air temperatures and atmospheric humidity—two heat wave must-haves—more readily available.