What Is Heat Lightning?

Lighting storm
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Heat lightning is the nickname given to the silent lightning strikes and faint flashes of light that are visible on the distant horizon on some warm, humid summer nights. To the naked eye, these flashes appear to occur without a nearby thunderstorm or rainstorm, which is why they’re also known as “dry lightning.” 

Most people have witnessed heat lightning before, but few realize that what they observed wasn’t a rare kind of electrical storm, but rather, ordinary lightning from a storm cloud too far away to be seen, and accompanied by thunder too far away to be heard.

Heat Lightning vs. Other Lightning Types

Weather-wise, heat lightning is no different than ordinary lightning. Its distance from an observer gives it several unique, albeit perceived, features that other lightning types lack. 

The most notable of these features is an obscured lightning bolt. According to The Weather Channel, a cloud-to-ground lightning strike can be seen as far as 100 miles from a storm; however, at such distances, mountains, trees, buildings, and even the curvature of the earth can obscure a clear view of the bolt. As a result of this, storm spotters only see the light from the strike that’s reflected off of a storm’s neighboring clouds, and not the full strike itself. Ordinary lightning, on the other hand, is typically observed at distances no more than a few miles away, which is close enough to not only see a white bolt, but also the red, orange, yellow, or violet hues along the bolt’s edges. (The bolt’s color depends on how hot it is. According to the visible light spectrum, the cooler lightning’s temperature, the longer the wavelengths of visible light it emits, and the more red the eye sees; the hotter its temperature, the shorter the wavelengths, and the more blue and violet the eye sees.)

Another distinct feature of heat lightning is a lack of audible thunder. Thunder — the sound of air around the lightning channel rapidly heating to temperatures of 50,000 degrees Fahrenheit — can only be heard within 10 to 15 miles of a storm’s center. Any farther away than this, and a thunderclap’s sharp crack lessens to a continuous rumble due to the sound waves refracting through the lowest layer of the atmosphere and reflecting off of the earth’s surface. Any farther away than that, and the sound is refracted and reflected to such a degree that voids are created through which the sound of thunder doesn’t propagate.

Heat Lightning Misconceptions

Virga falls from a dark, stormy sky.
Evaporating rainfall, or "virga," is often a sign of dry thunderstorms, which are not related to dry lightning.

Jon Sandstrom / Getty Images

A number of myths exist about heat lightning, including that it's a "real" type of lightning that differs from ordinary lighting. Here are a few others you shouldn’t believe. 

Myth 1: Heat Lightning Is Caused by Extreme Temperatures

The word “heat” in heat lightning doesn’t mean this lightning is created from excessive heat in the atmosphere. Rather, it’s a nod to the fact that this kind of lightning is often spotted on hot, summer nights. Like ordinary lightning, heat lightning forms when there’s a build-up of positive and negative charges in and around a thunderstorm cloud. 

Myth 2: Heat Lightning Only Occurs in Summer

Heat lightning frequently occurs in summer because that’s when thunderstorm activity peaks. (Summer’s warmer air temperatures and longer days allow more of the sun’s heat energy to trigger severe weather.) However, like thunderstorms, heat lightning can strike at any time of year, as long as the right conditions are in place.

Myth 3: Heat Lightning and Dry Thunderstorms Are the Same Thing

Heat lightning, or “dry lightning” as it is sometimes called, shouldn’t be confused with dry thunderstorms, as these two are different phenomena. Dry lightning is called “dry” because it appears to occur sans thunderstorm or rainstorm; but in reality, dry lightning is associated with a rainstorm, the rain is just too distant to be visible. Dry thunderstorms produce thunder, lightning, and precipitation, but are called “dry” because their rainfall evaporates before reaching the ground.

Is Heat Lightning Dangerous?

According to the National Weather Service, lightning injured nearly 250 people and killed 27 people, on average, over the 10-year period from 2009 to 2018. Heat lightning is responsible for virtually none of these incidents, mainly because its strikes are too far away to do direct or indirect bodily harm. While lightning can travel impressive distances through the air and along the ground, these distances are usually limited to about three miles for the average thunderstorm, and up to 25 miles for "bolts from the blue" — lightning that exits the side of a storm cloud, travels horizontally, then strikes from a clear blue sky. Simply put, if you’re far enough away from a thunderstorm to observe its lightning as heat lightning, you’re likely too far away to be harmed.