The sky rumbles and snow falls: a rare weather phenomenon has struck Earth (video).

A rare weather phenomenon occurred over several eastern and central states in the U.S. this weekend—arctic air continues to bring heavy snowfall and freezing temperatures to millions of Americans. Researchers stated that snow thunderstorms are extremely rare and require specific conditions, as reported by Live Science.

A snow thunderstorm, also known as a winter thunderstorm, takes place when a snowstorm is accompanied by thunder and lightning. This phenomenon was observed by meteorologists across much of the Midwest, as well as in Maryland, Virginia, and Washington.

The National Weather Service in St. Louis (Missouri, USA) shared satellite images from NASA's GOES-East spacecraft, capturing lightning flashes over Arkansas, Kansas, Oklahoma, and Missouri on Sunday, January 5. Meteorologists also warned of the potential for wet snow and advised against travel.

Researchers explained what a snow thunderstorm is and the conditions necessary for its occurrence.

What Causes a Snow Thunderstorm?

Thunderstorms occur when warm, moist air rises and condenses in cooler, drier air above it, forming cumulonimbus clouds. Inside these clouds is a mixture of light rising ice crystals and soft hail, also known as graupel—which is denser and falls downward.

This causes particles to collide with one another, resulting in electrons being transferred from the ice crystals to the graupel. Ultimately, according to the National Oceanic and Atmospheric Administration (NOAA), a cumulonimbus cloud forms with positively charged ice crystals at the top and negatively charged graupel at the bottom.

This creates a negative charge at the base of the cloud, repelling electrons in the Earth below it. The difference in charges between the cloud and the ground accumulates until the potential energy is eventually discharged in the form of a lightning strike. The energy from this strike heats the surrounding air and causes it to expand outward, creating the sound we know as thunder.

If this weather system is in the midst of a snowstorm, a rare weather phenomenon known as a snow thunderstorm occurs.

Why Are Winter Thunderstorms Rare?

Researchers note that the rarity lies in the conditions required for thunderstorms to form—they seldom occur during the winter months. According to John Shonk, a senior scientist at the UK Met Office and a researcher at the University of Reading, there is simply much less energy in the atmosphere during winter, resulting in significantly reduced surface heating.

In the rare instances when thunder snow occurs, it typically happens around the Great Lakes. At the same time, meteorologist Kendall Smith noted that the winter thunderstorm observed this weekend is quite unusual—it's very strange to see thunder snow this far south. Consequently, scientists have labeled this weather event as "historic."

What Are the Dangers of a Winter Thunderstorm?

Like any other storm that produces lightning, a thunderstorm can be dangerous. However, the lightning and thunder accompanying a snowstorm usually occur less frequently and are less intense than in summer thunderstorms.

During summer thunderstorms, thunder can be heard from several kilometers away; however, snow can muffle the sound, limiting the distance over which thunder can be heard. As a result, people may be less vigilant about an approaching storm and are at greater risk of unexpected lightning strikes.

Researchers also point out that the frequency of lightning strikes during winter thunderstorms is significantly lower than during summer ones, although they can still be destructive. Another danger is that winter thunderstorms are typically accompanied by heavy snowfall.