Why is lightning with snowfall 'thundersnow' an unusual event?

• The meeting of warm and cold air, along with particle collisions, plays a part.
• Lightning strikes are caused by the difference between positive and negative charges.
• Human activities also have an impact on the occurrence of thundersnow.

In northern India, including Jammu and Kashmir and Himachal Pradesh, December and January usually saw very little rainfall. But things changed unexpectedly when winter snowfall started. This brought a unique experience for the people in the Kashmir valley as they witnessed something unusual - lightning (Thundersnow) during snowfall, a rare event that had never been seen before.

Thundersnow, a rare but exciting wintertime occurrence, could partly be because you typically don't consider or anticipate thunder and lightning during a snowfall. Thundersnow operates similarly to the thunder and lightning you see in your average thunderstorm. The formation of thunderstorms occurs when warm air at the surface rises upwards.

Lightning in winter, known as ‘thundersnow’, is rare. “Thundersnow can occur when an intense cold front slams into warmer air,” explains a National Severe Storms Laboratory meteorologist. This might happen as the seasons change, for example.

In some winter storms, shallow layers of warm air are lifted and continue to rise on their own, increasing snowfall and causing enough electric charge separation for lightning to occur. “You don’t see as much lightning in cold weather because you don’t often have the highly turbulent conditions inside clouds. Still, lightning can happen in winter, and it can happen during a snowstorm,” adds the meteorologist.

Thundersnow usually happens when there’s a lot of upward movement in the air. This is more likely when there’s a big difference in temperature in the middle of a storm. If warmer air gets pulled into the storm, there can be more lightning.

What causes thundersnow?

Thundersnow happens because of the same reasons as a summer thunderstorm. When the air near the ground is warm and wet, it goes up and forms clouds. These clouds have very cold water, small ice pieces, and a kind of soft hail called graupel. When these things move around a lot in the cloud, they can make an electric charge. This can cause lightning and thunder.

In a winter storm, the “warm” air near the ground is actually very cold, or below freezing. When this air goes up into the even colder air above, it makes less of the very cold water that is important for making summer thunderstorms.

Scientists are trying to understand how important this very cold water is for making thundersnow. Some early results from research by Harkema, funded by NASA, suggest that this very cold water and graupel are not as important for making lightning in the winter as they are in the summer. But it’s not clear why this is the case.

Experts said that lightning during snowfall is uncommon, it can be linked to changes in the atmospheric dynamics. Warmer air at higher altitudes interacting with colder air near the ground can generate the necessary instability, resulting in the surprising combination of lightning and snow.

Muhammad Sultan Bhat 82, a resident of Kangan said "I've lived in Kashmir my entire life, and I've never seen anything like this before. The lighting during snowfall was both beautiful and bewildering. It's a topic of conversation in our community, and everyone is curious about the science behind it."

He added, "We're used to heavy snowfall in winter, but lightning was unexpected".

Thundersnow needs moisture, heat, and instability

Michael Notaro, an atmospheric scientist and director of the Nelson Institute Center for Climatic Research at the University of Wisconsin in Madison, says, "Very intense winter storms can trigger the rare phenomenon of thundersnow. The heavy snowfall of thundersnow events usually muffles the sound, resulting in infrequent lightning flashes and quieter thunder."

Michael Notaro says, "The process that produces lightning is complex and we don't fully understand it. He further added, "In two respects, moisture plays a significant role: when water vapor in the air condenses into liquid drops, it releases heat. This heat subsequently provides energy to the thunderstorm."

The upper atmosphere, about 15,000 to 25,000 feet above sea level, needs interactions between supercooled liquid water droplets and ice crystals to generate the electrical charges that accumulate until a lightning bolt occurs. Typically, cold winter air doesn't contain a lot of moisture and therefore, doesn't support thunderstorms.

Johns explains that the formation of thunderstorms links to the instability of the air, which often results from convection when warm air rises from the Earth's surface on hot days, and cooler air replaces it. This system can generate strong updrafts, which move the warm, humid air upwards where it cools and produces rain or ice. These updrafts play a critical role in thunderstorms, enabling interactions between water droplets and ice crystals that can trigger lightning.

“In winter,” Johns concludes, “cold surface air temperatures and reduced sunlight mean there’s less surface heating, less convection, and thus fewer opportunities for thunderstorms.”

Where does thundersnow occur?

New technologies are helping scientists find out exactly where and when thundersnow is happening. A tool called the geostationary lightning mapper, which is on a satellite, can see lightning flashes all over one half of the Earth. When these flashes happen at the same time and place as snowfall, that’s thundersnow, says a scientist named Harkema.

In a thundersnow, the snow is the main thing falling from the sky, not rain. The storm has a lot of up and down movement in the air, which makes it possible for lightning and thunder to happen.

The lightning in a thundersnow is usually from cloud to cloud, not from the cloud to the ground. At night, the lightning can look really bright because it reflects off the snowflakes.

Thunder always happens when there’s lightning, but in a thundersnow, the sound is quieter because the snow muffles it. So, while you might hear thunder from a summer storm from many miles away, in a thundersnow, you’ll only hear it if you’re within 2 to 3 miles of the lightning. The thunder sounds more like a rumble than the loud booms you hear in summer storms.

The geostationary lightning mapper does more than just find out where a thunderstorm is happening. It also lets researchers see what these lightning flashes look like from about 22,000 miles above the Earth. Harkema says that even though there seem to be fewer lightning flashes in thundersnow than in summer storms, the ones in thundersnow are bigger. This could mean they’re more dangerous to people on the ground.

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