Thundersnow: winter thunderstorms

In many places, thunderstorms happen a lot in the summer. But can they also happen in the winter? Yes, they can!
Winter thunderstorms have a different name. They’re called thundersnow. Both thunderstorms and thundersnow have thunder and lightning. The difference is in the main form of precipitation. In a thunderstorm, it rains. In thundersnow, it snows.

Let’s learn more about thundersnow. But first, let’s make sure we understand regular thunderstorms.

Thunderstorms are known for dark clouds, heavy rain and lightning. And, of course, thunder!

What causes lightning?

Lightning is very exciting to look at! Many scientists think it’s caused by the build-up of electric charge in cumulonimbus clouds.

Inside these clouds, winds push water droplets from the bottom toward the top. There, the water droplets turn into ice crystals. That’s because the top of a cloud is cooler than the bottom. Larger ice crystals turn into hailstones. When they get too heavy, they fall back to the bottom of the cloud.

As the hailstones fall, they transfer electrons to the ice crystals going in the other direction. The hailstones going down become negatively charged. The ice crystals going up become positively charged.

How do you think all this affects the charge in the cloud? The cloud ends up with more positively charged particles at the top. And more negatively charged particles at the bottom. This creates an electric field, like the one around a battery.

Negatively charged particles are attracted to positively charged particles. Remember, the bottom of a cloud has more negatively charged particles. And the top of a cloud has more positively charged particles. But there are also positively charged particles on the ground.

These differences in electrons create static electricity. The charge builds up between the positively and negatively charged surfaces. Eventually, it becomes big enough for a discharge to happen.

What happens when the bottom of a cloud discharges negatively charged particles? The particles leave the cloud in pulses called stepped leaders. They head for the closest positively charged particles on the ground. That usually means tall trees or buildings.

What about the positively charged particles on the ground? They move up to meet the negatively charged particles. When they finally meet, they cause a very fast secondary flash. It balances the charge between the particles and creates the lightning you see during a storm!

What is thunder?

So what causes the sound of thunder? Well, lightning is extremely hot! It can reach about 30 000 degrees Celsius. This creates hot air that expands. As the air cools, it contracts again. This all happens very fast.

The rapid expansion and contraction of air create a loud crack. You hear it as thunder. Have you ever heard a rumbling sound after the initial crack of thunder? That’s the sound of the air vibrating as it cools!

Did you know?

Light travels faster than sound. That’s why you usually see lightning before you hear thunder. The shorter the time between the lightning and the thunder, the closer you are to the storm!

What is thundersnow?

When thunder and lightning happen in cold temperatures, it’s called thundersnow. Thundersnow is pretty rare. There was some in western British Columbia in June 2019. It sometimes happens in the central and western United States. And it has been reported at times in Finland, the United Kingdom, Norway, China and Japan.

Why is thundersnow so rare? Normally, cumulonimbus clouds only form in warm summer weather. These clouds create lightning because their tops reach the colder air where ice crystals form.

It all begins when the sun heats up the Earth’s surface. This causes warm, moist air to rise into the atmosphere. The moist air releases heat as it condenses into water droplets. This warms the air even more. Eventually, a very tall cloud starts to form.

But clouds tend to be flatter in winter. That’s because there’s not as much heat to make moist air rise. The tops of flatter clouds don’t reach the colder air where ice crystals form. And without the ice crystals, there’s no lightning. That’s why thundersnow is so rare.

Did you know?

Thunder happens when the air is heated to very high temperatures by a lightning strike. First, this creates a shockwave. Then, it creates a sound wave that you can hear.

But every now and then, a winter cloud grows tall enough to create lightning! Thundersnow usually happens in a small area and for a very short period of time. So keep an eye (and ear) out for this incredible kind of weather event. It’s pretty amazing!