Thundersnow

The other day, something quite unusual happened over Seoul, South Korea. It was snowing heavily (not unusual), when a low, deep growl rippled across the sky and lasted for at least 5 seconds. This is the relatively rare phenomenon known as thunder snow.

Most commonly, snow is accompanied by, well, more snow, and possibly wind. It’s not something that is new to science, it just seems like the news has only really made a big deal about it in the last few years. It is so rare that it can even throw off weathermen:

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So what causes it and why is it so rare?

During a normal thunderstorm storm, a mass of warm air rises within denser, cooler air. As it rises it starts to cool and the moisture in it condenses. The warmer the air is, the more moisture it can hold, which is why some places that are hot and have a lot of rain feel so horribly humid.

When moisture condenses it releases latent energy which allows the warm air to cool slower and continue rising. This first stage is the towering cumulus stage. If there is a large enough instability between cool and hot air the huge cumulonimbus clouds begin to form, known as the mature stage. This all happens within about 30 minutes on average. During the mature stage there is often enough instability caused by the cooling air and condensing water, to create an electric charge, which builds up and eventually will discharge in the form of lightning. Of course thunder being the sound of lightning, it travels slower than light, and often seems to come later.

Thundersnow takes place in much the same way, but the conditions have to be slightly different. Of course usually ground level will be freezing, and a difference of at least 25°C (45°F) between the ground and 1.5km(5000 ft.) up from the ground is also needed. Most commonly thudersnow forms over lakes, as a lake’s temperature often drops slightly faster than that of the sea/ocean, which is a much larger, saltier body of water. The warm air usually rises against a cold front which also provides the difference in temperature needed.

In addition, due to the cold temperatures at ground level, the body of water must have a minimum distance (fetch) of 30 miles (50kms) across (in the prevailing direction of the front), for the air to take on enough moisture. These are some of the main prerequisites, which among other things means the likelihood of such an event is very slim and constricted to specific areas of the planet only.

Calculating distance from storm to you.

Count the second between the lightning flash to the thunder peel. Divide the number of seconds counted by 5 for a rough distance in miles. Divide by 3 for a rough distance in Km.

In the U.S.A. perhaps only 5-10 instances of thundersnow are recorded annually, usually forming over the Great Lakes. In Korea it is very rare, but often starts over the Sea of Japan.

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