Above: Images © istockphoto.com/S_amado

In most places, thunderstorms are a fairly common occurrence in the summertime. But have you ever wondered why they so rarely happen in the winter? When thunder and lightning occur but the main form of precipitation is snow, not rain, it’s called thundersnow.

Did you know? Thundersnow refers to thunderstorms where the primary precipitation is snow. Although it is uncommon, thundersnow is an important weather phenomenon in North America. It is most often reported in the Great Lakes region, in the Intermountain West, and in the central United States. It is most likely to occur during the month of March. Thundersnow has also been reported in countries such as Finland, the United Kingdom, Norway, China, and Japan. The most recent reported thundersnow events in Canada occurred in Winnipeg (January 2014) and southwestern Ontario (February 2014).

In summer, when the sun heats up the Earth’s surface, warm air rises into the atmosphere. This warm, moist air rises up and condenses into water droplets, releasing heat that warms the air even more. This process creates the huge, anvil-shaped, cumulonimbus clouds that you probably associate with thunderstorms.

In winter, clouds tend to be flatter since there is not as much heat to make the air rise and form tall clouds. Lightning cannot form in these smaller clouds and therefore thundersnow is much less common than a spring or summer thunderstorm. However, there are rare cases of instability in the atmosphere where a cloud, or part of a cloud, can grow tall enough to form lightning and still be cold enough to make snow.

Did you know? Thunder is caused by extreme heating of the air by a lightning strike. A shockwave is sent out first, followed by a sound wave that you can hear.You may be wondering why lighting only forms in tall clouds. Although scientists don’t know exactly how lightning forms, their theories all rely on the idea that there can be different types of precipitation in the same cloud: liquid water droplets, ice crystals, and hailstones. To produce lightning, the cloud must be tall enough so that the lower portion is warm enough to have liquid water and the upper portion is cold enough to have ice. When the liquid water and ice collide, the water freezes instantly and heat is released, forming a hailstone with a warm surface.

When the warm hailstone collides with an ice crystal, there is a net transfer of charge, leaving the hailstones negatively charged and the ice crystals positively charged. As a result, the cloud has positively charged upper portion and a negatively charged lower portion. Since opposite charges attract, the negative lower portion of the cloud causes the ground below it to have a positive charge. The charge builds between the cloud and the ground until it is so great that it travels through the air, creating lightning.

Thundersnow is often localized and short-lived, but keep an eye (and ear) out for this incredible weather phenomenon. The sight of lightning and the crash of thunder are unexpected and rare in the winter, making it a remarkable experience.

References

General information

CNN cameras in Kansas capture “thundersnow” (CNN, YouTube) How A Thunderstorm Is Formed (The Weather Channel, YouTube) How Lightning Forms (The Weather Channel, YouTube) Probing Question: Why doesn’t it thunderstorm in the winter? (Meghan Holohan, Phys.org) Recent ‘thundersnow’ strikes southwestern Ontario (Dale Carruthers, Canoe.ca) Strong winds, blowing snow forces some planes headed to Winnipeg to land in Brandon (CTV News Winnipeg) Weather Wizard Re-Creates Thundersnow (The Weather Channel, YouTube)

Scholarly publications

Ahrens, CD. 2009. Meteorology Today: An Introduction to Weather, Climate, and the Environment. 9th Edition. Brooks/Cole. Market PS, Halcomb CE, Ebert RL. 2002. A Climatology of Thundersnow Events over the Contiguous United States. Weather and Forecasting. 17:1290-1295. Schultz DM, Vavrek RJ. 2009. An overview of thundersnow. Weather 64:274-277.

Rachel Hems

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