Reports of thundersnow are rare. Snow muffles sound, so any thunder can’t be heard more than about two miles from a snowstorm’s center. Thundersnow storms tend to be small and to last under three hours. Therefore, people often don’t hear thunder in a snowstorm unless they are right in the middle of a small brief storm.
In addition, thundersnow itself is rare, because the conditions that cause it are so unusual.
Ordinary summer thunderstorms come with towering thunderhead clouds that rise in warm moist air with very cold air over it. This unstable condition creates roaring updrafts within the thunderheads. Positive and negative electrical charges gather in different parts of the clouds, building up electrical potential like feet shuffling across a rug.
When the charge builds high enough in a part of the cloud, lightning bolts bridge the gap the way a shock leaps from someone’s finger to a metal doorknob. Bolts may leap from one part of the cloud to another, or strike the ground.
Thunder rumbles because a lightning bolt instantly heats the surrounding air to a temperature hotter than the surface of the sun. The heated air expands, compressing the air just beyond it, and creates a shockwave that moves away from the lightning at the speed of sound.
For thundersnow, conditions that cause snowstorms must combine with conditions that cause thunderstorms.
This can happen near the Great Lakes or Great Salt Lake in Utah. In fact, Utah is the American state that reports the most thundersnow. Ordinary lake effect snow happens when cold dry air blowing across a lake picks up heat energy and moisture. Snow falls when the water vapor rises high enough into cold air to cool and condense.
In the case of thundersnow, the cloud must be tall, and the storm must pass over as much as 30 miles of water. The top of the cloud must be cold enough to freeze any supercooled water in the cloud, meteorologists believe, because liquid water can interfere with electrical charges building up in the icy cloud.
Electrical charges must build up in different parts of the cloud, at the same time as water vapor becomes snow.
Thundersnow can also happen without a lake, when a relatively warm oceanic snowstorm moves on shore and suddenly encounters frigid air aloft. Then the clouds may pile up into tall thunderheads, and electrical charges build up in the clouds.
It can also occur when an ordinary thunderstorm leads a weather front in a winter environment. Then a thunderstorm may happen in the normal way, but precipitation may reach the ground as snow. It can also happen when a thunderstorm climbs high enough along a mountain front to reach regions that chill the water vapor it bears to fall as snow.
Thunder happens because of sharp differences in electrical charges. Snow happens because water vapor cools and crystallizes. When both happen at once, the result is thundersnow.
Thundersnow does not happen often. Witnesses are astonished at the power of nature, just as with summer lightning, or soft spring rain.