‘Thundersnow’ explained

    Philadelphia area residents got the rare treat of a winter thunderstorm Wednesday night. After it was dubbed “thundersnow,” we asked ourselves why thunderstorms happen infrequently enough in the winter to warrant their own nickname.

    Here’s the answer, stripped of all science-ese: Lightning is basically giant sparks in the atmosphere, caused by the separation of positive and negatively charged particles. That separation takes two things, both of which are much more common in the summer than the winter.

    The first key is strong upward velocity of air. In the summer, there is a lot of built-up heat near the Earth’s surface. When a storm moves in higher up in the atmosphere, bringing cold air with it, a large temperature differential is created. Warm air rises, so the air near the ground rushes upward.

    Nels Shirer, a meteorology professor at Penn State University, said vertical temperature differential doesn’t happen very often in winter.

    “In the types of storms we have in the winter, warm air is being brought up from the Atlantic, and it’s usually rising while it’s moving vertically,” Shirer said. “So you end up having the relatively cold air at the ground and the warm air rising up over it.”

    In the summer, when warm air forces itself upward, it causes water and ice particles to collide. That, according to the dominant scientific theory (it hasn’t been conclusively proved yet), separates the positively and negatively charged particles and throws them far apart, which makes the spark that is lightning. But in the winter, most of what is up in the clouds is frozen, so that reaction doesn’t often happen.

    “You often just have ice in the clouds,” Shirer said, “so you don’t get the mechanism for separating the charge to work.”

    That’s why thunder and lightning are so rare in the winter.

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