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为什么冬天会打雷呢？大家知道，雷电是雷雨云中的放电现象。这种云的底部离地面约1公里高，一般云顶带正电荷，云底带负电荷，相应的地面也产生与云底电荷相反的感应电荷。由于对流作用，云上下运动，云内的冰晶相互摩擦使电荷逐步增多，云的内部、云与云、云与地面之间的同电荷区形成了很强的电场，电场达到一定强度就要相互中和而发生放电，在光的通路上产生高温，使四周空气剧烈受热，突然膨胀，发生巨大的响声，这就是雷鸣。
【英文原文】
It's a common enough sound in summer, after the wind finally picks up on a hot and humid afternoon, as the rain starts spattering down in big, fat drops--you're in for a full-blown thunderstorm.
It's much less common in winter, even during a snow storm. Why is thunder so much more likely in summer than in winter? We'll learn about thunder snow, on today's Moment of Science.
In a summer thunder storm, the atmosphere is stacked up like a two-layer cake. The lower layer is full of warm, moist air; the upper layer is icy cold and dry. Because hot air rises, there are soon tremendous updrafts, carrying the warm moisture of the lower layer up into the cold, dry layer. The turbulence of these updrafts creates static electricity as the air masses rub against each other. Thunder and lightning occur when the static electricity discharges in a giant bolt of electricity.
In the winter, the layer of air near the ground is likely to be a lot colder and drier than it is in the summer. It's much harder to get the sharp divisions in the atmosphere that lead to turbulence, static electricity, and thunder. It can happen though, especially near the coast. A thunderstorm can form over the relatively warm and moist air over the ocean, then move into the icy atmosphere over the land. When this happens, you'll get a sharp atmospheric division in a snow storm, and you'll get a thunder snow.
