【英语时差8,16】下雪天打雷

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 turbulence¹ 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 relatively² warm and most air over the ocean, then move into the icy atmosphere over the land. When this happens, you'll get a sharp atmospheric³ division in a snow storm, and you'll get a thunder snow.  

1 turbulence   

n.喧嚣，狂暴，骚乱，湍流

参考例句：

• The turbulence caused the plane to turn over.空气的激流导致飞机翻转。
• The world advances amidst turbulence.世界在动荡中前进。

2 relatively   

adv.比较...地，相对地

参考例句：

• The rabbit is a relatively recent introduction in Australia.兔子是相对较新引入澳大利亚的物种。
• The operation was relatively painless.手术相对来说不痛。

3 atmospheric   

adj.大气的，空气的；大气层的；大气所引起的

参考例句：

• Sea surface temperatures and atmospheric circulation are strongly coupled.海洋表面温度与大气环流是密切相关的。
• Clouds return radiant energy to the surface primarily via the atmospheric window.云主要通过大气窗区向地表辐射能量。