To form a tornado, cold air must sit above warm and moist air, and the wind needs to go in different directions at different heights, creating shear forces.
形成龙卷风，寒冷的空气必须处在暖湿的空气上部，而风应该在不同高度向不同的方向运动，以此创造出剪切力。
Suddenly, there appeared a hole in their nest. The cold wind blew in through the hole and got away the warm air from the nest.
突然，他们的巢里出现了一个洞。冷风从洞里吹进来，把巢里的暖空气吹走了。
Gravity pulls down cold air, and warm air rises.
重力令冷空气下沉，暖空气上升。
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