在赤道地区，对流过程一般比较稳定，天气总是不错，而在温带地区，季节变化、地区差异要明显得多，缺乏规律性。结果，高气压体系与低气压体系之间展开了永无穷尽的搏斗。低气压体系是由上升的空气创建的，把水分子送到天空，形成了云团，最终形成了雨。热空气比冷空气更能携带水气，这是热带和反季下暴雨最多的原因。因此，低的地方往往与云雨关系密切，高的地方一般阳光灿烂，天气不错。当两个这样的体系相遇的时候，往往从云团的样子看得出来。

For instance, stratus clouds—those unlovable, featureless sprawls that give us our overcast skies—happen when moisture-bearing updrafts lack the oomph to break through a level of more stable air above, and instead spread out, like smoke hitting a ceiling. Indeed, if you watch a smoker sometime, you can get a very good idea of how things work by watching how smoke rises from a cigarette in a still room. At first, it goes straight up (this is called a laminar flow, if you need to impress anyone), and then it spreads out in a diffused, wavy layer. The greatest supercomputer in the world, taking measurements in the most carefully controlled environment, cannot tell you what forms these ripplings will take, so you can imagine the difficulties that confront meteorologists when they try to predict such motions in a spinning, windy, large-scale world.

比如，要是携带水汽的上升气流无法突破上面比较稳定的一层空气，就会像烟碰到了天花板那样向外展开，于是就形成了层云一那种不大讨人喜欢，毫无特色，弄得天空阴沉沉的云层。事实上，要是你观察一个人抽烟，望着烟雾怎样从一变香烟在无凤的屋子里袅袅上升，你就会有个很好的概念，知道这到底是怎么回事。起先，烟雾笔直上升（这被称之为“层流”要是你想在别人面前卖弄一下学问的话），然后向外展开，扩散成波形的一层。连用在精密受控环境里进行测量的世界上最大的超级计算机，也无法准确预测这类波形烟雾会成什么形状，而气象学家却要在一个不停自转、大而多风的世界里预测这种运动，你可以想像他们面对的困难有多大。