Somewhere inside of every raindrop is a tiny impurity a touch of salt,a speck of soot,a grain of clay- that's absolutely crucial to the raindrop's existence.In fact,without these microscopic pieces of dirt,there would be no rain,because water vapor can't condense into droplets on its own.
Which is kind of weird,because water molecules like each other if they didin't,they wouldn't cling to each other like this!And in the air,vaporized water molecules collide and stick together all the time but they also break apart all the time,thanks to bond-breaking heat energy.Only when the air cools down past a certain point-called the dew point-does this breaking-apart slow down enough for little clusters of water molecules to grow into droplets.
这很奇怪 因为水分子明明很相亲相爱 要不 它们不会像这样互相抱成团！空气中 汽化的水分子时刻发生碰撞凝聚 但与此同时 由于断键热能的作用，它们也时刻发生分散。只有在空气冷却到特定温度————即霜点温度————才能令断键过程减缓到可以让一小撮的水分子凝结成水滴。
But actually,that's only true if the cluster is big to start with.If it's too small,its surface is so curved that the molecules on the outside have few neighbors to bond to,which makes them easy to break off,so the cluster as a whole has higher chances of losing molecules than gaining them,even below the dew point.
但事实上。只有当这撮水分子足够多时 才能凝成水珠。如果数量太少 那么由于表面凹凸不平 位于表面的水分子可以结合的邻里就很少，这会使它们很容易脱离群体，因此即使使温度低于霜点温度 这堆水分子丢失伙伴的概率还是大于凝聚的概率。
Which means that up until that critical size,a cluster's odds of shrinking are much better than its odds of growing.Unfortunately,that critical size is 150 million molecules.And while there are millions of five-molecule clusters in a golf-ball sized volume of air at dew point,odds are that only one of those clusters will grow to a size of ten.
也就是说 在达到临界体积之前，这团水汽缩小的几率远大于增大的几率。不幸的是 这个临界体积是1.5亿个分子。在霜点温度下 一个高尔夫球体积大小的空气中存在无数由5个分子组成的水汽团，其中只有一个水汽团最终能聚到10个分子。
And you'd need a golf ball of air ten million miles across to find one 50 moleculd cluster!Which,basiclly ,means that clusters of water molecules never get to 150 million on their own.Fortunately,they don't have to they can start off at that critical size by condensing onto one of the badjillions of little pieces of dirt floating in our atmosphere,and then grow and grow until they're a droplet in a raincloud.
一个直径一千万英里的空气球里 只有一个水汽团能凝聚到50个分子那么大！也就是说 仅凭水分子自身无法聚到1.5亿个分子。幸运的是 它们无需孤军奋战 它们可以将附着到大气中的尘埃上作为 最终凝聚到临界体积的起点，不断凝聚 直到变成雨云中的一粒小水滴。
So,ultimately,it's these little pieces of dirt,surrounded by water,that make life possible on our big piece of dirt surrounded by water.