This satellite measures whether there were clouds or not, because think about it: If there are clouds, then you might have some rain, but if there are no clouds, then it's actually impossible for it to rain.
这种卫星用来测量 空中是否有云层， 因为你可以想一下： 如果空中有云层，那就有下雨的可能， 但是如果没有云层， 那么就一点可能也没有。