云 is a simplified form of a traditional character 雲. If you look closely at this latter, you’ll notice that it consists of the character for rain, 雨, on top of the very same 云. (One would think that it is more logical to place rain under the cloud, but here you are.) There is a variety of historical forms of this character, some of them have got rain and some don’t, just like some clouds bring rain and some don’t. Back to 云: the squiggle under two lines can be thought of as depicting a cumulus cloud under stratus clouds.