an indistinct or billowing mass

Leonardo da Vinci said that clouds were like 'bodies without surface.' And in Folding Chair, Regina Spektor wisely points out, 'the sea is just a wetter version of the skies.'
So a couple of weeks ago I returned from what has become an annual camping trip to the Isles of Scilly. Islands allow you to see so much wider and further and higher than you otherwise would. There's a clarity of vision that you don't get on the mainland, meaning you see the weather coming towards you, far in advance. It's like looking at your immediate future laid out before you. I rarely pay attention to keeping one step ahead of the weather when I'm in a city or working.

Interestingly, clouds exist, whilst not being visible… read an article on subvisible clouds.

And this site is brilliant for anyone who's in the least bit interested in the skies above them:

The Cloud Appreciation Society.