At a certain height, air cools enough for any water vapour to condense into droplets and form visible clouds. The droplets are liquid water, and therefore denser than air, but they are tiny, so they have a low terminal velocity and fall very slowly. A typical cloud only has about 0.5g of water per cubic metre in it, and if the droplets are small enough, they will be kept aloft by the thermals in the cloud as warm air rises from below. Once the droplets have fused together and grown large enough, gravity dominates over buoyancy and they fall as rain.
As for why water that evaporates rises up into the sky in the first place, air is mostly nitrogen (N2) and oxygen (O2), with an average density of 1.225kg/m3. A water vapour molecule is much lighter with just one oxygen atom and two hydrogen atoms (H), so its density (at standard temperature and pressure) is only 0.804kg/m3.
Why are clouds white?
Even though clouds are made up of air and water, they appear white.
It’s odd because clouds contain air and water – both transparent substances that hardly absorb any visible light.
Crucially, however, some of this water exists as tiny, sparsely dispersed droplets. Light can travel many metres into cloud, but eventually it’s likely to hit a droplet. That can scatter the light, changing its direction, although often only slightly. In thick cloud, each particle of light may hit many droplets in turn. Follow each circuitous trail, and you’ll see light is eventually thrown back out of the cloud in a random direction, often near the side it entered (explaining the darker bit of cloud on the non-illuminated side).
So a cloud’s colour is basically a mix of all the light put into it. Daylight is usually white: light straight from the Sun mixed with a little blue skylight. But you see non-white clouds at sunset, or over lit cities at night. Multiple scattering of small particles also explains the whiteness of milk, sugar, beer foam, and whisked Marmite.