A typical instable days' weather;
Usually the day starts with a clear sky, but when the sun gains some strength, the first clouds can be spotted.
These are cumulus clouds (usually abbreviated to Cu). They form above thermic bubbles (convection). This is air that rises from the now warmed earth. But when air rises up, and thus gets colder, it also loses it's capability to hold moisture and at some point becomes saturated. The moisture condensates and drops of water are formed around some core that it clings to (a grain of salt, dust, etc).
The first clouds are usually a bit 'shapeless' as the develop the level at which the water condensates becomes more visible. Because clouds usually form far apart from eachother, it looks like they're on a different height, but this in fact, is an optical illusion. Every cloud in the same category (cirrus, stratus, cumulus etc) is formed roughly on the same level.
As the morning-thermic fades, and the sun gains more strength, the earth warms up a lot quicker, which in turn causes more air to rise, condensate etc. Clouds develop quickly during this time of the day and begin to shield sunlight from the earth. As a result, the warming of the earth slows down, and so does cloud developement.
Sometimes, these Cu clouds join and form bigger structures. This way cumulonimbus (abbrev: Cb) clouds can form. These are massive cloud structures. And because they are so immense, they block a lot of light, which makes the base of the structure look dark-grey. These are the clouds lightning can form in.
So, when will it rain?
Clouds, Planes and Umbrellas
This is really hard to tell, but when you see a Cb or two, and jet propelled airplanes leave long white trails, you'd better pack your umbrella.
The wind factor
You might also want to have a look at a barometer, like everything else air wants to float down, from high pressure to low pressure areas. This is what we call wind.
Air usually drifts towards a depression in a swirl (much like water in a sink), when a depression is near (high wind speeds), this means the air is cold (and unable to hold a lot of water), this increases the chance that it'll rain.
Did you know: If you're on the northern hemisphere, and the wind is in your back, the depression (or low pressure area) is at your left. The center of the low pressure area is about thirty degrees before that. The high pressure area is at your right. Reverse this for the southern hemisphere. This little fact is validated by the 'Buys-Ballot' law.