Air is a relatively even, or homogeneous mix of gases; mostly nitrogen at 78% and oxygen at 21% Ironically, and a good thing for us, air is never totally dry: water vapor in its gas form can contribute as much as 4% of the volume of air. As we all remember when we drew little arrows between the oceans and the sky for the water cycle in grade school, water enters the atmosphere from plants, the ground and the sea surface. Every time you breathe out, you're adding more water vapor to the air around you. On average, water vapor will stay in the lower atmosphere for about 10 days, after which it condenses into rain, snow or dew.
An interesting fact about humid or wet air is that it's less dense than dry air; this is because water molecules are lighter than oxygen and nitrogen, so the more water in air, the lighter it is. Warm air can hold more water than cold air, and as anyone who opens a tire valve knows, expanding air is cold. Compressing air, like when you force air into your bike tires, warms it up. This means that warm air holding water will rise, and as it does it expands and starts to cool in the lower pressure of higher altitudes. When it gets cold enough, it can't hold on to its water vapor, and the water forms into water droplets, snow flakes, and clouds.
This droplet-forming process happens first around a 'seeding' particle, such as dirt or pollen, where the water molecules latch on and coalesce around the dust in a process called nucleation.
The more dusty the air, the faster water will condense out of the vapor form and into clouds.
Because of this process, scientists think that rising pollution in the air will create more clouds, and more powerful storms.
What drives air to rise and expand, or get warm enough to evaporate water in the first place?
The answer is, heat from the sun. Solar heating strength depends on the three things:
1. The angle of the sun above the horizon (the sun's angle at midday gives the most solar heat daily, and the sun's angle on the summer solstice gives the most energy yearly). In mid-latitudes in the northern hemisphere, about three times as much energy comes from the sun in June than in December.
2. The transparency of the atmosphere, which depends on cloud cover, the ozone layer and smog.
3. How reflective the Earth's surface is (snowy landscapes reflect a lot more heat than black pavement).
Cloud forming is probably one of the most important climate processes on Earth, as it takes water, and lots of heat energy, from the tropics and distributes it towards the poles. This prevents the ocean at the equator from boiling, and the polar regions from freezing solid!
The key to this Earth-moderating effect is a unique property of water, it's heat capacity (after ammonia it has the highest heat capacity of any known substance), which allows it to suck up a lot of energy when it vaporizes into a gas form. This allows water to take energy from the equators when it evaporates, and when the air mass eventually cools in the polar regions, it condenses into clouds. All the heat energy that was sucked up when the water turned into a gas is then released when it reverts to liquid or ice, and this is the energy that helps fuel high winds, tornadoes and other storms.
Clouds will form anywhere that warm, moist air is forced upwards (like when it encounters a mountain range, or denser air mass), causing it to expand and cool. The water in the cooler air then can't be supported as a gas any more, and if there is a lot of pollution in the area, water and ice will form into clouds. This can turn a blue, clear-sky day into a roiling thunderstorm as quickly as you can say "nimbus".
For more information about clouds and climate, see: Essentials of Oceanography by Tom Garrison.
For information on pollution and storms, see: http://discovermagazine.com/2008/may/30-wednesday-better-bring-an-umbrella/?searchterm=pollution%20storms