- dust particles
- temperature or pressure changes
Water molecules must have a surface on which to collect. The air is constantly full of water. When you look into the sky and see a cloud, it is actually moisture you are seeing. Most of the time, water vapor in the air cannot be seen unless it collects and condenses to form a cloud.
Clean air (without any dust or particles) will not produce clouds without supersaturation (or relative humidity above 100%). Luckily for us, Mother Nature and humans put lots of Cloud Condensation Nuclei or dust into the atmosphere. This dust comes from sources such as volcanoes, cars, sea spray from the ocean, and fires. Other particles in the atmosphere, including bacteria, can also play a role in serving as condensation nuclei.
When temperatures or pressures decrease, the air cannot hold as much water. Go to the dewpoint temperature definition to see a great diagram of relative humidity and dewpoint temperature. The lower the temperature, the greater the condensation of gaseous water to liquid water. Note: I have simplified the language on air "holding water", but there are inherent misconceptions that can be developed as a result. Please read Relative Humidity Misconceptions for a full explanation of the physics of relative humidity.
Once the stage is set, cloud formation happens as water coalesces (or joins together). Now, learn to simulate cloud formation by making a cloud in a bottle.