1. Water vapor: This is the invisible gas form of water that exists in the atmosphere.

2. Condensation nuclei: These are tiny particles like dust, smoke, salt, or pollen that water vapor can condense around.

Here's how it works:

* As warm, moist air rises, it cools.

* Cooler air can't hold as much water vapor, so the water vapor condenses around condensation nuclei.

* These tiny water droplets or ice crystals form the visible cloud.