The main substance in clouds is water vapor, which condenses into tiny water droplets or ice crystals when temperatures drop. This process occurs when warm, moist air rises and cools, leading to condensation around tiny particles like dust or salt in the atmosphere. As these droplets cluster together, they form clouds. The water droplets in clouds are usually small, which allows sunlight to pass through them, giving clouds their characteristic opacity. Clouds play a crucial role in the Earth's weather systems and water cycle, facilitating precipitation.