Clouds are comprised primarily of water and dust. They may contain various impurities in the air or the water which evaporated to provide the water vapor accumulated within the clouds. The relative density of this water vapor contributes to the way in which these clouds form. More water may create larger and fluffier clouds, while sparse water creates thin and high-flying clouds. The water cycle contributes to the way in which clouds are formed.
The water cycle is essential to the weather systems on earth. It begins with liquid water in the oceans or lakes. As this water becomes heated by the sun, it evaporates creating water vapor. The water vapor rises due to its lower density than the nitrogen rich air around it. As it reaches the upper atmosphere, the lower temperatures cause the water to condense around dust particles in the air. The resulting condensates are known as clouds.
The condensed water known as clouds come in several different forms. The vocabulary which defines clouds has a rigid structure. For example, "nimbus" comes from Latin meaning rain. Three distinct layers of clouds exist. The lowest level of clouds most often contain rain. These clouds are commonly known as "cumulous" clouds. Cumulous comes from a Latin root meaning heap. They are known as heaps because they form from the lowest level up essentially heaping on top of one another creating denser and denser clouds.
Light from the sun creates the water vapor which forms clouds, but beyond this, light is responsible for the appearance of clouds. When water passes from one medium (air) to another (water) it is "refracted." This process bends the light. As it bends, the water creates a prismatic effect. This effect is responsible for rainbows after a rain storm. Light can also be absorbed by solid materials such as the dust within the clouds.
As the light is refracted within the cloud, it is bent so much that it escapes in many directions instead of passing straight to the ground. As the clouds bend the light within themselves, the dust absorbs some of the light. The denser the clouds become the more light they reflect and refract, thus becoming darker and grayer. A dense cloud is more likely to become heavier than the air around it and to fall to the ground as rain.