Rain is composed of water molecules made up of hydrogen and oxygen. Each water molecule contains two hydrogen atoms and one oxygen atom. The hydrogen atoms have two electrons and the oxygen has eight electrons. This combination is called a compound and forms the water molecules necessary to create clouds.
Clouds form when water vapor cools down in the air that surrounds it. This cooling causes the vapor to change back into water droplets in the atmosphere. Those droplets bind together and form the clouds that eventually bring the water back into oceans, lakes, rivers and the ground.
Not all clouds are rain clouds. Cirrus clouds are clouds that form higher up in the atmosphere and appear to be feathery. Cumulus clouds are thick in appearance and have the ability to change rapidly. Clouds that rain have either "nimbo" or "nimbus" in the name. For example, cumulonimbus clouds are thick clouds that produce rainstorms and thunderstorms. Nimbostratus clouds are layers of clouds that are higher in the atmosphere; they appear like sheets of clouds across the sky and produce drizzle and snow.
Acid rain has deposited materials from the atmosphere that contain more nitric and sulfuric acids than what is determined to be normal by the Environmental Protection Agency. These deposited materials can come from volcanoes, decaying vegetation, or man-made sulfur dioxide and nitrogen oxide emissions that are produced during fossil fuel combustion. In the case of acid rain, the deposited materials mix with the elements in rain, creating a different compound than normal rain. This acid rain can be damaging to plant and human life.