Clouds are made of tiny drops of water or small crystals of ice. Water vapor rises into the air, cooling and condensing into droplets of water, or if the air is cold enough, crystals of ice. When enough water vapor condenses into billions of ice crystals or drops of water, a cloud forms. Depending on how it forms, it becomes one of several types of clouds. The three main types found in the sky are cumulus, stratus and cirrus. They each have many derivatives.
Cumulus clouds are white and fluffy, like cotton balls in the sky. These form when warm, moist air rises quickly from the ground and cools off rapidly. They can form in clusters, and often are seen over the sea at regular intervals. A cumulus cloud may break up in about ten minutes. When they turn dark gray, they are called cumulonimbus clouds, and can produce rain, hail, or lightning. If the name has the suffix nimbus, it means precipitation.
Stratus clouds are flattened sheets of cloud that may stay in place for some time. They cause overcast weather or rain. Nimbostratus clouds are formed when air rises very slowly over a large area, and promise long steady rain. They resemble heavy gray blankets stretched out in the sky.
Cirrus clouds form very high in the sky, and are made up completely of ice crystals. They are wispy and light, and look a bit like feathers in the sky. If enough are in the sky that they seem to run into each other, they are called cirrostratus clouds, which look like a white veil in the sky.
Fog is similar to clouds in that they are both are made of tiny droplets of water. Clouds form much higher in the sky than fog, which forms at ground level. Fog is formed on calm, cool nights, because the ground is cold. The water vapor in the air condenses into droplets of water near the ground, filling the air with these droplets, and creating fog. These droplets of water are so small that it takes 7 trillion of them to make 1 tablespoon (14.78 ml) of water.