Clouds are formed by condensation. Stratus clouds are a greyish color, and usually cover nearly the entire sky. They are similar to fog, although they don't touch the ground. Nimbostratus clouds usually are a dark grey, wet cloud that can cause snow. They usually can cause light rainfall. When the prefix 'alto' is used, it means the clouds have a mediam height in the air. Altocumulus clouds are puffy, grey or white and can cause droplets of rain. If these clouds are spotted in the early morning of summer, it usually means that there will be thunderstorm in the late afternoon. Altostratus clouds are blue-grey or grey clouds made up of water droplets and ice crystals. These clouds usually cover the entire sky, and when the cloud gets thinner, the sun appears as a disk in the sky. These clouds often form before a storm comes. The thin clouds are cirrus clouds, and are usually blown into small streamers by high winds. They move accross the sky from west to east, and are most commonly pleasant. The clouds that resemble pieces of cotton are the cumulus clouds. The top looks like a calaflower, and has a tall tower. These grow upward, and doing so, can form into a thunderstorm cloud, also called the cumulonimbus. They are formed if the cumulus congestus clouds continue to grow upward.
For more info, please go to eo.ucar.edu/webweather/cloud3.html
