As water vapor rises, it expands, cools, and condenses into water droplets. It is these water droplets that form clouds — like the cirrus clouds shown above. If it's cold enough, the water vapor freezes into ice crystals. Water vapor can raise due to a number of different reasons. First, the heating of the air close to the earth's surface can cause air to raise. Second, air can be forced to rise as it is pushed up a hill or mountain. Third, air may be forced to rise as it is pushed on top of another air mass. (For example, a warm air mass moving over a colder mass.) ALL OF THESE CONDITIONS CAN CAUSE CLOUDS TO FORM! Let's think; we often see clouds near the mountains and when weather fronts collide.

Some Latin Roots for Clouds Names

Alto - means medium height
Cirrus - means "curley, hooded, or feathery"
Cumulus - means "heap"
Numbus - usually refers to a rain cloud
Stratus - means "layered"

Types of Clouds

There are three major types of clouds:

1. Cumulus
2. Stratus
3. Cirrus

Cumulus are low level clouds (less than a mile high) and are those white fluffy clouds you often see on a summer's day. These clouds typically have a flat bottom. Cumulus clouds that form at higher altitudes are called alto cumulus, of mid-height culus clouds. These clouds are between two and four miles above the Earth. Cumulonumbus clouds are towering clouds stretching from one mile to eight miles high and can produce thunder, lightning, and rain.

Stratus clouds are the low "layered" clouds that produce gloomy, overcast days. When stratus clouds form between one and three miles high, they are called altostratus. Altostratus clouds are usually not as dense as stratus clouds so you may be able to see the Sun, which you may not be able to do if you were looking through Stratus clouds. Stratus ("layered") clouds that produce rain or snow are called nimbostratus. If the air is cold enough, ice crystals develop within nimbostratus clouds. When the ice crystals get big and heavy enough, they fall as (you guessed it) snowflakes!

The last major type of cloud is called cirrus, meaning curly or wispy. These clouds are usually high level clouds (at least three miles high and typically much higher). Because the air temperature is very low for high altitude clouds, these clouds are made of ice crystals rather than water droplets. Cirrus clouds that are layered are called cirrostratus clouds and cirrus clouds that form clumps are called cirrocumulus.

Can you guess what you call clouds that are on the ground?

Typical altitudes for various types of clouds

Cumulus Clouds

 

Cirrus Clouds

 

Nimbostratus Clouds

References
Elsom, Derek M., Weather explained:  a beginner's guide to the elements, Henry Holt and Company, Inc., 1997

McMillan, Bruce, The Weather Sky, Harper Collins, 1991.

Kahl, Jonathan D., Weatherwise:  Learning about the Weather, Lerner Publications Company, 1992