How Clouds Form

A cloud is composed of tiny water droplets or ice crystals that are suspended in the air. A series of processes have to happen in order for these water droplets or ice crystals to form into clouds in the atmosphere, and different types of clouds form from different processes. The four main ways that clouds can form are:

• Surface Heating

• Cloud Formation Due to Surface Heating

  Some clouds form due to the heating of the Earth's surface. First, the Sun heats the ground, which then heats the air. This warm air is lighter and less dense than the surrounding air, so it begins to rise. When this air rises, it expands because of the lower pressures that exists at higher levels in the atmosphere. Anytime air expands because of a drop in pressure, it also gets cooler. So when the rising air expands it also cools.
  The cooling air that is rising is no longer able to hold all of the water vapor it was able to hold when it was warm. This extra water vapor begins to condense out of the air parcel in the form of liquid water droplets. As the air parcel rises higher and higher, it continues to expand and get cooler, and more moisture condenses out of it.
  Eventually, enough moisture will condense out of the air parcel to form a cloud! The types of clouds that form from the process of surface heating are cumulonimbus (and associated mammatus clouds), cumulus, and stratocumulus.
  In contrast to surface heating, the cooling of the Earth's surface leads to the development of fog and stratus clouds.
   
• Mountains and Terrain
• Air Masses Being Forced to Rise
• Weather Fronts (cold or warm)

All of these processes involve the cooling of air. Warm air is able to hold larger amounts of water vapor than cool air, so when air cools it is no longer able to hold all of the water vapor it was able to hold when it was warm. This extra water vapor begins to condense out of the air into liquid water droplets.
Typically, water vapor needs some sort of particle, such as dust or pollen, to condense upon. These particles are called condensation nuclei. Eventually, enough water vapor will condense upon condensation nuclei to form a cloud. The water droplets in the cloud may eventually fall down to Earth in the form of rain or snow (or other forms of precipitation).

Cloud Formation Due to Mountains

Some clouds form when air encounters a mountain range or other types of terrain. When this happens, the air will rise and cool, and this cooler air is no longer able to hold all of the water vapor it was able to hold when it was warm. The extra water vapor begins to condense out of the air parcel in the form of liquid water droplets and a cloud is formed.
The types of clouds that form from encounters with mountains are stratus clouds and lenticular clouds.
The image on this page shows how winds can blow into a mountain range and then rise higher in the atmosphere. The side of the mountains where the wind starts is called the windward side. The side of the mountains where the wind leaves the area is called the leeward side.
Another way that mountains cause cloud formation is when air rises because the mountain is warmer than the surrounding air and causes the air to rise. Once the air rises, it follows the same process to form clouds as described above. The types of clouds that form in this case are cumulonimbus (and associated mammatus clouds), and cumulus.

Clouds Formed by Air Being Forced to Rise

Some clouds form when air at the surface of the Earth is forced to rise. There are three processes that force air to rise.
First, in a low pressure system, wind moves in towards the center from all directions because air moves from high to low pressure. When this air meets in the center, there is nowhere for the air to go but up. Air is also forced to rise when it is traveling over land that slopes upward. The air cools as it rises, and eventually clouds will form. Finally, weather fronts produce clouds by causing air to rise when the lighter warm air flows over the heavier cold air.
All of the cloud types are formed by these processes, especially altocumulus, altostratus, cirrocumulus, stratocumulus, or stratus clouds

Cloud Formation Due to Weather Fronts

Weather fronts can cause clouds to form. Fronts occur when two large masses of air collide at the Earth's surface.
Warm fronts produce clouds when warm air replaces cold air by sliding above it. Many different cloud types can be created in this way: altocumulus, altostratus, cirrocumulus, cirrostratus, cirrus, cumulonimbus (and associated mammatus clouds), nimbostratus, stratus, and stratocumulus.
Cold fronts occur when heavy cold air displaces lighter warm air, pushing it upward. Cumulus clouds are the most common cloud types that are produced by cold fronts. They often grow into cumulonimbus clouds, which produce thunderstorms. Cold fronts can also produce nimbostratus, stratocumulus, and stratus clouds.