11.3 CLOUDS AND PRECIPITATION
Condensation level Air masses can change temps without being heated or cooled. When temps change w/o an increase or
decrease in temp is called an adiabatic process. For example, an air mass heats when is sinks and cools when it rises.
Adiabatic heating occurs when air is compressed, and adiabatic cooling occurs when air expands.
Cloud Formation A cloud can form when a rising air mass cools. As an air mass is heated, it becomes less dense than the
surrounding air, therefore it gets pushed up by the more dense cold air around it. When the warm air rises, it expands and
cools adiabatically. This rising and cooling will cause water vapor to condense. The water vapor will condense around a
condensation nuclei. A condensation nucleus is a small particle in the atmosphere around which water droplets will form.
These can be either in liquid or ice formations. When enough of them become large enough, a cloud is formed.
Atmospheric Stability The stability of air masses determines the type of cloud that forms.
Stable Air When air masses stop rising (resists rising) and sink back down, the air is stable. Fair weather clouds are formed
this way.
Unstable air When air masses continue to rise, it indicates unstable air. The clouds produces by unstable air are associated
with thunderstorms.
Atmospheric lifting Clouds can form when moist air rises, expands and cools. When air rises and becomes warmer than
the surrounding air it is called convective lifting.
Orographic lifting when clouds form because air is forced to rise over elevated land such as mountains. The rising air mass
expands and cools, with water droplets condensing when the temp reaches the dew pt. This is the windward side of the
mountain and they are some of the rainiest places on Earth.
Convergence Air is lifted because air flows into the same area but from different directions. When a warm air mass comes in
form one direction and a cold air mass comes in from another, the warm air is forced up and this will produce thunderstorms .
Types of Clouds The different types and shapes of clouds is due to them forming at different altitudes, and through
different processes. Clouds are classified by their altitude in which they formed and by their shape. There are 3 classes of
clouds based on their altitudes: low - below 6561 feet, middle – between 6561 and 19685 feet and high – above 19685
feet.
Low clouds – When warm moist air rises, expands and cools. In stable conditions, the mass will stop rising when it reaches
surrounding air of the same temp. The cloud will flatten out and the winds will spread it horizontally into a stratocumulus or
layered cumulus cloud. Cumulus clouds are puffy clouds below 6561 feet. Stratus is another cloud that forms below 6561
feet. These are layered sheet like cloud that covers much of the sky in an area. Stratus clouds usually form when fog is lifted
from the ground.
cumulus
stratus
Middle clouds – Altocumulus and altostratus clouds form at altitudes between 6561 and 19685 feet. They are made up of
ice crystals and water droplets because of the colder temps at this altitude. These clouds are usually layered. Altocumulus are
usually white or gray and form large round masses or way rows. Altostratus clouds have a gray color and they form thin
sheets of clouds. These clouds will sometimes produce mild precipitation.
Altocumulus
Altostratus
High clouds – These clouds form above 19685 feet and are made up of ice crystals. Temps here are below freezing. Cirrus
clouds have a wispy appearance. Cirrostratus forms continuous layers that can cover they sky.
Vertical development clouds If the air is unstable then the cloud will be warmer than the surrounding air and it will
continue to rise. As it rises, water vapor will condense and the air continues to increase in temp. The cloud can grow and
become a towering cumulonimbus. If the conditions are right, it can reach the tropopause. Its top is then entirely ice crystals.
Strong winds will spread the top into an anvil shape, (flatten it out) and it is now capable of producing strong winds, torrential
rain, and hail. (thunderstorms)
Coalescence When cloud droplets collide and join together to form a larger droplet. When they become too heavy they fall to
Earth.