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Cirrus clouds are the most common of the
high-level clouds. These clouds are mainly
composed of ice crystals, which come from
the freezing of supercooled water droplets
and exist at heights (usually above six km)
where the temperatures are typically below
-38 C. Cirrus are thin, wispy and usually
white in appearance. They generally occur
in fair weather and move from west to east
across the sky, indicating the direction of
the prevailing winds. Cirrus can come in
several shapes and sizes, from the fingershapes seen during pleasant weather
conditions to the uniform texture of more
extensive cirrus clouds, which can be the
first sign of an approaching warm front.
Stratus Clouds
Stratus clouds are uniform grayish clouds that often cover the
entire sky. They resemble fog that does not reach the ground.
Usually no precipitation falls from stratus clouds, but
sometimes they may drizzle. When a thick fog "lifts," the
resulting clouds are low stratus.
Altostratus clouds are gray or blue-gray middle level clouds
composed of ice crystals and water droplets. These clouds
usually cover the entire sky. In the thinner areas of the cloud,
the sun may be dimly visible as a round disk. Altostratus
clouds often form ahead of storms that will produce continuous
Cumulus Clouds
Cumulus, the puffy, cottonball cloud most often seen on a summer
day, is probably the most familiar to cloudgazers. These clouds have a
flat base and distinct outlines, and their colors range from white to light
gray. The bases can be as low as 1 km above the ground, and they
usually have a diameter of about a kilometer. Given the right
conditions, cumulus clouds can develop into larger, more towering
clouds, which usually means precipitation is about to occur.
Cumulonimbus clouds are much larger and more vertically
developed than the fair weather cumulus cloud. It can exist as
a single towering cloud, or can even develop into a line of
such towers, also known as a "squall line." Fueled by
vigorous convection, these clouds can climb vertically in the
atmosphere to an altitude of 12 km or higher. The lower
portion of the cloud is made up of water droplets, while at the
cloud top, where temperatures are well below 0 C, there are
primarily ice crystals. These clouds are capable of developing
into great thunderheads which contain all forms of
precipitation: snowflakes, snow pellets, large raindrops, and
sometimes hailstones. Lightning, thunder, and even violent
tornadoes sometimes accompany intense cumulonimbus
Cumulonimbus Clouds
High Pressure
High pressure
systems mean
sunny, clear days.
Low pressure
systems bring
storms or days of
An air mass is a large body of air.
• Forms by staying in the same place for
several days
• Covers large areas
• Same temperature
• Same moisture throughout
Front: the boundary between 2 air
Cold Front
Warm Front
• Forms when a cold air
mass overtakes a warm air
• The warm air rises cooling
water vapor rapidly
forming cumulonimbus
• Storms form producing
heavy rain
• Forms when a warm air
mass overtakes a cold air
• Warm air slides over the
denser colder air
• Cirrus clouds first then
low gray stratus
• Steady rains lasting
several days
Warm front – not as
nice as it sounds
Cold front
A strong, persistent updraft of warm moist air is formed and
lifted by the approaching cold front. Speeds in an updraft can be
as fast as 90 miles per hour! The air cools as it rises,
condenses, and forms cumulus clouds. When condensation
occurs, heat is released and helps the thunderstorm grow.
The cumulus cloud has grown into a cumulonimbus cloud at
above about 30,000 feet (about 9 km).
Stationary Front warm and cold air
battle to a standoff
Cold Front
Cumulonimbus clouds start to
build creating thunderstorm
Write the type of front and complete the
white blanks on the graphic.