Download Weather Study Guide

Weather Study Guide
Ms. Coonan
2013
Air Masses
 A large volume of air in the atmosphere that is relatively uniform in temperature
and moisture.
 Air masses form over large surfaces with uniform temperatures and humidity, called
source regions.
 Low wind speeds let air masses remain stationary long enough to take on the
features of the source region, either hot or cold. When wind moves the air mass,
they carry the weather conditions (hot or cold, dry or moist) with them to a new
region. When one an air mass collides with another of varying conditions, this
causes a storm.
 Air masses can form over land, or over water. Air masses that form over water are
humid, and air masses that form over land are dry.
 Air masses can either be hot, or cold, depending on the region where they originate.
Air masses that develop near the Arctic are cold, and air masses that develop near
the equator, are hot.
Fronts
 A boundary separating two different air masses; one generally being denser, with
different temperatures and levels of humidity.
 Clashing air masses causes various types of weather; snow, rain, wind, cold days and
hot days.
 Cold fronts are often associated with thunderstorms or other severe weather. They
tend to travel from West to East. Cold fronts move faster than warm fronts because
the air is denser, meaning there are more molecules of material in cold air than in
warm air.
 Cold fronts tend to over take stationary warm fronts. Cold, dense are seeps inbetween the warm less dense air, causing the warm air to rise. Because the warm
air is lifted instead of being pressed down, the movement of a cold front, through a
warm front, is called a low-pressure system. Low-pressure systems often cause
rainfall or thunderstorms.
 Warm fronts are generally seen after precipitation. When warm fronts take over
cold air masses, they bring high-pressure systems, where warm air is pressed close
to the ground, causing calm, clear weather. Because warm fronts aren't as dense or
powerful as cold fronts, they bring more moderate and long-lasting weather
patterns.
Clouds
 Clouds are classified into three main groups: cirrus, stratus, and cumulus.  Cirrus clouds are wispy, curly, or stringy. They are found high in the atmosphere—
typically higher than 6,000 meters (20,000 feet)—and are usually made of ice
crystals. Cirrus clouds usually signal clear, fair weather. Their shape often indicates
the direction the wind is blowing high in the atmosphere. 
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Stratus clouds are horizontal and stratified, or layered. Stratus clouds can blanket
the entire sky in a single pattern. They usually occur close to the Earth. Stratus
clouds often form at the boundary of a warm front, where warm, moist air is forced
up over cold air. This movement produces clouds as the moist air is cooled across
the entire front. The presence of stratus clouds usually means a chilly, overcast day.
If precipitation falls from stratus clouds, it is usually in the form of drizzle or light
snow. Cumulus clouds are large and lumpy. They can stretch vertically into the
atmosphere up to 12,000 meters (39,000 feet) high. Cumulus clouds are created by
strong updrafts of warm, moist air. Most forms of heavy precipitation fall from
cumulus clouds. The weather they bring depends on their height and size. The
higher the base of a cloud is, the drier the atmosphere and the fairer the weather
will be. Clouds located close to the ground mean heavy snow or rain.
Clouds are also classified according to how high they are in the atmosphere and
what kind of weather they produce.
The prefix "cirro-" refers to clouds that lie more than 6,000 meters (20,000 feet)
above the Earth.
Cirrocumulus and cirrostratus clouds are two examples of these “high-level” clouds.
The prefix "alto-" indicates clouds whose bases are between 2,000 and 6,000 meters
(6,500-20,000 feet) above the Earth, such as altocumulus and altostratus clouds.
They are considered "mid-level" clouds and are mostly made of liquid water
droplets, but can have some ice crystals in cold enough temperatures.
The prefix "nimbo-" or the suffix "-nimbus" are low-level clouds that have their
bases below 2,000 meters (6,500 feet) above the Earth. Clouds that produce rain
and snow fall into this category. Two examples are the nimbostratus or
cumulonimbus clouds. Nimbostratus clouds bring continuous precipitation that can last for many hours.
These low-level clouds are full of moisture. Cumulonimbus clouds are also called thunderheads. Thunderheads produce rain,
thunder, and lightning. Many cumulonimbus clouds occur along cold fronts, where
cool air is forced under warm air. They usually shrink as evening approaches, and
moisture in the air evaporates. Cumulonimbus clouds gradually become
stratocumulus clouds, which rarely produce rain