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WEATHER
SYSTEMS
AIR MASSES
• A huge expanse of air covering
thousands of square kilometers
• Relatively uniform horizontally in
temperature and humidity
• The properties of an air mass depend
upon the type of surface over which it
develops
• This surface called the source region
TYPES OF AIR MASSES
•
•
•
•
•
Continental Polar (cP)
Maritime Polar (mP)
Continental Tropical (cT)
Maritime Tropical (mT)
Arctic (A)
TYPES OF AIR MASSES
Region Stability
Type Winter
Summer
A
Stable
cP
Stable
Stable
cT
Unstable
Unstable
mP (Pacific)
Unstable
Unstable
mP (Atlantic)
Unstable
Stable
mT Unstable
Unstable
Characteristics
Winter
Summer
Bitter cold, dry
Very cold, dry
Warm, dry
Hot, dry
Mild, humid Mild, humid
Cold, humid Cool, humid
Warm, humidWarm, humid
CHANGES TO AIR MASSES
• Air masses move, which changes
their temperature, humidity, and/or
stability
• Changes result from
• (1) exchange of heat or moisture
• (2) radiational heating/cooling
• (3) adiabatic heating or cooling
WEATHER FRONTS
• A front is a narrow zone of transition
between air masses that differ in
density
• This is usually due to temperature
differences, so the most common
fronts are warm and cold fronts
• Two other types: stationary and
occluded fronts
STATIONARY FRONT
• Little or no horizontal movement
• That trailing air mass slowly
advances on the original air mass
• Sometimes associated with clouds
and precipitation
• When precipitation falls, it is usually
on the cold side of the front
WARM FRONT
• A warm air mass advances onto a
cooler air mass
• As it advances, clouds develop and
gradually lower and thicken
WARM FRONT
• A warm air mass advances onto a
cooler air mass
• As it advances, clouds develop and
gradually lower and thicken
• If precipitation forms, it is usually not
intense but may last for hours
COLD FRONT
• A cold air mass advances pushing
out the warmer air mass
• If the advance is slow, precipitation is
usually showery and brief
• If the warm front ahead is unstable,
severe weather is likely to form as tall
cumulonimbus clouds develop
OCCLUDED FRONT
• Also called an occlusion
• Results when a cold front advances
on another cold front
OCCLUDED FRONT
• There are 3 types of occlusions, due
to 3 combinations of cold air masses
• All 3 types will lift warm air, which can
lead to precipitation
• Occluded fronts can be difficult to
detect from surface data, because
the temperature difference may be
small
HIGHS AND LOWS
• Air masses usually have either higher
or lower pressures compared to the
surrounding air
• These are marked on a map with
either High or Low
HIGHS
• Also called “anticyclones” or “fairweather systems”
• Viewed from above (N Hemi), surface
winds blow clockwise and outward
• Highs usually track east and
southeast
LOWS
• Also called “cyclones” or “stormyweather systems”
• Viewed from above (N Hemi), surface
winds blow counterclockwise and
inward
• Lows usually track east and
northeast
SUMMARY