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GY205 Weather and
Climate
Lecture 5
(Please turn in homework on the table)
Air Masses
• Very large bodies of air with fairly uniform
temp and humidity characteristics
• Form in high and low latitudes, not midlatitudes
• Air takes on the characteristics of the
source region
• Air masses migrate from their source
regions, bringing changes in weather to
other places
Air Masses
Fronts
• Fronts are boundaries
between air masses
• Cold air is more
dense than warm air,
they don’t easily mix
• Colder air will stay on
the surface, warmer
air will be forced
above cooler air
• Adiabatic cooling of
the warm air can lead
to clouds and precip
Map symbols
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Cold Front
Cooler air displacing warmer air
Fastest moving of fronts
Steep frontal surface marked by cumuliform clouds
Can produce short-lived heavy precip, t-storms
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Warm Front
Warmer air displacing cooler air
Move slower than cold fronts
Gently sloping surface marked by stratus-type clouds
Can produce extended periods of precip
Cold Front and Warm
Fronts
Stationary Front
• Nonmoving boundary between warmer and
cooler air masses
• Often gray skies, possibly prolonged precip
Occluded Front
• An occluded front forms
when a cold front catches
up with a warm front
• Warm air is cut off from
the ground
• Often produces heavy
precip
Drylines
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Boundary between humid and drier air
Humid air is less dense, will be lifted by advancing drier air
Form during spring and summer in the southern Great Plains
Produce t-storms, often tornadoes
Mid-latitude cyclones (MLC’s)
• Large systems that travel thousands of miles over
several days, bringing precipitation, sometimes severe
weather
• Consists of well-developed fronts circulating about a lowpressure center
• Not to be confused with tropical cyclones (hurricanes)
Distinctive comma-shaped cloud pattern of a mid-latitude cyclone
Life Cycle of MLC’s
• Begin along linear stationary polar front
Cyclogenesis (A Storm is Born)
• A disturbance (topographic or from upper-level
winds) along the polar front causes a bend, or
wave, to form
• Warm air pushes north, cold air pushes south,
starting counterclockwise rotation
Mature Cyclone
• Distinct fronts develop and low pressure
intensifies
• Weather associated with fronts develops
• Storm migrates in a generally eastward direction
Occlusion Develops
• Faster cold front begins to catch up with slower
warm front
Cyclone Dissipates
• Occluded front grows, eventually the storm
system dies out
MLC Life Cycle
MLC in Motion
Upper-level Winds and MLC’s
• Rossby waves – long waves in the flow of the midlatitude Westerlies, formed by air flow along troughs and
ridges
• Air flow along Rossby waves produces high pressure
(anticyclones) and low pressure (cyclones) at the surface
• Air entering a trough produces upper-level
convergence, pushing air downward to create
high pressure at the surface
• Air leaving a trough produces upper-level
divergence, drawing air upward to create low
pressure at the surface
Weather Associated with MLC’s
• As a mid-latitude cyclone migrates eastward, it
brings changes in the weather along its fronts
• Clouds and precip move through, winds shift,
pressure changes, temps rise then fall
Paths of MLC’s
• MLC’s are steered by upper-level winds,
following the path of the jet stream
Common Winter Storm Tracks
GY205 Weather and
Climate
End of Lecture 5