AOS 100: Weather and
Climate
Instructor: Nick Bassill
Class TA: Courtney Obergfell
Miscellaneous
• Exams & Homeworks
Review of September 29th: Intro to
Cyclones and Fronts
• Cyclones are areas of low pressure that
have winds which rotate counterclockwise
around the center (in the northern
hemisphere)
• Extratropical cyclones affect areas roughly
north of 30º N
• These cyclones exist because of the
different amounts of heating across the
planet, so they act to move warm air
poleward and cold air equatorward
Review Continued
• Extratropical cyclones often have fronts
• Fronts are areas that separate two airmasses,
often warm vs. cold, and/or dry vs. moist
• Cold fronts are locations where warm air is being
replaced by cold air
• Warm fronts are locations where cold air is being
replaced by warm air
• Cold fronts are often sharper than warm fronts,
partially because it is easier for more dense air to
replace less dense air
Fronts
• The intersection of different airmasses are
called fronts
• The most common type are Warm Fronts
and Cold Fronts
• Fronts typically
move in the direction
the triangles or
semicircles are
pointing
Fronts Continued
• Fronts are commonly located near regions
of precipitation
• This is due to the fact that fronts are often
areas where warm and moist air is forced
to rise
• There are four types of fronts:
- Cold Fronts
- Warm Fronts
- Occluded Fronts
- Stationary Fronts
Cold Fronts
• This often acts to focus upward vertical motion,
leading to a narrow band of precipitation
• Severe weather is commonly observed in
advance of the cold front
Warm Fronts Continued
• A warm front has a much shallower slope
compared with cold fronts
• Precipitation associated with warm fronts is often
lighter and has a greater coverage than with
cold fronts
A Comparison
Occluded Fronts
• Due to the different densities of
cold and warm air, cold fronts
typically move faster than warm
fronts
• Therefore, sometimes the cold
front “catches up” to the warm
front, especially with strong, longlived cyclones
• When this occurs, an occluded
front is formed (often depicted in
purple)
Occluded Fronts
Continued
• Recall that
precipitation is
produced in regions
where warm moist air
is forced to rise
• Once an occluded
front has formed,
there is no source of
warm air, so
precipitation often
weakens in intensity
From: http://www.ux1.eiu.edu/~jpstimac/1400/FIG09_008.jpg
Stationary Fronts
• As the name implies, stationary fronts are very
slow moving (if they move at all)
• Stationary fronts separate cold air from warm
air, and can be associated with precipitation
• Stationary fronts frequently turn into cold fronts
or warm fronts
From:
http://www.ux1.eiu.edu/~jpsti
mac/1400/FIG09_008.jpg
Cyclone Life-cycle and Front
Evolution
Finding Fronts
• Fronts will often be associated with all or most
of:
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–
–
A locally strong temperature gradient
Relatively lower pressure
Clouds and precipitation
Converging (coming together) winds
Positive vorticity (i.e. a sharp turning of the winds
cyclonicly)
– Locally strong moisture gradients
• It is often helpful to first locate the location of
lowest pressure, since fronts normally originate
from it
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L
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Fronts in Other Images
• Besides looking at surface observations,
fronts can often be seen in other ways
• If precipitation is associated with the front,
they can be seen on radar images
• The clouds associated with fronts can be
seen on satellite images also
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How Do Low Pressures Form?
• Keep in mind that pressure is basically just
a measure of the mass of air above the
surface (or some other level)
• So a surface pressure of 990 mb means
that the weight of the atmosphere above
that location is less than a spot with a
pressure of 1000 mb
• This means that to decrease surface
pressure, the mass of the atmosphere
above that location needs to decrease
Low Pressure Formation Continued
• In order to decrease the mass of the column of air
above the surface (i.e. to form a low pressure
system), you need a net divergence of air in that
column
• Conversely, to strengthen a high pressure system,
you need a net convergence of air in that column
• This is where features in the upper levels of the
atmosphere become important
• But first, we need an understanding of atmospheric
forces
Why Are Forces Important?
• When we speak of “forces,” we’re really
describing why the air in the atmosphere
moves the way it does
• This describes both how the air moves
horizontally (which direction it moves), and
why the air moves vertically
• For example, precipitation occurs because
something forces air upward until it
condenses
Horizontal Movement
• When a studying the horizontal movement
of air, three primary forces are often
discussed:
- The Pressure-gradient force (or PGF)
- The Coriolis force
- The Frictional force