Download Chapter 8: Air Masses, Fronts, and Middle

Chapter 8: Air Masses, Fronts, and
Middle-Latitude Cyclones
Air masses
Fronts
Middle-latitude cyclones
Air Masses
• Air mass– an extremely large body of air
whose properties of temperature and
humidity are fairly similar in any horizontal
direction at any given latitude
• Source regions– regions where air masses
originate
– Over water?
– Over dry land?
– In a cold region?
– In a warm region?
•
Air mass classification
– Air mass classification uses a two letter classification scheme
– First letter:
• c – continental source region
• m – maritime source region
– Second letter:
• P – polar source region
• T – tropical source region
– There are four basic air mass types:
• cP – continental polar air mass
• mP – maritime polar air mass
• cT – continental tropical air mass
• mT – maritime tropical air mass
– There are two additional air mass types for extreme air masses:
• cA – continental Arctic air mass
• mE – maritime equatorial air mass
•
Air Masses of North America
– cP and cA air masses
• Air mass characterized by very cold and dry conditions
•
•
–
•
Source region is northern Canada and Alaska
What types of weather are associated with cP and cA air masses?
– Cold, dry air
– Lake effect snow
cP and cA air masses
• Air mass
characterized by
very cold and dry
conditions
• Source region is
northern Canada
and Alaska
• What types of
weather are
associated with cP
and cA air masses?
– Cold, dry air
– Lake effect snow
Lake-Effect Snow
Cold air goes over warm lake water, picks up moisture, then rises,
creates clouds and snow.
mP air masses
– These air masses are characterized by cool, moist, and conditionally
unstable conditions.
– Source region is the N. Pacific and N. Atlantic oceans
– What types of weather are associated with mP air masses?
• Rain or snow
•
mT air masses
– mT air masses are characterized by warm,
moist conditions
– The source regions for mT air masses are
the sub-tropical Pacific and Atlantic
oceans and the Gulf of Mexico
– What types of weather are associated with
mT air masses?
Figure 8.7 Winter and early spring surface weather patterns that
usually prevail during the invasion of cold, moist mP air into the
mid-Atlantic and New England states. (Green shading area
represents light rain and drizzle; pink region is freezing rain and
sleet, whit is snow)
•
cT air masses
– cT air masses are characterized by hot,
dry conditions
– cT air masses form over the deserts of
Mexico and the southwestern United
States
– What types of weather are associated
with cT air masses?
Fronts
• Front – a transition zone
between two air masses of
different densities
• There are four basic types of
fronts:
– Cold
– Warm
– Stationary
– Occluded
•
•
Stationary Front
– A nearly stationary (not moving) boundary between two air masses
– A stationary front is drawn on weather maps as alternating triangle and
semi-circle symbols on opposite sides of the front
– What types of weather are associated with stationary fronts?
• Clear or cloudy skies
• Light rain possible
Cold Front
– Cold, dry air replaces warm, moist air
– A cold front is drawn on weather maps as a line with triangle symbols,
with the triangles pointing in the direction that the front is moving
towards.
–
•
How do meteorologists
locate fronts on a weather
map?
• Large change in
temperature over a
short distance
• Change in moisture
content of air
• Shift in wind
direction
• Pressure and
pressure changes
• Cloud and
precipitation patterns
Warm Fronts
– Warm, moist air replaces cold, dry air
– A warm front is drawn on weather maps
as a line with semi-circle symbols, with
the semi-circles pointing in the direction
that the front is moving towards.
– What types of weather are associated
with warm fronts?
– Overrunning – rising of warm air up
over cold air
•
Occluded Fronts
– Form when a cold front catches up to a warm front
– An occluded front is drawn on weather maps as a line with triangle and
semi-circle symbols on the same side of the line, with the symbols
pointing in the direction that the front is moving towards.
– Cold-type occluded front (cold occlusion)
• Air behind the cold front is colder than the air ahead of the warm
front
– Warm-type occluded front (warm occlusion)
• Air behind the cold front is not as cold as the air ahead of the warm
front
– Often it is difficult to identify occluded fronts from a surface weather map
only.
Figure 8.17 The formation of a cold occluded front. The faster moving cold front in (a)
catches up to the slower-moving warm front in (b) and forces it to rise off the ground (c).
Figure 8.18 The formation of a warm-type
occluded front. The faster-moving cold
front in (a) overtakes the slower moving
war front in (b). The lighter air behind the
cold front rises up and over the denser air
ahead of the warm front. Diagram (c)
shows a surface map of the situation.
Middle-latitude Cyclones
• Middle-latitude cyclones are also called extratropical cyclones or mid-latitude
cyclones
• Polar Front Theory – a theory that explains the life cycle of mid-latitude cyclones
and their associated fronts
– Polar Front - the boundary that separates cold polar air from warm subtropical air (our old friend from the three-cell model of global atmospheric
circulation)
– Frontal wave – the initial kink that forms on the polar front (b)
–
–
–
•
Wave cyclone – the name given to cyclones that evolve in the manner
shown above
Open wave – fully developed cyclone (c and d)
Warm sector – warm air mass located between the cold and warm fronts
(c, d, and e)
Where do mid-latitude
cyclones tend to form?
– Named after the
region where they
form
– Nor’easters (a
favorite of weather
fans in the
northeastern United
States)– stroms that
fomr along the
eastern seaboard of
the US and then
move
northeastward
Developing Mid-latitude Cyclones and Anticyclones
• Cyclogenesis– development or strengthening of a mid-latitude cyclone
•
•
•
•
What causes mid-latitude cyclones and anticyclones to develop? (or how can we
change the air pressure at the surface?)
Remember, air pressure is just the weight of the overlying air. So, we can change
surface pressure by changing the amount of air over a certain location.
The secret lies in convergence and divergence
Figure 8.23 Convergence, divergence, and
vertical motions associated with surface
pressure systems. Notice that for the
surface storm to intensify, the upper trough
of low pressure must be located to the left
(or west) of the surface low.
Typically we have convergence on the west side of an upper level trough and
divergence on the east side of an upper level trough.
• Convergence (or divergence) can occur due to a change in wind direction and/or a
change in wind speed.
• For a mid-latitude cyclone to intensify (surface pressure decreasing) we need
more divergence than convergence over the center of the cyclone.
• Rising air is found above areas of convergence near the surface or below areas of
divergence aloft.
• Sinking air is found above areas of divergence near the surface or below areas of
convergence aloft.
Waves in the westerlies
• Longwave – widely spaced troughs in upper level flow (typically spaced 1000s of
km apart)
– Can have 3-6 longwaves in the upper level flow of the Northern
hemisphere at one time.
– Tend to move from west to east fairly slowly.
– Meteorologists sometimes refer to longwaves as Rossby waves.
• Shortwave - small
disturbance (trough)
embedded in upper level flow
– Tend to move rather
quickly from west to
east.
• These short and longwaves
are associated with areas of
convergence and divergence
and as such play an important
role in the life cycle of midlatitude cyclones.
The jet stream and mid-latitude cyclones
• Jet streak – core of strongest jet stream winds
• Areas of upper level convergence and divergence occur near jet streaks.
• Therefore, jet streaks are also important in the development of mid-latitude
cyclones.
• What seasonal changes occur in the jet stream and how does this affect midlatitude cyclones?
Figure 8.24 (a) As the polar jet stream and its area of maximum winds (the jet streak or MAX), swings
over a developing mid-latitude cycone, an area of divergence D draws warm surface air upward, and
an area of convergence (C) allows cold air to sink. The jet stream removes air above the surface storm,
which causes surface pressures to drop and the storm to intensify. (b) When the surface storm moves
northeastward and occludes, it no longer has the upper-level support of diverging air, and the surface
storm gradually dies out.