AIR MASSES, FRONTS AND MIDLATITUDE CYCLONES
Vicki Drake
Department of Earth Sciences
Santa Monica College
I.
Air Masses
a. Large body of air whose properties of Temperature and Humidity are
similar in any horizontal direction at any given altitude.
b. Covers thousands of km2
II.
Source Regions for Air Masses
a. Must be flat (no topographic features, i.e., mountains, valleys, etc.)
b. Must be of uniform composition (all water or all landmass – no mixture
c. Must have little to no surface winds
d. Areas usually dominated by High Pressure Cells – air can “stagnate”
and take on source region characteristics (dry, wet, hot, cold, etc.)
III Classification of Air Masses
a. cP – continental polar – low temperatures and low humidity
1. Alaska, Canada, Siberia, Northern Russia
2. Mixes over “warmer” Great Lakes = “Lake-Effect Snows”
in Buffalo, New York (for example)
b. cA – continental arctic – very low temperatures and very low humidity
1. Northern Canada/Arctic Circle, Greenland
c. mP - maritime polar – low to medium temperatures, and high humidity
1. Gulf of Alaska, North Atlantic Ocean
2. cP air moving offshore modified in Gulf of Alaska – cool,
moist and unstable air
d. mT- maritime tropical – medium to high temperatures, and high humidity
1. Gulf of Mexico, East Coast of Florida, West Coast of Baja
2. Winter source of Pacific mT is subtropical East Pacific as
air travels 1600 km towards S. California
3. “Pineapple Express” may produce heavy ppt in CA
4. East of Rockies – source region – Gulf of Mexico; confined
to southern states, but may move into Central Plains in
winter producing mild temperatures and fog, heavy ppt.
e. cT – continental tropical – very high temperatures and very low humidity
1. Northern Mexico and arid Southwest USA (Sonoran
Desert) – summer-time
2. Hot, dry and unstable at low levels
3. Low RH means air must rise >10,000 ‘ for saturation,
creating situation of clear skies, stable air aloft and little
to no ppt.
4. cT duration too long = drought
IV .Fronts
a. Transition Zone between 2 air masses of different densities
a. Density differences may be temperature or humidity
b. Air masses as separated by fronts of different temperatures and humidity
c. Frontal Zone: area that extends upward of front due to horizontal/vertical
air mass extent.
© Vicki Drake
SMC F-2000
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V. Locating a front on a surface weather map
a. sharp temperature changes over a relatively short distance
b. changes in humidity content of air mass
c. shifts in wind direction
d. air pressure changes
e. clouds and precipitation patterns
VI. Types of Fronts
a. Stationary Front – no movement, marked on a map by single line with
alternating triangles and half-circles (on opposite sides of line)
a. Triangles represent cold air, half circles represent warm air
b. Marks the boundary between warm/cold air masses
c. Surface winds blow parallel to stationary front, but in opposite
directions on either side of front
d. May represent dry weather with no ppt, or widespread cloudiness
with light or residual ppt.
b. Cold Front – marked on map by single line with triangles, pointing to the
direction cold air mass is moving TOWARDS
a. Zone where cold air replaces either warm or cool air
b. Lowest temperatures and pressure readings usually just as front
passes over
c. Cold dense air wedges beneath warm or cool air, forcing air to rise
d. Moist unstable air = cumuliform clouds
e. Upper level winds push cirrostratus and cirrus clouds ahead of
front, usually first indication a storm is coming
f. Along front – Thunderstorms may develop, creating downpours of
showery ppt, and gusty winds
g. Leading edge of front is steep due to friction slowing the air flow
near the ground.
c. Warm Front – marked on map by single line with half-circles, pointing to
the direction warm air mass is moving TOWARDS
a. Zone where warm air replaces cold or cool air
b. Warmest temperatures usually just as front passes over
c. Warm, less dense air tends to ‘ride’ up and over cool or cold, more
dense air (OVERRUNNING)
d. Clouds and precipitation produced well in advance of the front’s
surface boundary
e. Atmosphere usually more stable with warm air aloft – cloud types
usually altostratus, stratus type, with some cirrocumulus if air lifted
high enough
f. Associated with light to moderate rain, drizzle over widespread
area
d. Occluded Front –marked by alternating triangles and half-circles on the
same side of the line
a. Zone where either a cold front catches up and overtakes a warm
front, or a warm front overtakes a cold front
b. Cold type or Warm Type occluded Fronts possible
1. cold types: temperature dropping
2. warm types – temperature rising
c. Most violent weather occurs just where cold air overtakes warm
front due to temp. differences and continued release of Latent Heat
(Cold Type)
© Vicki Drake
SMC F-2000
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d. (Warm Type) – warm air “sandwiched” between two cold air
masses – producing widespread precipitation (similar to ordinary
Warm Front)
e. Primary difference between warm/cold type occluded fronts:
1. Warm Occluded front – upper level warm front
precedes the surface occluded front by as much as
200 miles
2. Cold Occluded Front – upper level warm front follows
surface occluded front, usually by 30 miles or so
VII. Development of Mid-Latitude Cyclones along Polar Front (aka Polar Front
Theory)
a. A Model explaining the “lifecycle” of an extra-tropical storm (storm outside
of tropical waters)
b. Development of this cyclone begins along the Polar Front (the boundary
between cold polar air and warm subtropical air)
a. Normal conditions – Polar Front represented as a Stationary Front
(no movement of air masses, warm air pushing “northerly”, while
cold air pushes “southerly”, wind blowing parallel to front in
opposite directions, Low pressure cells generated where surface
winds converge)
b. A frontal wave develops along this Stationary Front (under certain
conditions) pushing cold polar air across the front and warm
subtropical air pushing across in the opposite direction.
c. Region of lowest pressure at juncture of cold air/warm air –
generating Low Pressure Cell, with counterclockwise rotation
(Northern Hemisphere Coriolis Effect)
d. As warm air pushes into cold air region, overrunning occurs,
widespread ppt results
e. System moves slowly east by NE (Jet Stream interference)
f. Low Pressure develops – more isobars encircling the wave’s apex –
creating a strong pressure gradient and PGF
g. Along the front of cold polar air – Thunderstorms are developing,
h. Widespread ppt in advance of warm front, with region of warm dry
air advancing behind the warm front (WARM SECTOR)
i. Energy for storm: (1) transformation of PE → KE as warm air rises,
cold air sinks (2) release of latent heat from condensation (3)
increased wind speeds due to PGF
j. Central pressure continues to drop with increasing wind speeds
resulting
k. Fast moving cold front “squeezes” out WARM SECTOR catching up
to and overtaking warm front (OCCLUDED FRONT)
l. Storm most intense at this stage – dissipation not far away, as
OCCLUDED FRONT ‘chokes’ off source of energy
VII. Cyclogenesis
a. Development/strengthening of cyclones – anywhere two contrasting air
masses can collide
a. Regions within US with greatest tendency
1. Eastern slope of Rockies
2. Great Basin
3. Gulf of Mexico
© Vicki Drake
SMC F-2000
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© Vicki Drake
SMC F-2000
4