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Atmospheric Lifting Mechanisms
Convergent Lifting
Convectional Lifting
Orographic Lifting
Frontal Lifting (Cold and Warm Fronts)
Atmospheric Lifting Mechanisms
Figure 8.6
Local Heating and Convection
Figure 8.7
Convection over Florida
Figure 8.8
Orographic Precipitation
Figure 8.9
Chinook, Foehn and Santa Anna Winds
Chinook Winds: The warm and dry winds move down the east slope
of the Rockies. As the air descends adiabatically, it is warmed up
rapidly. When this occurs in the winter or spring, the winds can melt
the snow rapidly.
Santa Ana: During the autumn season, an anticyclone (High
pressure center) can reside over the Great Basin of California, the
clockwise flow directs air from Arizona and Nevada westward
through the Santa Ana Canyon . The already warm and dry winds
become warmer and drier (90-100oF) as they descent the mountain
slopes. Southern California is dry in the summer, the strong Santa
Ana wind (40mph) makes it ready for catastrophic fire.
Foehn Winds: Similar winds created by the Alps in center Europe.
Figure 8.9
Orographic
Patterns
Figure 8.10
Frontal Lifting
Font: The boundary between two air masses ,
i.e. the “battle” front between two air masses.
Cold Fronts
Cold air forces warm air aloft
400 km wide (250 mi)
Warm Fronts
Warm air moves up and over cold air
1000 km wide (600 mi)
Cold Front
1.
2.
3.
4.
Cold air advances into the warm air, forcing the warm air the rise.
Cold air is heavier than warm air, thus the warm overruns the cold air.
Slope is 1:100, i.e. the frontal surface rise 1 km in height over 100 km distance on the ground.
Intense precipitation over shorter period of time compared to warm front.
Figure 8.11
Cold Front
and
Squall Line
This is a cold front seen from
space. Due to the rapid lifting
of warm air by the advancing
cold air, a sharp line of
cumulonimbus clouds can
occur, called the squall line.
Figure 8.12
Warm Front
1.
2.
3.
4.
warm air advances, pushing the cold air to retreat.
Cold air is heavier thus more difficult for the warm air to displace.
Slope is 1:200, i.e. the frontal surface rise 1 km in height over 200 km distance on the ground.
Light-to-moderate rain over large area for an extended period.
Figure 8.13
Life Cycle of a Midlatitude Cyclone
Midlatitude cyclones are low pressure
systems with diameter exceeding 1000 km
that travel west to east across the planet in
the middle latitude regions, lasing several
day to more than a week.
Cyclogenesis
Open stage
Occluded stage
Occluded front
Stationary front
Dissolving stage
Cyclones and Anti-Cyclones
Cyclones and Anticyclones
Midlatitude Cyclone
Several factors can
cause the front line
to wave:
(1) Topography
(2) Temperature contrast
(3) Intensification of
divergence in the
upper atmosphere by
Jet Stream,
particularly when it
waves from north to
south.
Figure 8.14
Midlatitude Cyclone
Figure 8.15
Average and Actual Storm Tracks
(1) The general west-to-east movement of the air in the context of global air circulation.
(2) The counter-clockwise movement of the air on the ground around a cyclone.
Figure 8.16