Weather Systems
• Air masses
• Types of weather fronts (cold, warm, occluded)
• Traveling cyclones and anticyclones
• Tornadoes
• Tropical Weather Systems
• Poleward Transport of Heat and Moisture
Air Masses
E = equitorial
T= tropical (warm)
P = polar (cold)
A = Arctic (cold) (from Arctic
oceans and fringing lands)
AA = Antarctic
c = continental (dry)
Figure 6.1, p. 187
m = maritime (wet)
North American Air Masses
North American air
masses
acquire their
temperature
and moisture
characteristics over
their
source regions,
then move
across the
continent and
interact with other
air masses
Figure 6.2, p. 188
Frontal Activity
where air masses with different temperatures come
together
warm air lifted by cold dense air along a weather fronts
cold air mass warm air mass
Weather Fronts
Warm fronts
Cold fronts
Occluded fronts
Warm Fronts
Warm air is ramped over the colder air resulting in cloud formation
(condensation) and precipitation
Warm air
Cold air
Warm Front: In a warm front, warm air advances toward cold air and rides up and over the cold air. A notch
of cloud is cut away to show rain falling from the dense stratus cloud layer.
Cold Fronts
Cold air rapidly intrudes stationary warm air and lifts it
along a steep cold front resulting in cloud formation (condensation)
and precipitation (often thunderstorms)
Cold air Warm air
Cold Front: At a cold air front, a cold air mass lifts a warm air mass aloft
Cold Front Cumulus Clouds
A line of cumulus clouds marks the advance of a cold front, moving from left to right. The cold front pushes warmer,
moister air aloft, triggering cloud formation.
Occluded Fronts
In an occluded front, a warm front is overtaken by a cold front.
the warm air is pushed aloft and it is not longer in contact with the
ground
Warm air
Cold air
Cool air
Occluded Front: A warm front is overtaken by a cold front. The warm air is pushed aloft, and it no longer
contacts the ground. Abrupt lifting by the denser cold air produces precipitation.
Conditions for
formation of a wave
cyclone: Two
anticyclones, one with
warm subtropical air
and the other with cold
polar air, are in contact
on the polar front. The
shaded area is shown in
block (a) and the
following figure as the
early stage of
development of a wave
cyclone.
Wave Cyclones
wave cyclones are
the dominant form
of weather systems
in middle and high
latitudes
large inspirals of air
that repeatedly
form, intensify and
dissolve
Figure 6.9, p. 192
Weather Changes Associated with Wave Cyclones
Figure 6.10, p. 193
Cyclone Tracks
• Wave cyclones
tend to form in
certain areas
and travel
common paths
• Mid-latitude
wave cyclones
tend to travel
eastward
• Tropical
cyclones tend
to move
westward
Figure 6.11, p. 194
Traveling Cyclones and Anticylones on a daily weather map of the world
Figure 6.12, p. 195
Tornadoes
appear as dark funnel cloud
hanging down from
cumulonimbus clouds
Wind speeds may be as high
as 100 meters per second (225
miles per hour)
Cause great damage
Sometimes very ominous in
appearance, mammatus clouds are
harmless and do not mean that a
tornado is about to form; a commonly
held misconception. In fact,
mammatus are usually seen after the
worst of a thunderstorm has passed.
Tornadoes
Waterspout
- over water
- less power
- smaller temp
gradient
Funnel Cloud v. Tornado
Tornado Funnel Cloud
Tornadoes
a small but very
intense cyclonic
vortex in which air
spins at a
tremendous speed
associated with
thunderstorms
spawned by fronts
in mid-latitude
regions of North
America
Figure 6.14, p. 196
1960-1989 Data
USA Tornado
Occurrence
• About 800-1000
tornadoes spotted
each year.
USA Tornado Season
Most common
when temperature
gradient in central
U.S. is greatest.
Tornado Classification
• Fujita F-scale
– F0 (winds 980 1.2-1.7 33-42
2. Moderate 965-979 1.8-2.6 43-49
3. Strong 945-964 2.7-3.8 50-58
4. Very Strong 920-944 3.9-5.6 59-69
5. Devastating 5.6 >69
Table 6.2, p. 201
Impact of Tropical Cyclones
low pressure, high winds and the shape of bays can
produce sudden rise in water level (storm surge)
flooding may occur inland
activity varies from year to year (number and
strength)
season usually from May to November in the south
Atlantic region
The atmospheric circulation
the atmospheric circulation transfers heat from
equatorial regions toward the polar regions by:
the Hadley cell circulation,
air mass movement
Rossby waves
tropical cyclones
The Atmospheric Circulation
Figure 6.21, p. 205