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Chapter 9 Weather Patterns
Tuscon, AZ
New York Feb 2003
Weather Patterns
1. The primary weather producer at midlatitudes (e.g., North America) is the midlatitude cyclone.
2. Weather is interpreted in terms of the
polar front or Norwegian Cyclone
Model.
Weather Patterns - 2
1. In general, Fronts are boundary
surfaces that separate air masses of
different densities.
2. A polar front separates cold Arctic air
from warm subtropical air.
3. Fronts are normally between 15 and 200
km wide - where the two air masses fight
each other.
Weather Patterns - 3
1. It is always the warmer, less dense air
that is forced aloft.
2. Cool air acts as a wedge on which
lifting takes place. Called overrunning.
3. Types of front are:
warm, cold, stationary, occluded
& drylines.
Warm Fronts
1. The ground position of the warm air occupies
territory previously held by the cold air.
2. On weather maps - red line with red
semicircles pushing into the colder air.
3. As the colder air retreats, friction with the
surface gradually slows down the surface
position of the front compared to its position
aloft - less dense warm air has difficulty in
replacing more dense cold air.
Warm stable air
Warm conditionally unstable air
Warm Fronts - 2
1. The front therefore attains a slope, with a
wedge of warm air lying over a wedge of cold
air. Slopes are 1:200 on average.
2. As the warm air rises over the cold air, it
expands & cools adiabatically, causing
condensation and precipitation
3. First sign of an approaching front is cirrus
clouds, which form where the overrunning
warm air has ascended high up the wedge of
cold air. Surface front could be 1000 km
behind.
Light rain, warm front
Warm Fronts - 3
1. Approaching fronts can also be recognized
from aircraft contrails that last hours (need
moist air).
2. Warm fronts normally produce light to
moderate precipitation over a large area for
an extended period.
3. As the front approaches a given point, the
cirrus clouds grade into cirrostratus, then
thicker stratus and nimbostratus clouds,
which bring the rain.
4. Precipitation occurs well ahead of the
surface location of the front.
Warm Fronts - 4
1. As the rain falls through the cold air in the
lower wedge, some evaporates, saturating
the cloud base. Then get a stratus cloud
deck. These clouds can move rapidly
downwards, giving frontal fogs.
2. If the cold air is below freezing, rain drops
become supercooled as they fall. When
they hit the Earth, they flash-freeze to form
glaze.
3. When the warm front passes, the
temperatures rise, and the winds shift from
the east to the southeast. (more later).
Cold Fronts
1. When cold continental polar air advances into
a region occupied by warm air, the zone of
discontinuity is called a Cold Front.
2. Friction with the ground slows the advance of
the surface position of the front relative to its
position aloft.
3. Cold fronts normally move in from the north or
northwest (blame Canada).
Cold Fronts - 2
1. The cold air forces the warm air to rise. If this
rise is rapid, the moisture in the warm air
freezes, releasing latent heat (of fusion). So
the air gets warmer, and rises even faster.
2. The arrival of a cold front is often preceded by
altocumulus clouds, followed by cumulonimbus
clouds and rain.
3. Cold air behind the front is usually subsiding,
so the weather is clear (and cold).
Cold Fronts - 3
1. Cold fronts are about twice as steep as warm
fronts, and move about 50% faster.
Consequently cold-front weather is more
violent than warm-front weather.
2. Intensity of precipitation is greater than for a
warm front, and has a shorter duration.
3. Represented on weather maps by blue line +
blue triangular symbols that point into the
retreating warm air.
Cold Front Thunderstorm Development over Great Plains
Figure 9-7
Stationary Fronts
1. Sometimes neither the warm air or cold air
wins the battle, and we get a stationary front
(stalemate).
2. Air flow is parallel to the line of the front.
3. Represented on weather maps by blue
triangular symbols on one side of the line
(cold), and red semicircles on the other.
4. Gentle to moderate precipitation, but possible
floods if the front remains in the one place for
too long.
Occluded Fronts
1. One front overtakes another.
2. Cold-type occluded front - cold front
overtakes warm front. e.g. East of Rockies.
Expect heavy rain - warm air is forced aloft.
3. Warm-type occluded front - warm front
overtakes cold front. e.g. Along the Pacific
coast. Mild maritime polar air invades frigid
continental polar air.
4. Represented on weather maps by a purple
line with alternating purple triangles &
semicircles pointing in the direction of the
movement.
Drylines
1. Drylines arise when dry air overtakes and lifts
not so dry air.
2. Generate a band of severe thunderstorms.
3. Weather maps show that dewpoints are much
lower behind a dryline. (The air is drier.) See
also Chapter 10.
4. E.g. southern Great Plains. Dry continental
tropical air from SW US meets moist maritime
air from the Gulf of Mexico.
Mid-Latitude Cyclone
1. Development & intensification of a
midlatitude cyclone is explained in terms
of the "polar-front" theory .
2. Cyclones form along fronts. Life cycle
last about 3 to 7 days.
Mid-Latitude Cyclone - 2
1. Six stages:
1. Formation (cyclogenesis)
2. Development of wave in the front
3. Cyclonic circulation established
4. Occlusion begins
5. Occluded front develops
6. Cyclone dissipates
Mid-Latitude Cyclone - 3
1. Stage 1. Two air masses of different densities
& temperatures are moving parallel to the
front, and in opposite directions. Typically - cP
associated with polar easterlies on the north of
the front, mT driven by westerlies on the south.
(This causes counterclockwise rotation of the
air mass.)
2. Stage 2. Under suitable conditions, the front
takes on a wave shape that is usually several
hundred km long.
Mid-Latitude Cyclone – 4
1. Stage 3. For a cyclone to develop, the wave
must steepen and "break" (just like surfing
beaches). The warm moves pole-wards into
the cold air, establishing a warm front, while
cold air moves equator-wards, establishing a
cold front. This sets up a circular flow pattern in
a counterclockwise direction - the cyclone.
2. Stage 4. Cold front normally travels about 50%
faster then the warm front, and overtakes it,
causing occlusion. Ends of the fronts in center
of cyclone occluded first.
Mid-Latitude Cyclone - 5
1. Stage 5. The size of the occluded front grows
in length, displacing the warm front aloft. The
storm usually intensifies, the central pressure
falls, and wind speeds increase. During Winter,
can get heavy snowfalls and blizzard-like
conditions (because of the high winds).
2. Stage 6. Once all the warm air has been
displaced, we have just the cold air and no
temperature gradient. There is thus no energy
left to drive the cyclone, which therefore
dissipates.
Idealized Weather
in a Midlatitude
Cyclone
Winds as a forecasting tool…
Cyclogenesis / Formation
1. Polar-front model shows that cyclone
formation occurs where a frontal surface is
distorted into a wave-like discontinuity.
2. Wave is caused by surface factors, such as
topographic irregularities, temperature
contrasts, ocean currents.
3. The flow aloft is usually intensified before the
formation of a surface cyclone, so the flow aloft
is a very important contributor to cyclone
formation.
Cyclogenesis - 2
1. When the flow aloft is generally zonal (east to
west), little cyclonic activity occurs at the
surface.
2. When the upper air forms waves, these waves
drive the air north and south as well as eastwest and set up alternating troughs and ridges.
Surface cyclones tend to form below such
regions.
3. When surface cyclones form, they are usually
centered below the jet stream, and downwind
from an upper level trough.
Cyclogenesis - 3
1. Cyclonic & Anticyclonic Circulation cyclones & anticyclones generally appears
in pairs, below the jet stream .
2. The surface air that flows into the low pressure
center of a cyclone is forced aloft where it
diverges. On the other hand, the air above an
anticyclone converges, sinks to the surface,
and becomes the outflow from the anticyclone.
Cyclogenesis – 4
1. It is quite common for diverging air above a
cyclone to become the converging air above
an anticyclone, with the reverse situation on
the ground.
2. Divergence & Convergence Aloft divergence aloft tends to flow generally west to
east along sweeping curves. (At the surface,
divergence is in all directions over the surface.)
Cyclogenesis - 5
1. This flow is controlled by the jet stream.
2. On entering the zone of high wind speed
around the jetstream, air accelerates and
stretches out (divergence).
3. On entering a zone of low wind speed, air
slows down and piles up (convergence)
Cyclogenesis - 6
1. Upper level divergence and surface
cyclonic flow around a Low generally
develop downstream (to the east) from
an upper level trough.
2. Upper level convergence and surface
anticyclonic flow around a High generally
develop downstream (to the east) from
an upper level ridge.
Travelling Cyclones
1. Cyclones generally move over the surface of
the Earth at about 25 to 50 kph.
2. The path of a cyclone is controlled to a large
extent by the air flow aloft (500 mb level). The
upper level flow "steers" the surface cyclone.
3. It is important to be able to predict the path of
a surface cyclone.
4. We must understand and be able to predict the
wavy flow in the jetstream.
Travelling Cyclones - 2
1. In general, cyclones form where large
temperature contrasts occur in the lower
troposphere.
2. Main sites for cyclone formation: lee side
of Rockies; along Atlantic coast, east of
the Appalachians; North Pacific; North
Atlantic.
Major sites of
cyclone formation
Patterns of Movement
1. Cyclones that form east of the Rockies
tend to migrate to the east, and then
northeast.
2. Cyclones that influence western North
America originate over the Pacific, and
move northeastwards towards the Gulf of
Alaska, where they merge with the
Aleutian low.
Typical cyclone tracks
Panhandle Hook
1. Panhandle Hook - cyclones develop in
southern Colorado near the TX and OK
panhandles, move to S.E. and then
northwards.
Patterns of Movement - 2
1. During the Winter months, these storms form
further south, and often reach the west coast
of the US, sometimes as far south as
California (which therefore gets a short winter
rainy season.) These storms can reform east
of the Rockies, such as in Colorado.
2. Some cyclones form over the Great Plains,
and are associated with an influx of mT air
from the Gulf of Mexico.
Anticyclones & Blocking
Highs
1. Although high pressure systems generally
produce clear skies and calm conditions, they
are not always a "good thing".
2. Large anticyclones often develop over the
Arctic in winter. If these migrate south over the
plains (no blocking mountains), their dense
frigid air can bring atypical cold weather.
Anticyclones & Blocking
Highs - 2
1. Occasionally, large midlatitude
anticyclones will stagnate, staying at the
same location for 2 or more weeks.
2. They deflect the normally eastwards flow
of air towards the poles.
3. If they stay at the same location, they are
called Blocking Highs.
Anticyclones & Blocking
Highs - 3
1. Blocking highs cause good weather to exist
around themselves, but can cause extensive
rain by not allowing cyclones to continue to the
east as usual.
2. Blocking highs can also cause air pollution
episodes. The subsidence within an
anticyclone can produce a temperature
inversion (with warm air lying above cooler air)
that acts like a lid to trap pollutants. Winds
near the center of the anticyclone are light, so
do not disperse the pollutants.
March 23
Midwest Floods of 2008 & 2003
• June 2008 floods
• The Great Flood of 1993
Rainfall based on 10 days of
satellite data
Cedar River, Cedar Rapids, OH
Monroe County, 1993
Violent Spring Weather
Conveyor Belt Model
1. Norwegian (polar front) model talks about
conflicts between air masses along frontal
boundaries.
2. Conveyor Belt model consists of 3 interacting
air streams
Two that originate near the surface, and
ascend aloft;
One that originates in the upper troposphere.
Warm conveyor belt
1. Warm conveyor belt carries warm air from
Gulf of Mexico northward into warm sector of
cyclone. Convergence causes it to rise.
2. When it reaches the warm front, it rises even
more rapidly over the cold air.
3. Adiabatic cooling of warm moist air gives
clouds and precipitation.
4. When this air reaches the middle troposphere,
it turns right and joins the general west to east
flow.
Cold conveyor belt
1. Cold conveyor belt is airflow that starts at the
surface ahead of the warm front, and flows
westward towards the center of the cyclone.
2. Air is moistened by evaporation of moisture
falling through it.
3. Airstream rises because of convergence,
becomes saturated, and contributes to the
overall precipitation.
Cold conveyor belt - 2
1. When this air reaches the middle troposphere,
some of it rotates cyclonically around the low
to produce the "comma head" of the mature
storm system.
2. Remaining air turns right (clockwise) and joins
the general west to east flow.
3. Runs parallel to the warm conveyor belt, and
may generate precipitation.
Dry conveyor belt
1. Dry conveyor belt (yellow) originates in upper
troposphere, where it is part of the general
west to east flow.
2. This air is relatively cold and dry.
3. As this airstream enters the cyclone, some
descends behind the cold front, and provides
clear, cool conditions.
4. Other branch of this airstream flows west, and
forms the dry slot (no cloud)