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Chapter 9
Weather Patterns & Midlatitude Cyclones
Frontal Weather
• Fronts are boundary
surfaces between
different air masses.
– Warm front
– Cold front
– Stationary front
– Occluded front
– Drylines
Frontal Weather
• Warm fronts:
– A warm front occurs when warm air moves
into colder air.
– It is shown on a weather map by a red line
with red semicircles on the line in the direction
of travel.
– Warm fronts have relatively gentle slopes.
Frontal Weather
• Warm fronts
Frontal Weather
• Warm fronts
Frontal Weather
• Cold fronts:
– When cold air advances into warm air, the
zone of discontinuity is called a cold front.
– It is represented on a weather map by a blue
line with blue triangles pointing in the direction
of travel.
– Their slopes are about twice as steep as
warm fronts.
Frontal Weather
• Cold fronts
Frontal Weather
• Cold fronts
Frontal Weather
• Backdoor cold front (nor’easter):
– Rather than moving from west to east, this
type of cold front moves from east to west in
the northeast U.S.
– They occur primarily in the spring.
– The weather goes from warm to cool and
moist.
Frontal Weather
• Cold front
Man cross-country skiing in Boston – nor’easter Dec. 2009
Credit: REUTERS/Brian Snyder
Frontal Weather
• Stationary front:
– This occurs when airflow is neither toward
cold air nor a warm air mass.
– On a weather map, it is represented by red
semicircles and blue triangles alternating
sides.
– They may remain over an area for a long
time.
Frontal Weather
• Occluded front:
– When rapidly moving cold air overtakes a
warm front, an occluded front occurs.
– These are represented by a purple line with
alternating semicircles and triangles pointing
in the direction of movement.
• Occlusion:
– As cold air wedges a warm front upward, a
new front forms between the advancing cold
air and the warm air.
Frontal Weather
• Occluded front
Frontal Weather
• Drylines:
– Drylines are boundaries, which develop most
often over the southern Great Plains.
– They occur when dry (cT) air meets moist,
(mT) air from the Gulf of Mexico.
– They most often generate thunderstorms.
Midlatitude Cyclones and
the Polar Front Theory
• Midlatitude cyclones:
– These are low pressure systems.
– Their diameters often exceed 1000 km.
– These cyclones usually travel west to east.
– Most have a warm front and a cold front
extending from center.
– They frequently produce precipitation.
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.
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
•
•
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.)
Stage 2. Under suitable conditions, the front
takes on a wave shape that is usually several
hundred km long.
Mid-Latitude Cyclone – 4
•
•
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.
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
•
•
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).
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
Map of a
Midlatitude
Cyclone
Using winds as a forecasting tool.
Midwest Floods of 2008 & 1993
• June 2008 floods
• The Great Flood of 1993
Rainfall based on 10 days of
satellite data
Cedar River, Cedar Rapids, OH
Monroe County, 1993
Flash Floods are exactly what they
sound like. You cannot out run rushing
water.
http://wmp.gsfc.nasa.gov/projects/project_FlashFlood.php
Violent Spring Weather
Case Study of a Midlatitude Cyclone
• March 21
– The storm reached the west coast of U.S.
• March 23
– It was centered over the Kansas-Nebraska border.
– It was well developed.
– Air pressure was at 985 millibars.
Case Study of a Midlatitude Cyclone
• March 24
– Air pressure deepened to 982 millibars.
– The storm curved northward through Iowa.
– An occluded front developed when the cold front
overtook the warm front.
– Severe weather was produced.
March 25
Pressure moved
up to 1000
millibars.
The storm was
situated over the
Great Lakes.
It dissipated
within 24 hours.
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
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
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
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.
End Chapter 9
Chapter 9 Homework: GIST: 2, 4, 5, 8 Problems 3 & 4