Transcript Chapter 9

Chapter 9:
Weather Patterns
Notes
Maps of Storm Pattern in 1993
Polar-Front Theory (Norwegian Cyclone Model)
The middle latitudes – a region between southern Florida and Alaska
Mid-latitude (Middle-latitude) cyclone – the primary weather producer of this
region. Also called a low-pressure system.
Mid-latitude cyclones:
• Low-pressure systems with diameters that can exceed 1000 kilometers
• Travel west to east
• Last from a few days to more than a week
• Have a counter-clockwise rotation
• Most have cold and warm fronts extending from the central area of low
pressure
• Surface convergence and upward flow initiate cloud development that
frequently produces precipitation
In the Norwegian cyclone model, mid-latitude cyclones develop in conjunction
with the polar front, which separates cold polar air from warm subtropical air,
Idealized Mid-Latitude Cyclone
Fronts – boundary surfaces that separate air masses of different
densities.
• One air mass is usually warmer, and often contains more
moisture than the other
• Fronts can form between any two contrasting air masses
• Typically are 15- to 200-kilometer wide bands of discontinuity
(relatively narrow)
• Represented on weather maps as a broad line
As one air mass moves into another, limited mixing occurs along the
frontal surface, but for the most part, the air masses retain their
identity as one is displaced upward over the other.
Overrunning – term applied to warm air gliding up along a cold air
mass
Warm Fronts
Warm front – when the surface (ground) position of a front moves so that warm air
occupies territory formerly covered by cooler air.
Warm air, being less dense than cold air, has a hard time displacing the heavier, cold
air. As a result, the boundary separating air these air masses acquires a very gradual
slope.
As warm air ascends above the retreating wedge of cold air, it expands and cools
adiabatically, causing moisture in the ascending air to condense into clouds that often
produce precipitation.
There is a cloud sequence that typically precedes a warm front:
1) Cirrus clouds are the first sign of an approaching warm front. They form where the
overrunning warm air has ascended high up the wedge of cold air.
2) As the front nears, cirrus clouds grade into cirrostratus that gradually blend into
denser sheets of altostratus clouds.
3) About 300 kilometers ahead of the front, thicker stratus and nimbostratus clouds
appear and precipitation starts.
Cold Front
Cold front – the boundary where cold, continental polar air actively advances into
a region occupied by warmer air.
Because of friction, the surface of the cold front moves more slowly that the air
aloft, and the front steepens as it moves.
The arrival of a cold front is sometimes preceded by altocumulus clouds. The
forceful lifting of warn, moist air along a cold front results in the formation of
cumulonimbus clouds accompanied by heavy downpours and vigorous winds.
Usually the weather behind a cold front is dominated by subsiding air within a
continental polar air mass, and the drop in temperature is accompanied by
clearing that begins soon after the front passes.
Stationary and Occluded Fronts
Stationary front – when the airflow on both sides of a front is neither toward
the cold air mass nor toward the warm air mass, and the surface position of
the front does not move or moves very slowly.
Occluded front - forms what a rapidly moving cold front overtakes a warm
front, and a new front forms between the advancing cold air and the air over
which the warm front is gliding.
Types of Occluded Fronts
Cold-type occluded front - occurs when the air behind the cold front is
colder that the cool air it is overtaking. These fronts generally resemble
cold fronts in the type of weather generated.
Warm-type occluded front – occurs when the air behind the advancing
cold front is warmer than the cold air it is overtaking.
Stages of the Life-Cycle of a Middle-Latitude Cyclone
Life Cycle of a Mid-latitude Cyclone
The polar-front theory describe the development and intensification of a midlatitude cyclone (a low pressure system).
a)
b)
c)
d)
e)
f)
Formation – clash of two air masses (cyclogenesis)
Wave develops- caused by the opposing motion of the air masses
Cyclonic circulation established (warm front leading, cold front trailing)
Occlusion begins – the cold front overtakes the warm front
Occluded front forms
Cyclone dissipates – occurs when the horizontal temperature difference that
existed between the contracting air masses has been eliminated
Conditions Aloft Associated with a Middle-Latitude Cyclone
Cyclone Formation
The polar-front model shows that cyclogenesis occurs where a frontal
surface is distorted into a wave-shaped discontinuity. Several surface factors
produce this wave in a frontal zone, including topographic irregularities
(mountains) and temperature contrasts (as between sea and land).
There is a close relationship between surface disturbances and the flow of
air aloft. Whenever the winds aloft flow directly west to east there is little
cyclonic activity at the surface. However, when the upper air begins to
meander widely from north-to-south, forming high amplitude waves of
alternation troughs and ridges, surface cyclonic activity intensifies.
Surface cyclones are invariably centered below the jet-stream axis and
downwind from an upper-level trough.
Circulation of Mature Mid-Latitude Cyclone
The Conveyor Belt Model
The modern view of cyclogenesis called the conveyor belt model,
provides a picture of the airflow within a cyclonic system. It consists of
three interacting airstreams: two that originate near the surface and
ascend, and a third that originates in the uppermost troposphere.
The warm conveyor belt carries warm, moist air from the Gulf of
Mexico into the warm sector of the mid-latitude cyclone. This airflow
is the main precipitation producing air stream in a mid-latitude
cyclone.
The cold conveyor belt is airflow that starts at the surface ahead of
the warm front and flows westward toward the center of the cyclone.
The dry conveyor belt originates in the uppermost troposphere and is
relatively cold and dry.