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 mid-latitude cyclone (a low pressure system).

a)Formation – clash of two air masses (cyclogenesis)b)Wave develops- caused by the opposing motion of the air massesc)Cyclonic circulation established (warm front leading, cold front trailing)d)Occlusion begins – the cold front overtakes the warm fronte)Occluded front formsf)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.