Mid-latitude cyclone

Whatis a mid-latitude cyclone?

-Themid-latitude cyclone is a synoptic scale low pressure system that has cyclonic(counter-clockwise in northern hemisphere) flow that is found in the middlelatitudes (i.e., 30�N-55�N)


o There is a location (tropics vs.mid-latitudes) and size difference between hurricane and mid-latitude cyclone

�Typicalsize of mid-latitude cyclone = 1500-5000km in diameter

�Typicalsize of a hurricane or tropical storm = 200-1000km in diameter

Here is a picture of a typical mid-latitudecyclone and hurricane. Notice the size difference.

You are watching: How does surface air flow in a middle-latitude cyclone in the northern hemisphere


How does the mid-latitudecyclone form (for a longer description read pages 219-228)

1.Frompolar front theory, we know that in the mid-latitudes there is a boundary betweencold dry (cP) air to the north and warm moist (mT) air to the south

2.Alongthis boundary a counter-clockwise circulation can set up at the surface, whichacts to take warm air up from the south and cold air down from the north. Thisis called cyclogenesis.

3.Inthe center of this circulation, there is mass convergence (all the air ismoving in toward the center of the circulation much like when you make a whirl-poolin a pool, all the leaves and stuff floating around moves toward the center).When all that air hits the center, we have rising motion because it has nowhereelse to go.

4.Ifthe upper levels are favorable for cyclone development, then there is a regionof divergence aloft above the developing Low-pressure center. This will helppull the air that is converging at the surface upward and continue to develop thesurface cyclone. (The upper levels also steer the system and make it progresseast (like we learned earlier)).

a.Ifthe upper levels aren�t favorable for cyclone development, the cyclone won�tgrow and the mass convergence into the Low at the surface will just pile up andfill in the Low and it will decay.

b.Referto the second figure below for the optimal situation.

5.Ifthe upper levels are favorable, then the mid-latitude cyclone will continue todevelop and bring up mT air in the warm sector and bring down cP air in thecold sector.

a.ThemT air rises as it moves out ahead of the low helping to deepen the Lowpressure center to the east and help it move along

b.ThecP air sinks behind the system and fills in the Low on the backside alsohelping to move the system along

i.Itis this transfer of energy that both strengthens and propagates themid-latitude cyclone!

6.Oncethe mid-latitude cyclone is fully developed, well-defined fronts appear.

7.Asthe mid-latitude cyclone reaches maturity, the central pressure will be at itslowest and the occluded front will begin to form (as the cold front catches upto the warm front).

8.Oncethe system is occluded (all the warm air is above the cold air) the mass convergenceacts to fill in the Low and therefore the pressure increases in the Low and thesystem decays.

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Evolution of a mid-latitude cyclone. Each figurefollows the description above 1-8



Theabove figures were taken from www.u.arizona.edu/~korine/230/chap10_figs.htm


Diagram of flow (circulation) throughout thedepth of the troposphere


Wheredo mid-latitude cyclones typically form (in North America in winter)? (Remember that cycloneslike to develop along air mass boundaries)

-Leeside of the Rockies = Lee Cyclones

o Alberta Clippers

�FASTMOVING and usually don�t have too much precip associated with them because theyare far from a moisture source


�IntenseLow, with strong warm air advection in the warm sector, very cold temps in thecold sector. If there is a lot of gulf moisture to work with, they there isusually sleet, freezing rain and rain associated with the warm front, strong thunderstormsalong the southern edge of the cold front and snow along the backside and tothe NW of the Low (even BLIZZARDS)

-Alongthe East Coast

o Gulf Low

�Formalong the southern coast where there is a thermal boundary between the warmocean and cool land. Usually have a lot of precip associated with them because theyare so close to the ocean

oHatterasLow and Nor�easters (aka ��bomb�cyclones)

�Theseare the MOST INTENSE systems and they form along the thermal boundary betweenthe warm Gulf Stream and the cold Atlantic coast. They can bring flooding rainsalong the coast and several feet of snow further inland as they use the oceanas a vast source of the moisture. These also develop very quickly and sometimeshave pressure drops of 24mb in a single day. With a pressure drop of thismagnitude you can imagine how fast the winds are around these things.

What are their typical tracks? See diagrambelow


Precip patterns (and types),winds, temperatures, fronts, upper level flow and clouds that are around a typicalmid-latitude cyclone in winter.