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polar front theory

the theory postulated in the early 20th century describing the formation, development and dissipation of midlatitude cyclones. Many of the features of the theory are still valid today.


the beginning of cyclone formation with the disruption of the linear frontal boundary between a warm and cold front

mature cyclone

With this type, strong storms exist along the cold front. Also, during occlusion of this type, the faster moving cold front catches the slower moving warm front and cold wair forces the warmer air up.


represents the end of the cyclone's life cycle

Rossby wave

large, alternating troughs and ridges that establish patterns of upper-level divergence and convergence


rotation of a fluid

absolute vorticity

the overall rotation of the air and has two components: relative and Earth vorticity (both complement each other and increase the total)

relative vorticity

vorticity relative to Earth's surface, depends on air motions with respect to Earth's surface-2 sources: shear and curvature of the flow

Earth vorticity

vorticity due to Earth's daily rotation about its axis, is a function solely of latitude higher the latitude, the greater the vorticity

dynamic low

surface low-pressure systems resulting from upper-tropospheric motions, typically exist beneath regions of decreasing vorticity in the upper atmosphere, associated with low pressure aloft

thermal low

caused by localized heating of the air from below, associated with high pressure aloft

speed divergence

the spreading of air due to increasing wind speed in the downwind direction

diffluence and confluences

a type of horizontal divergence that occurs when steamlines spread apart in the downstream direction

short waves

smaller ripples superimposed on the larger Rossby waves. They migrate downwind within the Rossby waves and exert their own impact on the life cycle of midlatitude cyclones. They can either enhance or reduce the local divergence or convergence.

temperature advection

the horizontal transport of warm or cold air by the wind, formation of short waves depends on this

warm air advection

the horizontal movement of relatively warm air, where this occurs air is warmer and more buoyant than the air ahead of it and rises, air turns slightly to the right

cold air advection

the horizontal movement of relatively cold air, typically occurs behind a cold front thereby enhancing the temp contrast found on either side of the front, air turns slightly to the left


when the height contours and isotherms are in alignment the atmosphere is considered this


when the height contours and isotherms intersect the atmosphere is considered this

baroclinic instability

warm and cold air advection causes vertical motions called this

cutoff low

an upper-level area of low pressure that takes on a circular flow distinct from the general flow around it

conveyor belt model

the modern description of air flow through midlatitude cyclones, depicts cyclones in terms of three major flows

Why can storms move in a different direction than the direction the wind is moving?

B/c of the counterclockwise spiral within the midlatitude cyclone. Regardless of the direction in which a system is moving, winds at different points within midlatitude cyclone flow in different directions.

positive vorticity

counterclockwise rotation in the Northern Hemisphere

negative vorticity

air rotating clockwise


occurs when the speed of a fluid varies across the direction of flow

speed convergence

compaction of air due to decreasing wind speed in the downwind direction

warm conveyor belt

originates near the surface in the warm sector and flows toward and over the wedge of the warm front, the air rises and turns anticyclonically, also associated with the could cover.

cold conveyor belt

lies ahead of the warm front, enters the storm at low levels, air rises and turns anticlyclonically, it gains moisture from evaporation

dry conveyor belt

originates in upper troposphere as part of the generally westerly flow, brings the coldest air into the cyclone, it separates the cloud bands from the warm and cold conveyor belts giving them their comma shaped appearance


as much influences by upper-level conditions as are cyclones, they bring clear skies and calm conditions b/c the cool air wihtin them slowly sinks toward the surface

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