Extratropical / Midlatitude Cyclones

• Cyclone = any circulation around low-pressure.

• Extratropical Cyclone = large low-pressure system that often form in the mid-latitudes.

• Day-to-day weather in the mid- and high-latitudes is closely related to the location, development, and movement of cyclonic storms.

• Cyclones are responsible for:
• Changing air masses (temperature, dew point, wind, etc)
• Bringing precipitation opportunities

• Most weather is linked to extratropical cyclones, rather than being discrete weather events.

• Vertically speaking, they're found in the tropopause.

• Needed to balance temperature differences between the poles and equator (between the cold upper troposphere and the warm lower troposphere).

• Most intense in the late fall, winter and spring due to the temperature gradient between the tropics and the north pole being the strongest during that time.

• Winds blow counterclockwise (cyclonically) in the northern hemisphere and clockwise (cyclonically) in the southern hemisphere.

• It forms at intersecting fronts:
• Cold front is typically south and west, to east of the low.
• Cold air is "behind" the low, while warm air is "ahead" (lows and highs both move in the direction of the jet stream winds).
• Thunderstorms are often ahead of the cold front.
• Steady precipitation is often ahead of the warm front.

• Cloud Sequence: cirrus, cirrostratus, altostratus, nimbostratus

Norwegian / Bergen Cyclone Model

• During WWI, Vilhelm Bjerknes identified that midlatitude cyclones formed along the boundary separating polar air from the warmer air to the south.

• Widely used by weather forecasters to intercept and anticipate changes in the surface synoptic chart.

• Most real cyclonic disturbances do not fit the model perfectly.

•  Things that are difficult to identify:
• Warm fronts
• Problems with topography distorting / obscuring fronts.
• Some storms develop away from the stationary front.
• Fronts sometimes develop by themselves.

Life Cycle of a Midlatitude Cyclone. (a) According to the Norwegian model, the stationary polar front separates opposing masses of cold air and warm air. (b) Cyclogenesis first appears as a disruption of the linear frontal boundary. (c) The cyclone becomes mature; distinct warm and cold fronts extend from a low-pressure center. (d) Occlusion begins as the cold front catches up to the warm front. (e) Occlusion intensifies as more of the cold front catches up to the warm front. [Aguado and Burt 291]

Aguado, Edward, and James E. Burt. Understanding Weather and Climate. 7th ed.,  Pearson, 2014.