Cyclones and Anticylones
Learning Objectives
Midlatitude cyclones (extratropical cyclones) are low pressure systems outside of the tropics. Air in the cyclone moves counterclockwise around a low pressure center.
Cyclones move eastward in the Northern Hemisphere carried by prevailing winds.
Cyclones last several days to a week.
Depicted as an "L" on the weather map, cyclones bring rain and wind to an area.
Cyclones are fueled by the temperature differences (hence pressure gradients) that exist along frontal boundaries. They are usually strongest during cold months when temperature differences between air masses can be most extreme.
Formation:
Most cyclones start as an "upper atmosphere disturbance." Air moving through ridges of high pressure and troughs of low pressure converges and diverges. Surface air moves up into an area of divergence. As the surface air rises it begins to rotate as a cold air mass moves southeastward and a warm air mass moves northward.
Characteristics:
The cyclone is a center of low pressure. Lifting occurs and clouds form. Severe storms can be common if the temperature difference between the warm sector and the cold sector is extreme. The cyclone encompasses a cold front, a warm front and eventually it can even form an occluded front.
Weather:
The weather experienced as a cyclone moves over an area depends upon what portion of the storm passes overhead.
If the cyclone center passes to the north an area will first experience a warm front (clouds and rain followed by warming temperatures) and then a cold front (heavy rain or thunderstorms followed by colder temperatures).
If the cyclone center passes to the south an area will experience clouds and rain but the temperature will not change much and severe storms are uncommon.
As the cyclone develops it often forms a characteristic "comma shaped" cloud mass. An occluded front forms as the cold front overtakes the warm front. Occlusion usually acts to break up the storm dissipating the clouds and rain.
Cyclones that affect the United States originate from different areas bringing distinctly different weather patterns.
- Cyclones originating over the Pacific can strike anywhere along the Pacific coastline but are most common from northern California northward. These storms usually weaken as they move over the mountains but can restrengthen bringing snow to the Plains States.
- Alberta Clipper is the term used for a cyclone that originates over the Canadian interior and moves into the Plains. This storm will result in large snowfalls over the Midwest.
- Cyclones originating over the Gulf of Mexico will travel either to the east or the west of the Appalachians.
- Inside Runners: these storms travel up the west side of the Appalachian Mountains. They bring snow to the mountains and inland and rain to the coast. Do not bring particularly cold temperatures.
- Outside Runners: these storms move up the coast east of the mountains. They often strengthen as they encounter the warmer ocean waters (especially those off Cape Hatteras.) If they are positioned along the coast or just offshore the counterclockwise rotation brings wind from the northeast...these are the NorEasters. NorEasters bring heavy rain to the coast. The large snowfalls that Hampton Roads occasionally experience are the result of outside runners.
Anticyclones:
As the name suggests, anticyclones are the opposite of cyclones. Air converging at altitude sinks bringing dry conditions to the surface. Air moves clockwise in a descending spiral around a high pressure center. Depicted as an "H" on the weather map, anticyclones bring clear, pleasant weather.
Formation: form from convergence in the upper atmosphere forcing air to descend.
