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Mid-Latitude Cyclones

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Mid-Latitude Cyclones

Where were you on Halloween 2011?

If you live along the northeastern United States you may remember Halloween being postponed in 2011. A large and atypically early nor'easter dropped as much as 32 inches of snow, caused over three million people to lose power, and brought on 39 deaths. Like hurricanes, nor'easters are cyclones, but they form much further north.

Mid-Latitude Cyclones

Cyclones can be the most intense storms on Earth. A cyclone is a system of winds rotating counterclockwise in the Northern Hemisphere around a low pressure center. The swirling air rises and cools, creating clouds and precipitation.

Mid-latitude cyclones form at the polar front when the temperature difference between two air masses is large. These air masses blow past each other in opposite directions. Coriolis effect deflects winds to the right in the Northern Hemisphere, causing the winds to strike the polar front at an angle. Warm and cold fronts form next to each other. Most winter storms in the middle latitudes, including most of the United States and Europe, are caused by mid-latitude cyclones ( Figure below ).

A hypothetical mid-latitude cyclone affecting the United Kingdom

A hypothetical mid-latitude cyclone affecting the United Kingdom. The arrows point the wind direction and its relative temperature; L is the low pressure area. Notice the warm, cold, and occluded fronts.

The warm air at the cold front rises and creates a low pressure cell. Winds rush into the low pressure and create a rising column of air. The air twists, rotating counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Since the rising air is moist, rain or snow falls.

Mid-latitude cyclones form in winter in the mid-latitudes and move eastward with the westerly winds. These two- to five-day storms can reach 1,000 to 2,500 km (625 to 1,600 miles) in diameter and produce winds up to 125 km (75 miles) per hour.

Nor'easters

Mid-latitude cyclones are especially fierce in the mid-Atlantic and New England states, where they are called nor’easters because they come from the northeast. About 30 nor’easters strike the region each year. ( Figure below ).

The 1993 “Storm of the Century” was a nor’easter that covered the entire eastern seaboard of the United States

The 1993 “Storm of the Century” was a nor’easter that covered the entire eastern seaboard of the United States.

An online guide to mid-latitude cyclones from the University of Illinois is found here: http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cyc/home.rxml .

Summary

  • A cyclone is a system of winds rotating counter-clockwise (in the Northern Hemisphere) around an area of low pressure.
  • A mid-latitude cyclone forms at the polar front when the temperature difference between air masses is very large.
  • Nor'easters are mid-latitude cyclones that come from the northeast.

Practice

Use this resource to answer the questions that follow.

http://www.slideshare.net/lschmidt1170/midlatitude-cyclones

1. What is a midlatitude cyclone?

2. What is at the center of a midlatitude cyclone?

3. What does a mature midlatitude cyclone have?

4. Where is the heaviest precipication located in a midlatitude cyclone?

5. What does wind direction tell you?

6. What is a meteogram?

Review

1. Describe the circumstances that result in a nor'easter.

2. What is a cyclone?

3. What are the motions of air in a mid-latitude cyclone?

Vocabulary

cyclone

cyclone

Wind system that rotates around a low pressure center.
mid-latitude cyclone

mid-latitude cyclone

A cyclone that forms in the middle latitudes at the polar front.
nor'easter

nor'easter

Mid-latitude cyclones that strike the northeastern United States.

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