Cyclogenesis: The Making Of An Extratropical Low Pressure System

By

Home / Cyclogenesis: The Making Of An Extratropical Low Pressure System
The low pressure center on the east coast is intensifying. The cold front has begun to catch up to the warm front. Analysis courtesy of NOAA

The low pressure center on the east coast is intensifying. The cold front has begun to catch up to the warm front. March 30 analysis courtesy of NOAA

The Weather Channel calls it Bombogenesis, but the image is not one that should be connected with a weather system.

The damage is occasionally similar to that done by a bomb, but a more careful choice of words is in order.

Cyclogenesis may be a tongue-twister, but it has the right connotation — the genesis, or beginning, of a cyclone.

First Things First: What’s a Cyclone?

In its broadest sense, a cyclone is a circulation of wind around a low pressure center. Meteorologists generally use the term in connection with large-scale systems as opposed to tornadoes and dust devils.

Confusion arises because storms that are called hurricanes in the Atlantic and eastern Pacific Oceans, and typhoons in the western Pacific, are called tropical cyclones in the Indian Ocean. Therefore we have to use the awkward term extratropical cyclone to refer to low pressure centers in the middle latitudes. From this point forward, in this article, I will use the word cyclone to mean a mid-latitude low pressure system.

Why Does A Cyclone Form?

The simple answer is — to keep meteorologists busy. The right answer, however, is not so simple.

On a global scale, cyclones form to maintain the energy balance on a rotating planet with differential heating between the equator and the poles. On a scale of hundreds of miles, a cyclone forms to reduce the potential energy that exists locally due to a large temperature gradient (difference in temperature with distance).

The cyclone redistributes the air so that warm air lies atop cold (a lower energy state), and turns the lost potential energy into wind (kinetic energy). The wind is then dissipated by friction into the ground and becomes heat.

The Role Of Pressure In Cyclogenesis

Atmospheric pressure is the weight of air. The atmosphere is not distributed uniformly; there is less air above some places than others. On a spinning globe, wind blows with low pressure on its left and high pressure on its right. But not exactly. There can be a component of wind that blows towards or away from high or low pressure. A net flow of air away from a developing low pressure center will cause the pressure to continue to fall.

The Role Of The Jet Stream In Cyclogenesis

The dynamics of an atmosphere on a spinning globe include a jet stream at middle levels of the atmosphere. The theory of how this comes about is extremely complicated, but the fact is not: At an altitude of about three miles in the middle latitudes (30 to 50 degrees) winds blow from the west at speeds up to 150 miles per hour. Within this jet stream flow, waves move from west to east at an average of 25 miles per hour.

On the forward edge of a wave, a zone of divergent winds forms. If wind flows away from a region above the ground, the pressure at the ground will drop. This is the center of the cyclone.

If the condition persists, the pressure will fall enough to form a powerful storm. The average atmospheric pressure around the world is about 1013 millibars, which is equivalent to the weight of 29.92 inches of mercury. The pressure in a very intense cyclone such as the recent Winter Storm Xenia is around 955 millibars or 28.20 inches of mercury.

Fronts and Cyclones

Last week's Winter Storm Xenia is off Nova Scotia and the fronts have occluded. Analysis courtesy of NOAA.

Last week’s Winter Storm Xenia is off Nova Scotia and the fronts have occluded. Analysis courtesy of NOAA.

A cyclone forms on the boundary between warm and cold air masses, called a front.

As the cyclone starts spinning, the warm air ahead of the low pressure center moves northward and slides over the cold air. Behind the front, the cold air undercuts the warm. Eventually the cold front catches up to the warm front and squeezes the warm air upward to create the lower energy state.

The line that marks the meeting of the two fronts is called an occluded front.

Storms Winding Down

When you see an occluded front on a weather map, the storm has matured and should be ready to wind down. There is no single word to describe the final stage of a cyclone, so I will coin one: cyclodissipation.

Leave a Comment