Extratropical Cyclone

The storms we usually associate with the word "cyclone" are large storms which occur in the tropics, and may move North into the temperate zones in the form of hurricanes or typhoons. However, cyclones can and do occur at any latitude and in any climate. Those that are born within 30 degrees of the equator (north or south of that line) are tropical cyclones. Those found above 60 degrees north or south of the equator are arctic or polar cyclones.

However, many cyclones also happen every year between 30 and 60 degrees -- these are called extratropical cyclones. Each kind of cyclone forms, develops, and acts in a different way, since different climatic forces are at work shaping it, but all cyclones -- as their name suggests -- are storms which show rotation over a large area.

Where a tropical cyclone is almost perfectly round and has very little temperature difference from one part of the storm to another, an extratropical cyclone includes both a warm front and a cold front (which may combine into an occluded front when the cold front catches up with its warmer counterpart) and is a bent oval shape which is often described as a comma. A tropical cyclone contains no weather fronts, either cold or warm, while extratropical cyclones are the exact opposite, with weather fronts being the basis for their whole structure and existence.

It is an interesting fact, however, that tropical cyclones often transform into extratropical cyclones after they move out of the tropics, towards the end of their life cycle.

We experience extratropical cyclones as changeable, stormy weather, often with first warmer, then colder air entering the area, accompanied by rain and thunderstorms. The complex weather patterns of these storms mean that the bad weather they bring can be both long-lasting and various, and low pressure is likely to remain for quite some time while they are are active in our region. Extratropical cyclones are also very large, since they contain two fronts and several air masses, which also helps to make their weather effects last for a long time.

Extratropical cyclone causes

An extratropical cyclone forms because of the interaction between a weather front (or frontal boundary) and an active upper level jet stream. The area of low pressure which is the cyclone's core is strengthened by the jet stream, which pulls the already-rising air of the low pressure system upwards. Since the low pressure area is being intensified by air being pumped upwards into the jet stream, it can grow very intense and the air will begin to rotate around the low pressure center, much as water going down a bathtub drain rotates, but in reverse.

Since the atmosphere around the low begins to revolve around it, this generates both a cold front and a warm front. The cooler air to the north of the low rotates around it to the south, becoming a cold front. On the other side of the low, the opposite is occurring -- warm air is being rotated north, turning into a warm front. The entire process of extratropical cyclone birth ensures that a cold front and a warm front will always form, since cold air is twisted south along one side of the cyclone while warm air is twisted north around the other side.

An extratropical cyclone, in short, is a very intense low pressure which is powered, at least to begin with, by interaction with a high level jet stream. The low pressure system itself can be caused by frontal disturbances, the landscape and high pressure systems, and so forth, but the cyclone that develops always follows a similar pattern and needs jet stream intervention to strengthen into a true extratropical cyclone.

What an extratropical cyclone does

Once it has formed, an extratropical cyclone moves across the planet's surface as a large storm system. The warm and cold fronts each produce weather systems that they would ordinarily generate. However, the cold front almost always moves faster than the warm front. This is because cold air is denser than warm air. The warm front pushes more slowly, since it is moving a denser mass out of the way, while the cold front, with its greater mass, moves quickly, displacing the lighter warm air ahead of it with ease.

Sooner or later, the cold front catches up to the warm front, since they are rotating around a common center, but at different speeds. When this happens, they blend together into an occluded front. This pushes the warm air aloft, but also lessens the cooling effect of the cold air. An occluded front is marked by rain and thunderstorms, but raises or lowers the temperature only slightly. Drier air usually follows behind an occluded front, however.

Eventually, the low pressure area moves far enough from the jet stream to weaken, and the inward rotation 'fills' the low with more air, weakening it yet more. A few extratropical cyclones are restored to full power at this point by intersecting the jet stream again, but most melt away as the low weakens and can no longer support the cyclonic rotation needed for a cyclone.

Extratropical transition -- from tropical to extratropical

Tropical cyclones that move north into the temperate zones eventually become extratropical cyclones, with a warm and cold front pair rotating around their core. This is the fate of hurricanes which penetrate far enough north. Since they are no longer in a region dominated by tropical conditions, they begin to pull cold air along one edge and warm air along the other around their center of rotation. In time, they begin to behave much like an extratropical cyclone, lose their round shape, and become the distorted oval of an extratropical cyclone before dissipating.

Once in a very long time, an extratropical cyclone will move out over warmer waters, usually in the autumn, and transform into a tropical cyclone -- a process called tropical transition.

Violent weather and extratropical cyclones

Extratropical cyclones aren't quite as strong as hurricanes, since they form differently and usually move over land rather than water, limiting the moisture they can pick up. However, they can still produce very intense thunderstorms, squall lines, powerful winds, hail, and tornadoes. If you check the weather map and see the pattern of an extratropical cyclone -- a warm front and cold front at right angles, meeting at a common center -- headed your way, then you should be prepared for wet weather, thunderstorms, and high wind.