Extratropical cyclone

Extratropical cyclones are massive, large-scale weather systems. They are also known as mid-latitude cyclones or wave cyclones. These systems drive much of the weather in the Earth's middle latitudes. This area is located between 30° and 60° latitude on both sides of the globe.

These storms are powerful and can produce many different weather conditions. They can cause wind, clouds, and mild showers. They can also cause severe hail, thunderstorms, blizzards, and even tornadoes.

Because they are areas of low atmospheric pressure, people often call them "lows" or "depressions."

These cyclones are classified as baroclinic systems. This means they form along frontal zones where there is a sharp change in temperature and dew point. The process of forming these low-pressure areas is called cyclogenesis. It often begins near a jet streak. A jet streak is a faster-moving part of the high-altitude jetstream.

Within certain quadrants of a jet streak, air moves away from each other. This is called divergence. As air moves away at high altitudes, the mass in the air column decreases. This causes the atmospheric pressure at the surface to drop. This drop in pressure draws in more air at the surface, which creates convergence. This upward motion of air makes the system cloudy.

As an extratropical cyclone matures, it develops distinct weather fronts. A cold front moves toward the equator and travels around the back of the system. A warm front moves more slowly because the denser, cooler air ahead of it is harder to move. Eventually, the cold front catches up to the warm front. This process is called occlusion. When this happens, a tongue of warm air is forced upward, which is called a trowal.

This process causes the cyclone to become "cold-core." This means the air near the center is colder than the surrounding air. This is the opposite of a tropical cyclone, which has a warm core.

Sometimes, these storms grow with incredible speed. If the atmospheric pressure falls very rapidly, it is called explosive cyclogenesis. Scientists call these extremely fast-growing storms "bombs." These systems can become incredibly intense. In the North Atlantic and North Pacific, they can reach hurricane-force winds during December and January.

For example, an extratropical cyclone near Iceland in December 1986 dropped to a pressure equivalent to a Category 5 hurricane. Such intense systems show how much energy these mid-latitude storms can hold.

Tropical cyclones can also transform into extratropical cyclones. This process is known as extratropical transition. It usually happens between 30° and 40° latitude. During this transition, the storm's size typically increases, but its central core weakens.

The storm also changes shape and becomes less symmetric. The energy source shifts from the latent heat of condensation to baroclinic processes. Scientists use the XT technique to estimate the intensity of these transitioning storms. This technique helps them track the storm as it loses its warm core and becomes a cold-core system.

Extratropical cyclones are very common across the planet. A study using reanalysis data between 1979 and 2018 found massive numbers of these systems. Researchers detected between 49,745 and 72,931 cyclones in the Northern Hemisphere. They also found between 71,289 and 74,229 in the Southern Hemisphere.

In the Southern Hemisphere, there are about 37 cyclones active during any six-hour period. In the Northern Hemisphere, about 234 significant extratropical cyclones form every winter.

The way these storms spin depends on where they are located. This is due to the Coriolis effect. In the Northern Hemisphere, the wind flows counterclockwise around the center. In the Southern Hemisphere, the winds spin clockwise.

The wind is usually strongest on the side of the cold front or the occlusion. These storms are a vital part of the Earth's atmospheric system. They help move heat and moisture around the middle latitudes of our world.