Extratropical cyclone

Big storms move across the Earth.

These storms bring wind and clouds. They can also bring rain or snow.

These storms happen in the middle parts of the world. They move air from one place to another. This helps change the weather fast.

A storm can start when air moves away from the top. This makes the air below feel light. The light air makes the storm grow.

Some storms are very strong. They can even have wind like a hurricane.

These storms can change shape too. They can even grow from a tropical storm. It is amazing how the weather works!

Large storms called extratropical cyclones drive much of our weather.

These storms happen in the middle parts of the Earth. They bring wind, clouds, and rain. Some can even bring snow or tornadoes.

These storms form along weather fronts. A front is a line where temperatures change fast. The storm starts when air moves away from the top of a column. This is called divergence. As air moves away, the air pressure at the bottom drops. This low pressure pulls more air in. This makes the storm grow stronger.

Sometimes, a tropical storm changes into one of these storms. This is called extratropical transition. The storm grows larger but its center gets weaker. It also changes shape. It loses its warm center and becomes a cold-core system. This means the air near the center is very cold. Some storms grow so fast they are called bombs. They can have winds as strong as a hurricane.

Extratropical cyclones are huge weather systems that drive much of the world's weather.

These storms happen in the middle latitudes of the Earth. This is the area between 30 and 60 degrees latitude. They can bring many different kinds of weather. You might see wind, clouds, or mild showers. Sometimes, they bring much harder weather like heavy hail, thunderstorms, or blizzards. Some even cause tornadoes.

People often call them "lows" or "depressions" because of their low pressure.

These storms work by using temperature changes. They form along frontal zones. A front is a line where temperature and moisture change quickly. The process starts with something called cyclogenesis, or low pressure formation.

It often happens near a jet streak. This is a fast-moving part of the jetstream high in the sky. At the top of the storm, air moves away from each other. This is called divergence. When air moves away at the top, the air pressure at the bottom drops. This low pressure pulls more air in from the sides. This upward motion makes the system cloudy.

As the storm grows, it develops fronts. A cold front sweeps toward the equator. A warm front moves more slowly. Eventually, the cold front catches up to the warm front. This is called occlusion. The storm then becomes a "cold-core" system. This means the air near the center is colder than the air around it.

Some storms grow very fast. If the pressure drops quickly, scientists call it a "bomb." These can be very powerful. They can even have winds as strong as a hurricane.

Scientists have studied these storms for a long time. They use special tools to track them. For example, they use the XT technique to see when a tropical storm changes. This is called extratropical transition.

During this change, the storm grows larger but its center weakens. It also loses its warm center. In the Northern Hemisphere, these storms spin counterclockwise. In the Southern Hemisphere, they spin clockwise.

This happens because of the Coriolis effect.

There are many of these storms on our planet. A study found thousands of them in both hemispheres. Between 1979 and 2018, researchers detected many thousands in the Northern and Southern Hemispheres.

In the Southern Hemisphere, there are about 37 cyclones active at any time. In the Northern Hemisphere, about 234 significant storms form each winter. You can see how they look on satellite images. They often look like a giant comma shape in the clouds.

This shape helps weather forecasters understand where the storm is going.

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.