Amy has taught university-level earth science courses and has a PhD in Geology.
Cyclonic Behavior of the Atmosphere
You've likely seen dramatic satellite photos of hurricanes in the Atlantic Ocean off the coast of the U.S. These storms mean strong winds, flooding, and damage if they struck land. You also may have heard local weather reporters discussing the storms losing strength as they impacted the land. What you likely did not hear the reporters discussing is that all of these storms are rotating in the same direction, or that they are associated with strong low-pressure systems.
In this lesson, we're going to learn how cyclone systems develop in the atmosphere and how different types of cyclonic storms are produced. Next time you see coverage of hurricanes on the national news, you'll be able to understand a little more about where the storm came from.
Developing Storm Systems
Before storm systems become fully developed cyclones or anticyclones, they first form as ridges and troughs. To understand these terms, let's review air pressure isobars. Isobars are lines marking areas of equal air pressure just like contours mark areas of equal elevation on a map. In ridges, the isobars are elongated towards the north. In troughs, the isobars are elongated towards the south. You can think of them as pressure topography of atmosphere, with the ridges being high-pressure mountains and the troughs being low-pressure valleys.
While these low-pressure troughs can start as elongated features, they can also develop into closed cells through spinning of the winds via the Coriolis force and pick up enough energy to become cyclones. The same is true of high-pressure ridge systems, which will form into closed anticyclone cells. This is the formation of the closed high and low-pressure isobars that you've likely seen in the local weather reports across the country.
So, in review, cyclones are closed cells with a low-pressure center, and anticyclones are closed cells with a high-pressure center.
So, you can think of ridges and troughs as beginner cyclones. These ridges and troughs would not develop into fully formed cyclones or anticyclones without the Coriolis force. The Coriolis force directs winds away from their original path due to the rotation of the Earth. Because the Earth is a sphere and is rotating to the east, moving winds in the Northern Hemisphere are deflected to the right and winds in the Southern Hemisphere are deflected to the left.
Now, if there were no Coriolis force, air would choose to move along the pressure force gradient, which is the shortest distance between high pressure and low-pressure zones. Therefore, air would want to rush directly into a low-pressure trough or directly away from a high-pressure ridge. But because of the Coriolis effect, these winds will not travel directly along their intended path.
Remember that winds in the Northern Hemisphere are directed to the right and winds in the Southern Hemisphere are directed to left. So, in the Northern Hemisphere, as winds are rushing to the center of the storm's eye in low-pressure systems, they are deflected to the right. This produces the counterclockwise spinning of some storms that we witness in the Northern Hemisphere. The opposite happens in the Southern Hemisphere where the deflection of winds to the left creates a clockwise spinning.
These counterclockwise spinning cells in the Northern Hemisphere and clockwise spinning cells in the Southern Hemisphere are also called cyclones. 'Cyclone' is the general term for rotation around a low-pressure system and thus eliminates the confusion over the direction of the rotation due to the hemispheric difference. In fact, here in the U.S., we call low-pressure storms in the Atlantic hurricanes, but they call the same storms cyclones in the Southern Pacific. According to the definition of cyclone, though, all of these storms are cyclones, despite where they occur, or what the locals choose to name them.
So, that covers low-pressure systems. What about high-pressure systems? Although high-pressure systems are generally associated with calmer weather patterns, there are still some associated winds due to the pressure gradient force from the center of the pattern to the outside. However, in this case, the higher pressure is in the center. Thus, air is moving from the center outward. Just as in low-pressure systems, the air is deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Therefore, high-pressure systems in the Northern Hemisphere will spin clockwise and counterclockwise in the Southern Hemisphere.
These spinning storms around high-pressure systems are called anticyclones because they have the opposite rotation to low-pressure systems. Anticyclones spin clockwise in the Northern Hemisphere and counterclockwise in the Southern.
In review, cyclones are rotation around a low-pressure system that spin counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. This is because the Coriolis effect directs winds away from their original path due to the rotation of the Earth and deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Anticyclones are spinning storms around high-pressure systems. Therefore, anticyclones have the opposite spin than cyclones.
We also learned that before storms are full-fledged cyclones or anticyclones, they start out as either ridges or troughs. Ridges occur when isobars are elongated towards the north. Troughs are when isobars are elongated towards the south. These ridges and troughs can develop further into fully closed cells according to the Coriolis force.
Cyclones & Anticyclones Terms & Definitions
|Isobars||lines marking areas of equal air pressure just like contours mark areas of equal elevation on a map|
|Ridges||isobars are elongated towards the north|
|Troughs||isobars are elongated towards the south|
|Cyclones||closed cells with a low-pressure center|
|Anticyclones||closed cells with a high-pressure center|
|Coriolis force||directs winds away from their original path due to the rotation of the Earth|
|Pressure force gradient||the shortest distance between high-pressure and low-pressure zones|
With careful study of this lesson's content, you can go on to exhibit your ability to:
- Contrast cyclones and anticyclones
- Compare ridges to troughs
- Describe isobars
- Determine what makes the Coriolis force
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