Most people in the world will never experience a tropical
cyclone. This is because the majority of the population lives in the mid-latitudes,
which is typically considered to be between 30 and 60 degrees latitude. If this
is the case, what are those giant weather making spirals that regularly roll
through? Well, those are Extratropical Cyclones, also called Mid-Latitude
Cyclones (or MLC, which is what I will typically refer to them as). These
storms play a significant role in world weather, especially during the winter. Although
they appear somewhat like their tropical counter parts, they are vastly
different. For this post I’m going to start off with just a comparison between
the two types of cyclones, the specific features of MLCs are each interesting
on their own, so I’ll reserve detailed descriptions of them for later posts.
-Size:
Perhaps the most apparent difference between tropical cyclones and MLCs is their size. While tropical cyclones certainly look impressive, the main cluster of clouds near the center is only on the order of 100 km or so. In atmospheric science terminology this size is labeled meso-scale (as in middle), alongside other weather phenomena such as squall lines. MLCs, on the other hand, are on the order of 1000 km or more, a size that belongs in the category of synoptic-scale. It should be noted that while tropical cyclones are considered meso-scale, they are very near the upper boundary of that category. Furthermore, some individual tropical cyclones do grow significantly larger, such as Hurricane Sandy, and might be considered synoptic-scale storms in some respects.
Perhaps the most apparent difference between tropical cyclones and MLCs is their size. While tropical cyclones certainly look impressive, the main cluster of clouds near the center is only on the order of 100 km or so. In atmospheric science terminology this size is labeled meso-scale (as in middle), alongside other weather phenomena such as squall lines. MLCs, on the other hand, are on the order of 1000 km or more, a size that belongs in the category of synoptic-scale. It should be noted that while tropical cyclones are considered meso-scale, they are very near the upper boundary of that category. Furthermore, some individual tropical cyclones do grow significantly larger, such as Hurricane Sandy, and might be considered synoptic-scale storms in some respects.
-Energy Source:
While not as apparent as some other factors, the source of the storm’s energy is a fundamentally defining characteristic. Tropical cyclones derive their energy from heat released by warm moist air as the water vapor inside it condenses. This causes the core to become very warm, which causes a drop in pressure. This in turn enhances the flow of the warm air into the system, thus acting as a positive feedback loop as long as the storm remains over warm water. The power behind MLCs is entirely different, although it is also organized about a low pressure center. The fuel for MLCs is the temperature gradient between the cold air descending from the polar regions and the warm air rising from the tropics. When a weak low pressure center forms, it twists the gradient cyclonically (counter-clockwise in the northern hemisphere). This allows the dense cold air to slip southward and around the low, while the warm air is allowed to proceed northward.
While not as apparent as some other factors, the source of the storm’s energy is a fundamentally defining characteristic. Tropical cyclones derive their energy from heat released by warm moist air as the water vapor inside it condenses. This causes the core to become very warm, which causes a drop in pressure. This in turn enhances the flow of the warm air into the system, thus acting as a positive feedback loop as long as the storm remains over warm water. The power behind MLCs is entirely different, although it is also organized about a low pressure center. The fuel for MLCs is the temperature gradient between the cold air descending from the polar regions and the warm air rising from the tropics. When a weak low pressure center forms, it twists the gradient cyclonically (counter-clockwise in the northern hemisphere). This allows the dense cold air to slip southward and around the low, while the warm air is allowed to proceed northward.
-Fronts:
Fronts are a feature exclusive to MLCs. In a basic sense, fronts are where the two air masses (warm and cold) meet and form a steep gradient of temperature. The cold front is often more dominant that the others. Due to its density, it plows straight into warm air, which is forced to rise sharply up the frontal boundary. The warm front is much more passive, it is caused by warm air gently sliding up the retreating cold air mass. Besides warm and cold fronts, there are also occluded fronts and stationary fronts. Occluded fronts form when a MLCs cold front ‘catches’ up to the warm front in a manner reminiscent of a zipper. At this point, warm air at the surface is no longer connected to the low’s center. Finally, a stationary front is similar to a cold front in structure, but the cold air mass isn’t really advancing into the warm air.
Fronts are a feature exclusive to MLCs. In a basic sense, fronts are where the two air masses (warm and cold) meet and form a steep gradient of temperature. The cold front is often more dominant that the others. Due to its density, it plows straight into warm air, which is forced to rise sharply up the frontal boundary. The warm front is much more passive, it is caused by warm air gently sliding up the retreating cold air mass. Besides warm and cold fronts, there are also occluded fronts and stationary fronts. Occluded fronts form when a MLCs cold front ‘catches’ up to the warm front in a manner reminiscent of a zipper. At this point, warm air at the surface is no longer connected to the low’s center. Finally, a stationary front is similar to a cold front in structure, but the cold air mass isn’t really advancing into the warm air.
-Weaknesses:
Perhaps the most dramatic display of the two cyclones’ differences are the mechanisms that weaken them. For starters, if a tropical cyclone encounters a high shear environment it will be torn apart, unlike a MLC thrives on shear, in fact they would die without it. Furthermore, while a tropical cyclone must remain over warm water, MLCs can exist over cold water, or land for that matter. Lastly, due to the heated center, tropical cyclones are referred to as warm core lows, which is just the opposite as most MLCs, which are appropriately referred to as cold core lows.
Perhaps the most dramatic display of the two cyclones’ differences are the mechanisms that weaken them. For starters, if a tropical cyclone encounters a high shear environment it will be torn apart, unlike a MLC thrives on shear, in fact they would die without it. Furthermore, while a tropical cyclone must remain over warm water, MLCs can exist over cold water, or land for that matter. Lastly, due to the heated center, tropical cyclones are referred to as warm core lows, which is just the opposite as most MLCs, which are appropriately referred to as cold core lows.
Below are a several images of MLCs of all shapes and sizes.
Notice that often, the clouds associated with these systems are not arranged in
anything that resembles a spiral.
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