An extratropical cyclone is an atmospheric system whose main source of energy comes from horizontal temperature gradients. These are low-pressure systems usually associated with cold, warm, or occluded fronts.
Unlike tropical cyclones, extratropical cyclones don’t have strong horizontal temperature gradients throughout the entire diameter of the storm at surface level. The winds in a tropical cyclone are powered by energy released during cloud formation and rainfall. Tropical systems are typically made up of one warm air mass, while extratropical cyclones involve two or more air masses—warm and cold—so they’re often tied to one or more frontal boundaries.
In the upper diagrams (not shown here), you’d see horizontal maps of surface temperature, pressure, and wind fields for both a tropical cyclone (left) and an extratropical cyclone (right). The colors indicate temperature:
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Blue = 15°C / 59°F
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Blue-green = 20°C / 68°F
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Green = 25°C / 77°F
Dotted lines represent surface wind speeds:
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34 knots = 39 mph = 63 km/h
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64 knots = 74 mph = 117 km/h
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96 knots = 110 mph = 174 km/h
Solid lines show isobars, or lines of equal atmospheric pressure.
The lower diagrams show vertical pressure surfaces, temperature anomalies, and circulation patterns at the surface and the tropopause (the boundary between the troposphere and the stratosphere).
One of the key differences:
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In tropical cyclones, the strongest winds occur near the surface.
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In extratropical cyclones, the strongest winds are found closer to the tropopause, roughly 12 km (8 miles) above the surface.
Why the difference?
Tropical cyclones have what's known as a "warm core" in the troposphere (just below the tropopause), meaning the center of the storm is warmer than the surrounding environment at the same atmospheric pressure level (also known as isobaric level—which is similar to measuring altitude from the ground).
Extratropical cyclones, on the other hand, have a cold core in the troposphere and a warm layer in the stratosphere above the tropopause. This difference in structure is what causes the different wind patterns and storm behaviors.
Contribution by: Stan Goldenberg
Source: NOAA