Tropical Cyclone, Subtropical Cyclone, Extratropical Cyclone – what’s the difference? They’re all vortex regions of low pressure with wind and rain, aren’t they? Yes, yes, but there are obvious differences that set them apart from each other.

You’ll hear these terms much more often as we approach the official start of Hurricane Season in the Atlantic Basin, so today we’re going to look at what classifies a hurricane as. one of three hurricanes. All satellite images we use will be obtained from weather.us database this.

Tropical storm

(Hurricane Michael – October 10, 2018)

Definition:

  • intense storms originating in warm/tropical oceans
  • warm core system
  • Formed at latitudes 5 degrees N and above
  • Characterized by low pressure
  • winds exceeding 74 mph

Featured:

  • Round storm
  • Quite compact – average diameter ~200 mi
  • Rotate counterclockwise (northern hemisphere)
  • Wind controlled by pressure drop
  • Pre-existing perturbation requirement with sufficient convergence
  • SST requirement of 26.5 degrees Celsius (80 degrees Fahrenheit) at a depth of 50 meters (150 feet) for fuel delivery
  • Requires a longitudinal shear force of 10 m/s-1 or less for reinforcement. Strong shear forces will tilt the circulation and destroy the storm.
  • Requires humid air. Dry air will kill the storm.
  • Area difference:
    • Outer Squall Road – fairly straight stream of showers/storms more than 50 miles ahead of the storm
    • Outer reflective band – usually in numbers 2-5, made up of showers/storms. Wind increases as the distance to the center decreases.
    • Rain Shield – heavy rain and increasing wind. Usually tropical storm force / hurricane force sustains the winds here.
    • Eyeglasses – strong convection around the eyes. Most of the fierce winds and rain are found here.
    • Eye – round center of tropical cyclone. Usually it’s mostly clear / clear, with light winds and no rain. The lowest pressure is found here.
  • Strengthens on warm waters, weakens/disappears on land

Subtropical Cyclones

(Subtropical Cyclone Melissa – October 11, 2019)

Definition:

  • Features of both tropical and extratropical cyclones – a mixed storm
  • Formed from an extratropical cyclone
    • a cold core system that is cut off from the flow under conditions more favorable for convection development (requires SSTs of at least 20 degrees Celsius (68 degrees Fahrenheit))
  • warm core

Featured

  • Convection near the center of the warm core
    • Convection initially warms the core/adds moisture allowing subtropical cyclones to form
  • There is a wider wind field – maximum winds extend much further away from the center than in tropical cyclones
  • The temperature is higher than that of a tropical cyclone
  • May be sustained by lower SSTs than tropical cyclones – typically 24 degrees Celsius (75 degrees Fahrenheit)
  • Can form at higher latitudes than tropical cyclones
  • Common in North Atlantic Ocean

A bit of history: Subtropical cyclones were not recognized by the NHC until 1972. They were also not named until 2002.

Extratropical cyclone

(North Atlantic extratropical cyclone – March 9, 2022)

Definition:

  • Also known as mid-latitude cyclone
  • Large scale low pressure system
  • Regulates much of the weather on Earth
  • Cold core

Featured:

  • Occurs from 30 degrees to 60 degrees latitude
  • Create a variety of weather
    • shower head
    • gales
    • thunderstorms
    • snow storm
    • tornado
  • There is both a warm front and a cold front
  • Rapid change in temperature / dew point along its front face
  • Cyclone formation occurs along baroclinic zones (areas with sharp temperature gradients/dewpoints) with high divergence.

Fun fact: tropical cyclones leave the tropics, lose all tropical features, and transform into extratropical cyclones commonly known as Post-tropical cyclone.

I hope you enjoyed my brief insight into the differences between the types of tornadoes – and I hope you learned something, too!

As always, if you want to know more about a certain topic feel free to tweet me an idea and I’ll see where I can in our blogs!

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