Tropical cyclones are the storms of a thousand names; each refers to a different stage of development, but all include the term "tropical" (which refers to their formation at tropical latitudes and within maritime tropical (mT) air masses). Not knowing the differences between each could put you at greater risk during hurricane season. But not to worry! The following list (in order of increasing intensity) will help keep you cyclone literate.
Tropical cyclones begin (and end) as extratropical. That is, they resemble low pressure systems, complete with a warm and cold front.
How, then, does a harmless "low" begin to transition into a damaging hurricane? There are two requirements necessary to make any cyclone tropical: (1) latent heat energy to power the storm and (2) a warm core. Extratropical cyclones have neither of these; they're powered by temperature contrasts between warm and cold air masses, and cold temperatures are found at their core. But by traveling over warm waters, extratropical systems can quickly strengthen into tropical disturbances or tropical storms (more on this later).
If an extratropical cyclone moves over warm waters, but not warm enough (at least 80°F (26.5°C)) to turn it fully tropical, it becomes what's called subtropical.
Subtropical cyclones possess a mix of both extratropical and tropical characteristics. The cold core still exists, but the introduction of warm sea air (at least 70°F (21°C)) into the storm's low levels begins to blend the warm/cold temperature contrast. This causes the warm and cold fronts to vanish, instability to occur, and thunderstorms to grow. As more and more thunderstorms "fire off" inside of the storm complex, its center becomes moist and warm from the release of latent heat.
If a subtropical cyclone's winds are under 39 mph, the storm is called a subtropical depression. If winds are at or above 39 mph, the storm is called a subtropical storm.Tropical Disturbance
Once the center is entirely warmed through the cyclone finally becomes fully tropical! But before it can be called a tropical disturbance, tropical characteristics must be maintained for at least 24 hours.
When maximum sustained winds increase to 39 mph and the system begins to noticeably spin counterclockwise (clockwise in the Southern Hemisphere) as it moves, it is declared a tropical depression.
When maximum sustained winds increase to between 39-73 mph, the tropical depression is upgraded to a tropical storm.
When maximum sustained winds are measured at 74 mph or more, the tropical storm is upgraded to a hurricane.
When a hurricane strengthens to a category 3, 4, or 5 storm on the Saffir-Simpson scale it assumes the title of "major." At this strength, catastrophic wind and flood damage occurs.
When a tropical cyclone begins to lose its tropical characteristics, it is deemed post-tropical.
Despite being weakened storms, post-tropical cyclones can still carry a threat of heavy rains and high winds. If conditions once again become favorable, they can also re-intensify.
A post-tropical cyclone is designated extratropical once its remaining tropical characteristics are lost. Quite often these storms will merge with a passing frontal system and travel poleward, towards the polar front (the sharp barrier separating cold polar air to the north from warm tropical air to the south). Once exposed to this cold pool of air, the storm's winds begin to wrap it inward, creating the warm/cold temperature contrast characteristic of an extratropical system. Eventually, a warm and cold front form.
Although rare, hurricane and tropical-storm-force winds can still exist at this weakened stage.
Once all thunderstorm activity has ceased and winds have subsided to 39 mph or less, the post-tropical cyclone becomes a remnant low. This is the final stage of dissipation.