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Our experts explain what is needed for hurricanes to form and why some end up more memorable than others.

Defining major storms: Hurricanes & everything in between

Scott Sutherland
Meteorologist/Science Writer

Friday, June 16, 2017, 1:27 - 'Hurricane' and 'Tropical Storm' are easy enough, but with terms like 'depression', the different categories and descriptors like 'post-tropical', 'extra-tropical' and even 'subtropical' - what do all of these mean?

'Tropical cyclone' is the blanket term for used by meteorologists and atmospheric scientists to describe all of these large, rotating storms that spin up over tropical ocean waters. However, several other terms show up in forecasts and news stories - hurricane, tropical storm, typhoon, etc - depending on what region of the world these storms form, which country is doing the reporting, and how strong the storms get. However, even with all the other possible factors involved in determining how strong they are - central pressure, rainfall, speed, etc - for exactly which term they go by, it all boils down to their sustained wind speeds.

Specifically for storms in the Atlantic and eastern Pacific oceans:

When one of these cyclones is just in the process of developing, so it's just a collection of clouds (storming or not) rotating around a low-pressure centre, with no distinct spiraling or central eye, but with wind speeds measured at up to 38 mph for at least 1-minute, it earns the term Tropical Depression. It doesn't yet earn itself a name, though. Every storm starts out this way, so technically there are several tropical depressions every year.

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When a tropical depression becomes a bit more organized, starts to take on the more familiar spiral shape as the clouds rotate around the core (but still lacks the distinctive eye), and the sustained wind speeds top 39 mph, it becomes a Tropical Storm. It's this point where the storm earns itself a name. Names for Atlantic and Pacific storms around North America go alphabetically, and alternate between male and female names.

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If a tropical storm still has warm ocean waters under it, to tap into as a source of energy, and the winds blowing across the top of the storm aren't driving too hard (ie: wind sheer is low), it will grow stronger. When sustained wind speeds in the storm reach 74 mph, it graduates to become a Hurricane. Technically, this branches off on its own scale, now, defined by what's known as the Saffir–Simpson hurricane wind scale (SSHWS). 

A Category 2 Hurricane, like Hurricane Juan in 2003, is one with sustained wind speeds of between 96-109 mph.

Beyond these are the 'major hurricanes' - Category 3 Hurricanes like Sandy in 2012, that have maximum sustained winds measured anywhere between 111-129 mph, Category 4 Hurricanes such as Ophelia in 2011 (and famous Hazel back in 1954), with maximum sustained winds between 130–156 mph, and the true monsters, Category 5 Hurricanes, where maximum sustained winds exceed 157 mph. In our record-keeping, there have only been 33 Atlantic storms that have reached Category 5. An excellent example of the devastation these are capable of is Hurricane Katrina in 2005, which was one of the five deadliest hurricanes to ever hit the United States, and the costliest natural disaster in the nation's history. Fortunately, we have not seen a storm this powerful since Hurricane Felix in 2007.

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As mentioned above, the strength of a storm depends on how much energy it has available to it, and the ocean surface is the primary source of that energy.

For storms making their way up the U.S. Eastern Seaboard, like Tropical Storm Hermine, the Gulf Stream supplies most of their energy. This warm ocean current hugs close to the east coast of the United States until it reaches North Carolina, and from there it flows on a fairly straight course to the northeast, out into the North Atlantic (shown to the right). The heat spreads out beyond this course, into the waters off of New England and past Nova Scotia, and the amount of heat that flows into these regions fluctuates, depending on shifts in the track of the main current, the time of year, weather patterns, and even climate change.

If a storm maintains a track along the east coast, exactly how long it maintains its strength typically depends on how warm the waters are off of New England. However, on its way, it will weaken any time it encounters land, due to friction with the ground and removal of its ocean energy source, or if it encounters cool waters, and it will strengthen again any time it moves back over warmer waters. This can cause the storm to change status - up and down between hurricane categories and even down to a tropical storm and back up to a hurricane again.

It's when the storm moves over a large tract of land or moves over persistent cool waters that it will quickly lose strength and status. It will scale back down through whichever categories it reached before that, until its maximum sustained wind speeds drop below 39 mph. At this point, the storm becomes Post-Tropical, meaning its just a leftover remnant of a tropical cyclone, but it can still pack some very strong short-term wind gusts and significant rainfall.

There are two more terms that get used once in awhile, which are applied to storms that transition from one type to another.

Although the term Extra-Tropical Cyclone is used to describe your everyday standard weather system - typically shown on weather maps by a Low center with a warm front and a cold front - a tropical cyclone can become extra-tropical if its vertical structure becomes more tilted. At that point, it no longer gets its energy primarily from below, but instead from the differences in temperature horizontally across the storm, it develops warm and cold fronts, and it typically joins up with any other frontal systems that happen to be in the vicinity. Hurricane Sandy went through this kind of transition just minutes before it made landfall on October 29, 2012, causing it to merge with a storm front that was moving in from the west, to become what one meteorologist called a 'Frankenstorm' - in honour of the upcoming Halloween celebration. As Sandy demonstrated, even when a tropical cyclone makes this transition, it can still be very powerful, continuing to pack hurricane force winds and enough moisture to produce heavy rainfall that can touch off major flooding.

The opposite situation to this is when a storm system moves from over land to over water and takes on a more vertical structure - like that of a tropical cyclone. This produces a Subtropical Cyclone. These can form much further north than tropical cyclones do, since these storms have colder temperatures at the storm's top, and thus don't need water temperatures quite as warm at their base to maintain their strength.

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Thumbnail image source: Creative Commons

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