With all the jargon for tropical storms and hurricanes, what does it all mean?
With all eyes on Hurricane Arthur this week, as it becomes the first hurricane to make landfall in the United States since Sandy in 2012, there have been a lot of terms being thrown around - tropical depression, tropical storm, category 1, category 2, and even "post-tropical system with elements of a hurricane and a nor'easter" - so what, exactly, do all of these really 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 62 km/h 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. However, since the term that sticks with any particular storm is the 'highest' one it reaches, Tropical Depression Eight, which developed in the Gulf of Mexico in early September 2013, is the latest example we've seen in North America. Its maximum sustained winds only reached 55 km/h, and it lasted for just around six hours after making landfall, but it still packed enough heavy rain to trigger flooding in several regions of southern Mexico, including Mexico City.
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 63 km/h, 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. Although Arthur is the latest Atlantic storm to become a tropical storm, it advanced beyond that, so the last official one in the Atlantic was Tropical Storm Melissa, in November 2013. In the eastern Pacific, where storm activity has been higher (likely due to the influence of the developing El Nino), the latest are Tropical Storms Douglas and Elda.
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 118 km/h, 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). The weakest storm on this scale is considered a Category 1 Hurricane. By far the most common strength of hurricane, Hurricane Ingrid (mid-Sept 2013) was the last Atlantic storm to top out in this category, and this was the strength of Hurricane Earl when it hit Nova Scotia back in 2010. A Category 2 Hurricane, like Hurricane Juan in 2003, is one with sustained wind speeds of between 154-177 km/h.
Beyond these are the 'major hurricanes' - Category 3 Hurricanes like Sandy in 2012, that have maximum sustained winds measured anywhere between 178–208 km/h, Category 4 Hurricanes such as Ophelia in 2011 (and famous Hazel back in 1954), with maximum sustained winds between 209–251 km/h, and the true monsters, Category 5 Hurricanes, where maximum sustained winds exceed 252 km/h. 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.
Next Page: Strength of the storm depends on how much energy