Weather Blog

The scoop on tropical storms vs. hurricanes

The scoop on tropical storms vs. hurricanes
 I figured with Tropical Storm Fay in the news wandering around Florida, now might be a good time to chat about hurricanes and tropical storms and answer some frequently asked questions we get in the ol' e-mail bin:

1) What is the difference between a "Tropical Storm" and a "Hurricane"?

It's all about the wind. A hurricane is any tropical based storm that has wind speeds greater than 74 mph. A "tropical storm" is a storm with winds of 39-73 mph. A "tropical depression" is when the Florida Marlins lose to the Atlanta Braves -- or when a storm is tropical in nature, but has winds under 39 mph.

We only name the storms when they get to "tropical storm" strength or higher. Tropical depressions just get plan numbers like "Tropical Depression 3". I guess that's because we only have 21 names and we don't want to run out as they are more common.

2) So, then what's a typhoon?

That is a hurricane, just in a different part of the world. The term "hurricane" is used for storms in the North Atlantic, Northeast Pacific Ocean east of the International Dateline, or the south Pacific Ocean east of 160 degrees East longitude.

A typhoon is a storm in the northwestern Pacific west of the International Dateline -- i.e., storms that strike the northeast Asian coasts like Japan, China and Taiwan.

A cyclone is officially a hurricane/tropical storm that occurs in the Indian Ocean. Although the term has also become colloquially associated with tornadoes in the Midwest, but that is not an official definition. Cyclone Nargis was in the news recently when it hit Myanmar.

3) Fay? Cristobal? Dolly? Who Names These Things? 

The World Meteorological Organization is in charge of selecting names for hurricanes. It was developed in the 1950s to make it easier to communicate storms to the public and mariners (instead of "that big storm at 34.423 degrees N and 78.232 degrees West, oops now 78.233").

It used to be just women's names were used, but in 1979, it was decided that men shouldn't miss out on being picked on for having a storm named after them, so now it alternates male/female. (And as of yet, no "Scott" on the list, whew!)

There are six lists that rotate every six years. When a hurricane does massive destruction, (like Katrina) the name is retired and a new name replaces it, (welcome, "Katia"). (See list of retired names here)

In the Atlantic Ocean, storm names are a mix of Anglo/English, Spanish and French names in deference to the languages of the countries that these hurricanes typically strike. A separate list is maintained for the Pacific Ocean, and there are several other lists used for storms in other parts of the world, again using names more common to each region. That link above has the entire lists.

As to how they come up with the names, that is a closely guarded secret. They typically pick names that are easy to understand, but beyond that, the WMO huddles in a corner and then mysteriously comes out with the names. (There's no truth to the long-standing rumor that it's ex- boy/girlfriend names of those on the panel :) )

What Happens When We Run Out Of Names?

You'd think with 21 names available that'd be enough. (We skip "Q", "U", "X", "Y" and "Z" just due to lack of names that start with those, or meteorologists just don't date very many Zekes, Yolandas or Xaviers.) Up until 2005, we'd never gotten past the "T" name. But then 2005 was tropical storm haven, with 27 storms.

But scientists are nothing if not great at planning ahead. The rule was to start using the Greek Alphabet once the next storm formed after "Wilma." So we had Alpha, Beta, Gamma -- all the way up to Zeta, becoming the first Atlantic storm ever to start with a "Z" (and sound like a UW frat at the same time.)

What Steers Hurricanes?

You might be wondering how hurricanes move after watching the meandering track of Tropical Storm Fay, which might be the first tropical system in history that should have been required to have a designated driver. It's made landfall in Florida three separate times, and could possibly make landfall a fourth and *fifth* time along the Gulf Coast.

It's the upper level winds that determine where a hurricane might go, but when you get a situation where the upper level winds are very weak and variable, as they are over the Gulf Coast and Florida right now, storms can drift or meander.

Here's another one with a wacky track: Hurricane Ophelia

There was another one, and I can't find which one it was but it was several years ago, where it made landfall into central Florida, then, as if it realized it made a wrong left turn at the light near Vero Beach, actually backed up and out into the Atlantic, and raced north and made landfall again farther north up the seaboard.

4) What does "Category 3" mean?

Hurricanes are rated by top wind speed on the Saffir-Simpson scale. Starting at 75 mph, you go up in category about each 20-25 mph in wind speed. A Category 3, for example, is a storm that has top winds of 111-130 mph. Anything over 155 is a Category 5.

So far, only three Category 5s have ever struck the U.S. -- Hurricane Andrew in 1992, Hurricane Camille in 1969, and the "Labor Day" hurricane that struck the Florida coast in 1935, but that was before storms were given names.

Note that Hurricane Katrina was a Category 5 in the Gulf of Mexico, but weakened to a Category 3 before making landfall at New Orleans. But it was the storm's surge and the levy failures that led to the widespread destruction along the Gulf Coast and New Orleans there, not so much the wind.

Why Don't We Get Hurricanes In The Northwest?

Hurricanes need warm, ocean water to survive, and the water temperatures off the Pacific Coast are generally in the 50-60 degree range -- way too cold for hurricanes. In fact, a hurricane has never struck the Pacific Coast of the United States, but a tropical storm once did hit the shores of  southern California near Long Beach in 1939. (There was also Hurricane Nora, which actually made landfall in Baja California, Mexico as a Hurricane, but was still an official tropical storm when it crossed north into California and Arizona.)

But We've Had Big Windstorms, Why Aren't They Named?

Washington and Oregon have both had their share of catastrophic windstorms. The storms of December 2006, and 2007, as well as the Inauguration Day Storm of 1993 and the Columbus Day Storm of 1962 have all had wind speeds over hurricane strength.

But those storms were not tropical in nature, so they are not official hurricanes. They are just big storms. We give them calendar or geographic names, but they don't warrant the attention of the World Meteorological Organization -- probably because it would be a nightmare to manage.

And maybe they don't have that many ex's in the group :)