The following is a list of storm terminology used by forecasters and atmospheric scientists when referring to hurricanes, tropical storms and other weather systems:
- Anemometer: A device that measures the speed and force of the wind.
- Cape Verde Season: Typically begins around mid-August of each year as tropical systems develop off of the western coast of Africa (near the Cape Verde Islands) and build up momentum over the Atlantic Ocean as they move towards North and Central America. All that warm open water unobstructed by land can give a storm a lot of time and fuel to develop. As an example: Hurricane Andrew started as a tropical wave in 2002 on August 14th, but didn’t hit the east coast of Florida until August 24th. Four of the five largest Atlantic hurricanes on record (in terms of gale diameter) were Cape Verde storms: Igor in 2010 (920 miles), Lili in 1996 (806 miles), Karl in 2004 (777 miles), and Helene in 2006 (748). The one exception was Olga in 2001 (863 miles) which was a rare late-season storm that formed east of Bermuda.
- Convection: In terms of severe weather, this can be described as surface heat moving vertically into the atmosphere. As the heat mixes with colder air, the water vapor in the air condenses resulting in the formation of clouds that can then lead to precipitation.
- Cyclone: An area of rotating atmospheric circulation in which the center is distinguished by low pressure. Cyclones derive their names from the cyclical rotation of wind: in the northern part of the planet storms rotate counterclockwise; in the southern part they rotate clockwise. The word cyclone is very generic; researchers, meteorologists and scientists usually paired the word with a qualifier such as tropical, subtropical, extratropical, polar or meso. In the United States (North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, eastern North Pacific Ocean), we call tropical cyclones hurricanes; in other parts of the world scientists refer to them by a number of different names (see hurricane below for a more detailed explanation). The center air of a tropical cyclone is warmer then the surrounding air and usually has its strongest winds towards the ground as opposed to the upper atmosphere.
- Depression: When you’re not feeling happy because another hurricane is coming (just seeing if you were paying attention). In meteorology, it is another name for an area of low atmospheric pressure, a low or a trough. A depression is the beginning stages of a storm; as the air pressure becomes lower than its surroundings, it starts to rotate in clockwise or counterclockwise directions.
- Dirty Side: This is the name given to the right side (easterly side) of a storm. The description sticks because the area is the most powerful part of any cyclone in the northern hemisphere with the strongest winds, heaviest rains and most energy that can lead to a high storm surge. When moving easterly, the dirty side of a storm also has increased wind speeds; meteorologists not only calculate how fast the winds of the storm are spinning, but they also take into account how fast the storm itself is moving. The extra energy on the dirty side of a system comes from moisture pulled in from the south; on the opposite end is what is known as the “clean side” of a storm (west side of a storm) with less energy and moisture (sometimes because of drier air from the north). Cyclones in the northern hemisphere spin counterclockwise; in the southern hemisphere storms spin clockwise so dirty and clean sides are reversed. Random fact of the day: author Martha Serpas has a book of poems called The Dirty Side of the Storm. Martha is a former colleague teaching at the University of Tampa.
- El Niño: the phenomenon describing the warming of Pacific Ocean currents along the coasts of Peru and Ecuador that influence dramatic changes in the weather across the globe. El Niño runs on a sort of schedule, about every three to five or seven years. The term is Spanish for “the boy;” the weather phenomenon got its name during the 19th century when fishermen noticed the pattern of weather changes. Those changes showed up around Christmastime- “the boy” was named after Jesus.
- On the flip side, La Niña is considered the polar opposite to El Niño: La Niña is unusually cooler water in the Pacific Ocean, leading to drier summers and warmer winters in the southeastern U.S. That drier air cools the trade winds and reduces or eliminates upper level winds. The result: storms have more of an opportunity to develop and can be more intense. La Niña is not a good thing if you live in a hurricane prone area (take note those on the eastern seaboard and the Gulf of Mexico).
- El Niño Modoki is a hybrid/modified version of the El Niño weather trend: although the signature is that of a typical El Niño weather pattern (warm weather heating up the waters), the results instead are more like that of a La Niña system. In instances of El Niño Modoki, development of Eastern Pacific warming (EPW) occurs further west leading to Central Pacific warming (CPW). The initial study, published in July of 2009 by researchers at Georgia Tech, was inconclusive as to why warmer weather in different parts of the Pacific produces such drastically different results, instead concentrating on the possible discovery of a trend leading to a new classification and a better distinction of the influence of weather patterns.
- Extratropical: Scientists and forecasters use this terminology to signify when a cyclonic storm system has no distinctive tropical (or polar) traits; for the U.S., this is the weather we see most of the time moving eastwards across the country. Unlike a tropical system, an extratropical storm has most of the higher winds in the upper atmosphere, has a distinctive “front” and has cooler air towards the center of the system.
- A subtropical cyclone (storm) on the other hand is kind of a hybrid: it has the traits of both a tropical and extratropical cyclone. As an example, a subtropical storm in hurricane season is weaker than a hurricane or a tropical storm, but exhibits hurricane/tropical storm traits. A subtropical storm will have a warm center, but the eye is usually not very well-defined and swirling winds are far from the center. The storms usually form around the warm water and when warm and cold fronts collide. Subtropical systems usually develop into tropical storms.
- Feeder Bands: These are the outlying spiral cloud formations that draw moisture from the earth’s surface and “feed” that moisture towards the center of a storm. Scientists and forecasters also refer to these as “outer convective bands.”
- Flash Flood Watch: A flash flood is possible in the area.
- Flash Flood Warning:A flash flood is imminent.
- Fujiwara (Fujiwhara) Effect: Two storms (hurricanes, tropical storms, tropical depressions) colliding over an area and then essentially spinning around each other as each feeds off of the other’s energy. This is not a good thing: think of it as a kind of “square dance” as the two storms lock arms and swing each other around. The phenomenon is named for Dr. Sakuhei Fujiwara who outlined this theory in 1921.
Has this ever happened? Yes- in 1995, Tropical Storm Iris hooked up with Hurricane Humberto down in the Caribbean around the Windward Islands. The storms danced around each other before Iris became a hurricane and Humberto dissipated. But Iris wasn’t done: the hurricane then interacted with Tropical Storm Karen, eventually absorbing Karen because that storm was much weaker.
A Fujiwara Effect was feared in 2004: when both Tropical Storm Bonnie and Hurricane Charley hit Florida, it was the first time in 98 years (since 1906) the state had been hit with two storms in a 24 hour period. Luckily, Bonnie hit Florida’s panhandle to the north and Charley made what can only be described as a crazy-ass sudden right turn and hit Charlotte County well to the south.
In the same 2004 season, four major storms were seen simultaneously spinning either over or in some proximity to the U.S. The four (Ivan, Jeanne, Karl and Lisa) were viewed on September 22nd, 2004. Ivan, after running over the Florida panhandle on September 16th, regenerated into a Tropical Depression and made landfall in Texas, Jeanne was re-strengthening and in the middle of its own crazy-ass loop and heading for the east coast of Florida, Karl was at Category 3/Category 4 strength out in the middle of nowhere and Tropical Storm Lisa was just starting to weaken into a Tropical Depression. Coincidentally, the only other time something similar to this had happened was on September 25th, 1998 when satellites photographed four hurricanes simultaneously spinning in the Atlantic basin (each with sustained winds of over 75 mph). For you trivia buffs, three of those storms had the same names as the 2004 storms (Ivan, Jeanne and Karl).
NOTE: the Fujiwara Effect can occur with non-tropical storms also (an example being two storms on the eastern coast of the U.S. around Christmas of 1994).
- Hurricane: (winds higher than 74 mph): An intense cyclonic tropical weather system. There is a well-defined circulation pattern. There are a number of terms used across the world for storms with a wind speed over 74mph: in the North Atlantic, Northeast Pacific (east of the international dateline) and South Pacific Ocean (east of the Hawaiian Islands) storms are called hurricanes. In the Northwest Pacific Ocean, west of the international dateline, hurricanes are called typhoons. In the Southwest Pacific Ocean (near the Philippines, eastern Indonesia and New Guinea) and Southeast Indian Ocean (near Western Australia and Western Indonesia) hurricanes are called severe tropical cyclones. In the Northern Indian Ocean hurricanes are called severe cyclonic storms. In the Southwest Indian Ocean, hurricanes are called tropical cyclones.
- Hurricane Warning: Hurricane conditions are expected in a specified area within 24 hours.
- Hurricane Watch: Hurricane conditions are possible within a specified area within 36 hours.
- Intertropical Convergence Zone (ITCZ): A weather loop of sorts that straddles the Earth around the equator acting as a dividing line between northern and southern hemispheric weather. As the northern and southern weather patterns collide, the thin band of clouds and thunderstorms is known as the ITCZ.
- Loop Current: Tropically warm to hot water that travels around the eastern portion of the Gulf of Mexico. It is created when water from the Caribbean Sea passes through the Yucatán Strait (between Mexico and Cuba), loops around clockwise in the Gulf of Mexico, and then exits through the Florida Straits. Once the current hits the Atlantic Ocean it is known as the Florida Current and then becomes part of the Gulf Stream waters heading north along the east coast.
The Loop Current is not static: it can reach as far west as Texas and as far north as the Mississippi River Delta. There are some circumstances when the current doesn’t specifically loop and water bypasses the Gulf of Mexico and instead flows from the Yucatán Strait directly into the Florida Straits.
Warm water in the Gulf can go be as deep as 100-150 feet, but in a Loop Current, that depth can be as much as 250 feet. Hurricanes strengthen over warm water and lose strength when cooler water rises to the surface. The high temperature water (about 80 degrees) is an almost never ending fuel supply for a hurricane; if that supply is hundreds of feet deep, colder water almost has no chance of getting to the surface and cutting short a hurricane’s energy supply.
Of note: both Hurricane Katrina and Hurricane Rita (2005) passed over a Loop Current strengthening each from a Category 1 to a Category 5 in a matter of days. Furthermore, Katrina remained a much more powerful storm because it crossed into a Loop Current Eddy, a separate ring of warm water that can break away from the Loop Current.
- Storm Surge: A spectacular phenomenon as seawater rises (up to 20 feet high and 50 to 100 miles wide) with the arrival of a hurricane. The stronger the hurricane and the shallower the offshore water, the higher the surge will be. Evacuation zones are specified according to susceptibility.
- Storm Tide: The storm tide is the combination of a storm surge and the astronomical tide. When a storm surge arrives at the same time as high tide, the water height will increase.
- Tropical Storm Watch: Tropical storm conditions are possible in a specified area within 36 hours.
- Tropical Storm Warning: Tropical storm conditions are expected in a specified area within 24 hours.
- Tropical Wave: Just as powerful as a tropical storm, by definition, a tropical wave has no clear-cut defined eye or center of rotation. Look for the term to pop up in the early developmental stages of a storm.
- Upwelling: A process by which a storm system churns colder water towards the surface reducing warmer water energy needed for fuel. A stationary storm can significantly weaken itself if upwelling occurs; upwelling can also reduce the severity of a second storm following the same path as a preceding system. NASA has a number of satellites (including Aqua, Jason-1 and TRMM) that can measure water temperature; water temperatures below 82 degrees Fahrenheit can weaken a storm. Follow this link to see a NASA animation of the process.