Tropical Terminology
Each year, residents of and visitors to the Gulf and Atlantic coasts are bombarded with various words and phrases pertaining to tropical weather. The nation’s natural fascination with weather combined with a 24/7 news cycle, an increased focus on effectively preparing for and responding to natural disasters, and social media have led to an explosion of meteorological terms in the public discourse. Who in the Florida Keys has not heard of “tropical waves”, the “Saffir/Simpson Scale”, and the “cone of uncertainty”. Indeed, one does not have to search many television stations, radio channels, or tweets to find someone using “tropical disturbance”, “tropical cyclone”, or even “invest”. A review of some of these terms may be helpful and timely given the season (and you know I am not referring to red snapper or lobster season).
So, I have chosen four tropical terms on which to elaborate. These terms are used frequently enough to warrant further explanation, and often are thrown about via the local air waves or tossed about in conversations among family, friends, or co-workers.
Tropical Wave
Use of this term has grown more popular every year since the National Hurricane Center introduced it during the early 1970s. The term essentially refers to a tropical storm or hurricane “seedling” disturbance. It has nothing to do with ocean “waves”. The technical definition: “a trough or cyclonic curvature maximum in the trade-wind easterlies. The wave may reach maximum amplitude in the lower middle troposphere”. What might you expect with a tropical wave passage in the Florida Keys? Most will be quite benign, and may pass unnoticed by many folks, with perhaps only a slight increase in daily shower coverage. Stronger tropical waves will result in the prevailing moist easterly breezes backing to northeast and freshening, but usually no more than 20–25 knots. The stronger waves will lead to a day or two of cloudy and stormy weather, followed by a wind shift from northeast to southeast.
Tropical Disturbance
The word “disturbance” is interesting, suggesting a deviation from the norm (“trouble”, “perturbation”, “disorder”, “unrest”, and “turmoil” all are synonyms). The technical definition: “a discrete tropical weather system of apparently organized convection – generally 100 to 300 nautical miles in diameter – originating in the tropics or subtropics, having a nonfrontal migratory character, and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation in the wind field”. This definition is at once both detailed and vague, suggestive of the uncertainty inherent in evaluating the constantly changing tropical atmosphere. We watch disturbances closely due to their persistence and potential. If a tropical disturbance arrives in the Florida Keys, it usually means rain and lots of it.
Tropical Cyclone
Hurricanes, tropical storms, and tropical depressions are all types of tropical cyclone. Hurricanes are the most intense, while tropical depressions are the weakest. The technical definition: “a warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined center. Once formed, a tropical cyclone is maintained by the extraction of heat energy from the ocean at high temperature and heat export at the low temperatures of the upper troposphere. In this they differ from extratropical cyclones, which derive their energy from horizontal temperature contrasts in the atmosphere (baroclinic effects).” Lots of information is packed into that definition. The section on energy is important though because the energy pathways help determine the unique structure of the tropical cyclone, resulting potentially in the rapid development of incredible wind speeds in the system core. Tropical cyclone effects vary dramatically depending on system intensity, size, structure, movement, and evolution, as well as your location in the system.
Invest
This is a newer term, having become prevalent during the Internet age. The technical definition: “a weather system for which a tropical cyclone forecast center (like the National Hurricane Center) is interested in specialized data sets (e.g., microwave imagery) and/or running model guidance. Once a system has been designated as an invest, data collection and processing is initiated on a number of government and academic web sites, including the Naval Research Laboratory (NRL) and the University of Wisconsin Cooperative Institute for Meteorological Satellite Studies (UW-CIMSS). The designation of a system as an invest does not correspond to any particular likelihood of development of the system into a tropical cyclone; operational products such as the Tropical Weather Outlook should be consulted for this purpose.” System-specific data and information increase dramatically once an “invest” is designated. Subsequently, this usually is when the phone starts ringing off the hook at the Florida Keys National Weather Service (305-295-1316).
This is just a small sampling of tropical weather terms. For more information, please visit the excellent glossary put together by the men and women of the National Hurricane Center (available online at http://www.nhc.noaa.gov/aboutgloss.shtml).
The Atlantic Basin hurricane season covers half the year (June through November). However, 87% of the activity historically occurs during the months of August, September, and October. During the last 170 years, numerous hurricanes have affected the Florida Keys during the months of September and October. Remember to periodically review your hurricane plan (if you don’t have one, get one!). As always, be weather-ready, and stay safe!
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