Subject: A1) What is a hurricane, typhoon, or tropical cyclone? NOAA Contributed by Chris Landsea The terms "hurricane" and "typhoon" are regionally specific names for a strong "tropical cyclone". A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation (Holland 1993). Tropical cyclones with maximum sustained surface winds of less than 17 m/s (34 kt, 39 mph) are called "tropical depressions" (This is not to be confused with the condition mid-latitude people get during a long, cold and grey winter wishing they could be closer to the equator ;-)). Once the tropical cyclone reaches winds of at least 17 m/s (34 kt, 39 mph) they are typically called a "tropical storm" and assigned a name. If winds reach 33 m/s (64 kt, 74 mph)), then they are called: • "hurricane" (the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E) • "typhoon" (the Northwest Pacific Ocean west of the dateline) • "severe tropical cyclone" (the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E) • "very severe cyclonic storm" (the North Indian Ocean) • "tropical cyclone" (the Southwest Indian Ocean) (Neumann 1993). Subject: A2) What is a "Cape Verde" hurricanes? Contributed by Chris Landsea Cape Verde-type hurricanes are those Atlantic basin tropical cyclones that develop into tropical storms fairly close (<1000 km [600 mi.] or so) of the Cape Verde Islands and then become hurricanes before reaching the Caribbean. (That would be my definition, there may be others.) Typically, this may occur in August and September, but in rare years (like 1995) there may be some in late July and/or early October. The numbers range from none up to around five per year - with an average of around 2. Subject: A3) What is a super-typhoon? What is a major hurricane ? What is an intense hurricane ? Contributed by Stan Goldenberg "Super-typhoon" is a term utilized by the U.S. Joint Typhoon Warning Center for typhoons that reach maximum sustained 1-minute surface winds of at least 65 m/s (130 kt, 150 mph). This is the equivalent of a strong Saffir-Simpson category 4 or category 5 hurricane in the Atlantic basin or a category 5 severe tropical cyclone in the Australian basin. "Major hurricane" is a term utilized by the National Hurricane Center for hurricanes that reach maximum sustained 1-minute surface winds of at least 50 m/s (96 kt, 111 mph). This is the equivalent of category 3, 4 and 5 on the Saffir-Simpson scale. "Intense hurricane" is an unofficial term , but is often used in the scientific literature. It is the same as "major hurricane". Subject: A4) What is an easterly wave ? Contributed by Chris Landsea It has been recognized since at least the 1930s (Dunn 1940) that lower tropospheric (from the ocean surface to about 5 km [3 mi.] with a maximum at 3 km [2 mi.]) westward traveling disturbances often serve as the "seedling" circulations for a large proportion of tropical cyclones over the North Atlantic Ocean. Riehl (1945) helped to substantiate that these disturbances, now known as African easterly waves, had their origins over North Africa. While a variety of mechanisms for the origins of these waves were proposed in the next few decades, it was Burpee (1972) who documented that the waves were being generated by an instability of the African easterly jet. (This instability - known as baroclinic-barotropic instability - is where the value of the potential vorticity begins to decrease toward the north.) The jet arises as a result of the reversed lower-tropospheric temperature gradient over western and central North Africa due to extremely warm temperatures over the Saharan Desert in contrast with substantially cooler temperatures along the Gulf of Guinea coast. The waves move generally toward the west in the lower tropospheric tradewind flow across the Atlantic Ocean. They are first seen usually in April or May and continue until October or November. The waves have a period of about 3 or 4 days and a wavelength of 2000 to 2500 km [1200 to 1500 mi.], typically (Burpee 1974). One should keep in mind that the "waves" can be more correctly thought of as the convectively active troughs along an extended wave train. On average, about 60 waves are generated over North Africa each year, but it appears that the number that is formed has no relationship to how much tropical cyclone activity there is over the Atlantic each year. While only about 60% of the Atlantic tropical storms and minor hurricanes ( Saffir-Simpson Scale categories 1 and 2) originate from easterly waves, nearly 85% of the intense (or major) hurricanes have their origins as easterly waves (Landsea 1993). It is suggested, though, that nearly all of the tropical cyclones that occur in the Eastern Pacific Ocean can also be traced back to Africa (Avila and Pasch 1995). It is currently completely unknown how easterly waves change from year to year in both intensity and location and how these might relate to the activity in the Atlantic (and East Pacific). Subject: A5) What is a tropical disturbance, a tropical depression, or a tropical storm ? Contributed by Chris Landsea These are terms used to describe the progressive levels of organized disturbed weather in the tropics that are of less than hurricane status. Tropical Disturbance A discrete tropical weather system of apparently organized convection - generally 200 to 600 km (100 to 300 nmi.) in diameter - originating in the tropics or subtropics, having a non-frontal migratory character, and maintaining its identity for 24 hours or more. It may or may not be associated with a detectable perturbation of the wind field. Disturbances associated with perturbations in the wind field and progressing through the tropics from east to west are also known as easterly waves . Tropical Depression A tropical cyclone in which the maximum sustained wind speed (using the U.S. 1 minute average standard) is 33 kt (38 mph, 17 m/s). Depressions have a closed circulation. Tropical Storm A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1 minute average standard) ranges from 34 kt (39 mph,17.5 m/s) to 63 kt (73 mph, 32.5 m/s). The convection in tropical storms is usually more concentrated near the center with outer rainfall organizing into distinct bands. Hurricane When winds in a tropical cyclone equal or exceed 64 kt (74 mph, 33 m/s) it is called a hurricane (in the Atlantic and eastern and central Pacific Oceans). Hurricanes are further designated by categories on the Saffir-Simpson scale. Hurricanes in categories 3, 4, 5 are known as Major Hurricanes or Intense Hurricanes. The wind speed mentioned here are for those measured or estimated as the top speed sustained for one minute at 10 meters above the surface. Peak gusts would be on the order of 10-25% higher. Last updated January 30, 2006 Subject: A6) What is a sub-tropical cyclone? Contributed by Chris Landsea and Sandy Delgado. A sub-tropical cyclone is a low-pressure system existing in the tropical or subtropical latitudes (anywhere from the equator to about 50°N) that has characteristics of both tropical cyclones and mid-latitude (or extratropical) cyclones. Therefore, many of these cyclones exist in a weak to moderate horizontal temperature gradient region (like mid-latitude cyclones), but also receive much of their energy from convective clouds (like tropical cyclones). Often, these storms have a radius of maximum winds which is farther out (on the order of 100-200 km [60-125 miles] from the center) than what is observed for purely "tropical" systems. Additionally, the maximum sustained winds for sub-tropical cyclones have not been observed to be stronger than about 33 m/s (64 kts, 74 mph)). Many times these subtropical storms transform into true tropical cyclones. A recent example is the Atlantic basin's Hurricane Florence in November 1994 which began as a subtropical cyclone before becoming fully tropical. Note there has been at least one occurrence of tropical cyclones transforming into a subtropical storm (e.g. Atlantic basin storm 8 in 1973). Subtropical cyclones in the Atlantic basin are classified by the maximum sustained surface winds: - less than 18 m/s (34 kts, 39 mph) : "subtropical depression", - greater than or equal to 18 m/s (34 kts, 39 mph) : "subtropical storm" Prior to 2002 subtropical storms were not given names, but the Tropical Prediction Center issued forecasts and warnings on them similar to those for tropical cyclones. Since 2003 they are given names from the tropical cyclone list. For more information see http://www.e-education.psu.edu/public/meteo/subtropical_cyclones.html Penn State University's write up on the Subtropical Cyclones. Last Revised : March 16, 2010 Subject: A7) What is an extra-tropical cyclone ? Contributed by Stan Goldenberg An extra-tropical cyclone is a storm system that primarily gets its energy from the horizontal temperature contrasts that exist in the atmosphere. Extra-tropical cyclones (also known as mid-latitude or baroclinic storms) are low pressure systems with associated cold fronts, warm fronts, and occluded fronts. Tropical cyclones, in contrast, typically have little to no temperature differences across the storm at the surface and their winds are derived from the release of energy due to cloud/rain formation from the warm moist air of the tropics ( Holland 1993, Merrill 1993). Structurally, tropical cyclones have their strongest winds near the earth's surface , while extra-tropical cyclones have their strongest winds near the tropopause - about 8 miles (12 km) up. These differences are due to the tropical cyclone being "warm-core" in the troposphere (below the tropopause) and the extra-tropical cyclone being "warm-core" in the stratosphere (above the tropopause) and "cold-core" in the troposphere. "Warm-core" refers to being relatively warmer than the environment at the same pressure surface ("pressure surfaces" are simply another way to measure height or altitude). Often, a tropical cyclone will transform into an extra-tropical cyclone as it recurves poleward and to the east. Occassionally, an extra-tropical cyclone will lose its frontal features, develop convection near the center of the storm and transform into a full-fledged tropical cyclone. Such a process is most common in the North Atlantic and Northwest Pacific basins. The transformation of tropical cyclone into an extra-tropical cyclone (and vice versa) is currently one of the most challenging forecast problems (i.e., Jones et al. 2003). References: Jones, S.C., Harr, P.A., Abraham, J., Bosart, L.F., Bowyer, P.J., Evans, J.L., Hanley, D.E., Hanstrum, B.N., Hart, R.E., Lalaurette, F., Sinclair, M.R., Smith, R.K., Thorncroft, C. 2003: The Extratropical Transition of Tropical Cyclones: Forecast Challenges, Current Understanding, and Future Directions. Weather and Forecasting, 18, 1052-1092. Merrill, R. T., (1993): "Tropical Cyclone Structure" - Chapter 2, Global Guide to Tropical Cyclone Forecasting, WMO/TC-No. 560, Report No. TCP-31, World Meteorological Organization; Geneva, Switzerland Web version of Guide Last updated August 13, 2004 Subject: A8) What is storm surge and how is it different from storm tide ? Contributed by NHC Storm Surge Unit Storm surge is an abnormal rise of water generated by a storm, over and above the predicted astronomical tide. Storm Tide is the water level rise during a storm due to the combination of storm surge and the astronomical tide. Storm surge inundation refers to the storm surge as height above ground level. For the (see F7.html) SLOSH model, this is done by subtracting the average elevation of each grid cell from the water level computed by the model referenced to a vertical datum. This helps alleviate confusion inherent in past use of of surge above an abstract geophysical reference level. Last Updated : May 14, 2010 Subject: A9) What is a "CDO" ? NRL - Monterey Contributed by Chris Landsea "CDO" is an acronym that stands for "central dense overcast". This is the cirrus cloud shield that results from the thunderstorms in the eyewall of a tropical cyclone and its rainbands. Before the tropical cyclone reaches hurricane strength (33 m/s, 64 kts, 74 mph), typically the CDO is uniformly showing the cold cloud tops of the cirrus with no eye apparent. Once the storm reaches the hurricane strength threshold, usually an eye can be seen in either the infrared or visible channels of the satellites. Tropical cyclones that have nearly circular CDO's are indicative of favorable, low vertical shear environments. Subject: A10) What is a "TUTT"? Fitzpatrick et al. 1995 Contributed by Chris Landsea A "TUTT" is a Tropical Upper Tropospheric Trough. A TUTT low is a TUTT that has completely cut-off. TUTT lows are more commonly known in the Western Hemisphere as an "upper cold low". TUTTs are different than mid-latitude troughs in that they are maintained by subsidence warming near the tropopause which balances radiational cooling. TUTTs are important for tropical cyclone forecasting as they can force large amounts of vertical wind shear over tropical disturbances and tropical cyclones which may inhibit their strengthening. There are also suggestions that TUTTs can assist tropical cyclone genesis and intensification by providing additional forced ascent near the storm center and/or by allowing for an efficient outflow channel in the upper troposphere. For a more detailed discussion on TUTTs see the article by Fitzpatrick et al. (1995). Subject: A11) What is the "eye"? How is it formed and maintained ? What is the "eyewall"? What are "spiral bands"? (Written with major assistance by Chris Landsea) The "eye" is a roughly circular area of comparatively light winds and fair weather found at the center of a severe tropical cyclone. Although the winds are calm at the axis of rotation, strong winds may extend well into the eye. There is little or no precipitation and sometimes blue sky or stars can be seen. The eye is the region of lowest surface pressure and warmest temperatures aloft - the eye temperature may be 10°C [18°F] warmer or more at an altitude of 12 km [8 mi] than the surrounding environment, but only 0-2°C [0-3°F] warmer at the surface (Hawkins and Rubsam 1968) in the tropical cyclone. Eyes range in size from 8 km [5 mi] to over 200 km [120 mi] across, but most are approximately 30-60 km [20-40 mi] in diameter (Weatherford and Gray 1988). The eye is surrounded by the "eyewall", the roughly circular ring of deep convection which is the area of highest surface winds in the tropical cyclone. The eye is composed of air that is slowly sinking and the eyewall has a net upward flow as a result of many moderate - occasionally strong - updrafts and downdrafts. The eye's warm temperatures are due to compressional warming of the subsiding air. Most soundings taken within the eye show a low-level layer which is relatively moist, with an inversion above - suggesting that the sinking in the eye typically does not reach the ocean surface, but instead only gets to around 1-3 km [ 1-2 mi] of the surface. The exact mechanism by which the eye forms remains somewhat controversial. One idea suggests that the eye forms as a result of the downward directed pressure gradient associated with the weakening and radial spreading of the tangential wind field with height (Smith, 1980). Another hypothesis suggests that the eye is formed when latent heat release in the eyewall occurs, forcing subsidence in the storm's center (Shapiro and Willoughby, 1982). It is possible that these hypotheses are not inconsistent with one another. In either case, as the air subsides, it is compressed and warms relative to air at the same level outside the eye and thereby becomes locally buoyant. This upward buoyancy approximately balances the downward directed pressure gradient so that the actual subsidence is produced by a small residual force. Another feature of tropical cyclones that probably plays a role in forming and maintaining the eye is the eyewall convection. Convection in tropical cyclones is organized into long, narrow rainbands which are oriented in the same direction as the horizontal wind. Because these bands seem to spiral into the center of a tropical cyclone, they are sometimes called "spiral bands". Along these bands, low-level convergence is a maximum, and therefore, upper-level divergence is most pronounced above. A direct circulation develops in which warm, moist air converges at the surface, ascends through these bands, diverges aloft, and descends on both sides of the bands. Subsidence is distributed over a wide area on the outside of the rainband but is concentrated in the small inside area. As the air subsides, adiabatic warming takes place, and the air dries. Because subsidence is concentrated on the inside of the band, the adiabatic warming is stronger inward from the band causing a sharp contrast in pressure falls across the band since warm air is lighter than cold air. Because of the pressure falls on the inside, the tangential winds around the tropical cyclone increase due to increased pressure gradient. Eventually, the band moves toward the center and encircles it and the eye and eyewall form (Willoughby 1979, 1990a, 1995). Thus the cloud-free eye may be due to a combination of dynamically forced centrifuging of mass out of the eye into the eyewall and to a forced descent caused by the moist convection of the eyewall. This topic is certainly one that can use more research to ascertain which mechanism is primary. Some of the most intense tropical cyclones exhibit concentric eyewalls, two or more eyewall structures centered at the circulation center of the storm ( Willoughby et al. 1982,Willoughby 1990a ). Just as the inner eyewall forms, convection surrounding the eyewall can become organized into distinct rings. Eventually, the inner eye begins to feel the effects of the subsidence resulting from the outer eyewall, and the inner eyewall weakens, to be replaced by the outer eyewall. The pressure rises due to the destruction of the inner eyewall are usually more rapid than the pressure falls due to the intensification of the outer eyewall, and the cyclone itself weakens for a short period of time. Last updated May 20, 2011 Subject: A12) What is a moat in a hurricane ? Contributed by Frank Marks The term "moat" usually refers to the region between the eyewall and an outer rainband, such as a secondary eyewall rainband. The moat is the relatively light rain region between the rainband and the eyewall. Subject: A13) What is UTC ? How do I tell at what time a satellite picture was taken ? Contributed by Neal Dorst UTC stands for Universal Time Coordinated, what used to be called Greenwich Mean Time (GMT) and Zulu Time (Z). This is the time at the Prime Meridian (0¡ Longitude) given in hours and minutes on a 24 hour clock. For example, 1350 UTC is 13 hours and 50 minutes after midnight or 1:50 PM at the Prime Meridian. The Greenwich Royal Observatory at Greenwich, England (at 0¡ Longitude) was where naval chronometers (clocks) were set, a critical instrument for calculating longitude. This is why GMT became the standard for world time. Meteorologists have used UTC or GMT times for over a century to ensure that observations taken around the globe are taken simultaneously. On most satellite pictures and radar images the time will be given. If it's not in local time then it will usually be given as UTC, GMT, or Z time. To convert this to your local time it is necessary to subtract the appropriate number of hours for the Western Hemisphere or add the correct number of hours for the Eastern Hemisphere. And don't forget the extra hour adjustment for Daylight Savings Time or Winter Time over Standard Time for your zone. Local Time Zone Time Adjustment (hours) Atlantic Daylight Time (ADT) -3 Atlantic Standard Time (AST) Eastern Daylight Time (EDT) -4 Eastern Standard Time (EST) Central Daylight Time (CDT) -5 Central Standard Time (CST) Mountain Daylight Time (MDT) -6 Mountain Standard Time (MST) Pacific Daylight Time (PDT) -7 Pacific Standard Time (PST) Alaskan Daylight Time (ADT) -8 Alaskan Standard Time (ASA) -9 Hawaiian Standard Time (HAW) -10 New Zealand Standard Time (NZT) International Date Line Time (IDLE) +12 Guam Standard Time (GST) Eastern Australian Standard Time (EAST) +10 Japan Standard Time (JST) +9 China Coast Time (CCT) +8 West Australia Standard Time (WAST) +7 Russian Time Zone 5 (ZP5) +6 Russian Time Zone 4 (ZP4) +5 Russian Time Zone 3 (ZP3) +4 Bagdad Time (BT) Russian Time Zone 2(ZP2) +3 Eastern European Time (EET) Russian Time Zone 1(ZP1) +2 Central European Time (CET) French Winter Time (FWT) Middle European Time (MET) Swedish Winter Time (SWT) Middle European Winter Time (MEWT) +1 Western European Time (WET) Greenwich Mean Time (GMT) 0 Last updated August 13, 2004 Subject: A14) How do I convert from mph to knots (or m/s), from inches of mercury to mb (or hPa), or convert degrees of latitude to miles (or kilometers) ? Contributed by Neal Dorst For winds: 1 mile per hour = 0.869 international nautical mile per hour (knot) 1 mile per hour = 1.609 kilometers per hour 1 mile per hour = 0.4470 meter per second 1 knot = 1.852 kilometers per hour 1 knot = 0.5144 meter per second 1 meter per second = 3.6 kilometers per hour For pressures: 1 inch of mercury = 25.4 mm of mercury = 33.86 millibars = 33.86 hectoPascals For distances: 1 foot = 0.3048 meter 1 international nautical mile = 1.1508 statute miles = 1.852 kilometers = .99933 U.S nautical mile (obsolete) 1¡ latitude = 69.047 statute miles = 60 nautical miles = 111.12 kilometers For longitude the conversion is the same as latitude except the value is multiplied by the cosine of the latitude. Subject: A15) How do tropical cyclones form ? Contributed by Chris Landsea To undergo tropical cyclogenesis, there are several favorable pre cursor environmental conditions that must be in place (Gray 1968,1979) : Warm ocean waters (of at least 26.5 degC [80 degF]) throughout a sufficient depth (unknown how deep, but at least on the order of 50 m [150 ft]). Warm waters are necessary to fuel the heat engine of the tropical cyclone. An atmosphere which cools fast enough with height such that it is potentially unstable to moist convection. It is the thunderstorm activity which allows the heat stored in the ocean waters to be liberated for the tropical cyclone development. Relatively moist layers near the mid-troposphere (5 km [3 mi.]). Dry mid levels are not conducive for allowing the continuing development of widespread thunderstorm activity. A minimum distance of at least 500 km [300 mi.] from the equator. For tropical cyclogenesis to occur, there is a requirement for non-negligible amounts of the Coriolis force to provide for near gradient wind balance to occur. Without the Coriolis force, the low pressure of the disturbance cannot be maintained. A pre-existing near-surface disturbance with sufficient vorticity and convergence. Tropical cyclones cannot be generated spontaneously. To develop, they require a weakly organized system with sizable spin and low level inflow. Low values (less than about 10 m/s [20 kts 23 mph]) of vertical wind shear between the surface and the upper troposphere. Vertical wind shear is the magnitude of wind change with height. Large values of vertical wind shear disrupt the incipient tropical cyclone and can prevent genesis, or, if a tropical cyclone has already formed, large vertical shear can weaken or destroy the tropical cyclone by interfering with the organization of deep convection around the cyclone center. Having these conditions met is necessary, but not sufficient as many disturbances that appear to have favorable conditions do not develop. Recent work (Velasco and Fritsch 1987, Chen and Frank 1993, Emanuel 1993) has identified that large thunderstorm systems (called mesoscale convective complexes [MCC]) often produce an inertially stable, warm core vortex in the trailing altostratus decks of the MCC. These mesovortices have a horizontal scale of approximately 100 to 200 km [75 to 150 mi.], are strongest in the mid-troposphere (5 km [3 mi.]) and have no appreciable signature at the surface. Zehr (1992) hypothesizes that genesis of the tropical cyclones occurs in two stages: stage 1 occurs when the MCC produces a mesoscale vortex. stage 2 occurs when a second blow up of convection at the mesoscale vortex initiates the intensification process of lowering central pressure and increasing swirling winds. Subject: A16) Why do tropical cyclones require 80¡F (26.5¡C) ocean temperatures to form ? Contributed by Chris Landsea Tropical cyclones can be thought of as engines that require warm, moist air as fuel (Emanuel 1987). This warm, moist air cools as it rises in convective clouds (thunderstorms) in the rainbands and eyewall of the hurricane The water vapor in the cloud condenses into water droplets releasing the latent heat which originally evaporated the water. This latent heat provides the energy to drive the tropical cyclone circulation, though actually very little of the heat released is utilized by the storm to lower its surface pressure and increase the wind speeds. In 1948 Erik Palmen observed that tropical cyclones required ocean temperatures of at least 80¡F (26.5¡C) for their formation and growth. Later work (e.g., Gray 1979) also pointed out the need for this warm water to be present through a relatively deep layer (~150 ft, 50 m) of the ocean. This 80¡F value is tied to the instability of the atmosphere in the tropical and subtropical latitudes. Above this temperature deep convection can occur, but below this value the atmosphere is too stable and little to no thunderstorm activity can be found ( Graham and Barnett 1987). See Question G3 for how this value might change if a significant global warming occurs. References: Graham, N. E., and T. P. Barnett, 1987: Sea surface temperature, surface wind divergence, and convection over tropical oceans. Science, No.238, pp. 657-659. Gray, W.M. 1979 : "Hurricanes: Their formation, structure and likely role in the tropical circulation" Meteorology Over Tropical Oceans. D. B. Shaw (Ed.), Roy. Meteor. Soc., James Glaisher House, Grenville Place, Bracknell, Berkshire, RG12 1BX, pp. 155-218 Palmen, E. H., 1948: On the formation and structure of tropical cyclones. Geophysica , Univ. of Helsinki, Vol. 3, 1948, pp. 26-38. Last updated August 13, 2004 Subject: A17) What is the Saharan Air Layer (SAL) ? How does it affect tropical cyclones ? Contributed by Jason Dunion The Saharan Air Layer (SAL) is a mass of very dry, dusty air which forms over the Sahara Desert during the late spring, summer, and early fall and usually moves out over the tropical North Atlantic Ocean every 3-5 days. The SAL extends between ~5,000-20,000 ft (~1500-6000 m) in the atmosphere and is associated with large amounts of mineral dust, dry air (~50% less moisture than a typical moist tropical sounding), and strong winds (~25-55 mph or ~10-25 m/s). These strong winds, or jets, are usually found between 6,500-14,500 ft (2000-4500 m) above the surface in the central and western North Atlantic and have a depth of ~1-2 miles (~1.6-3.2 km). The SAL can have a significant negative impact on tropical cyclone intensity and formation. Its dry air can act to weaken a tropical cyclone by promoting downdrafts around the storm, while its strong winds can substantially increase the vertical wind shear in and around the storm environment. It is not yet clear what effect the SAL's dust has on tropical cyclone intensity, though some recent studies have suggested that it can actually impact the formation of clouds. The SAL can cover an area the size of the continental U.S. and has been tracked as far west as the Caribbean Sea, Central America, and the Gulf of Mexico. Real-time satellite imagery for tracking the SAL can be found at http://cimss.ssec.wisc.edu/tropic2/real-time/salmain.php?&prod=splitEW&time References: Dunion, J.P., and C.S. Velden, 2004: The impact of the Saharan Air Layer on Atlantic tropical cyclone activity. Bull. Amer. Meteor. Soc. vol. 85 no. 3, 353-365. Last updated March 17, 2010 Subject: A18) What is a neutercane? Contributed by Neal Dorst A neutercane is a small (meso-)scale (< 100 miles in diameter) low-pressure system that has characteristics of both tropical cyclone and mid-latitude or extratropical cyclone. A subclass of sub-tropical cyclone, neutercanes are distinguished by their small size and their origination, sometimes forming within mesoscale convective complexes. The term was coined by Robert Bundgaard, after he participated in a research flight in the early 1970's. He witnessed a small cyclonic circulation over land, which appeared to have both tropical and extratropical characteristics. He used the term in later discussions with Dr. Bob Simpson, then director of the OAR. 'Neutercane' was meant to synthesize the word 'neutral' and 'hurricane' to imply a hurricane-like vortex which was midway between tropical and extratropical. Dr. Simpson observed similar circulations on geostationary satellite loops, and conducted an investigation with hurricane specialist Banner Miller. He presented a talk on them at the 8th AMS Conference on Hurricane and Tropical Meteorology in 1973. During the 1972 hurricane season, Simpson inaugurated use of the term in official bulletins, labeling the second (Bravo) and third (Charlie) subtropical cyclones observed that year as Neutercanes. (Neutercane Bravo transformed into Hurricane Betty.) However, objections in the press to the term as possibly sexist led to NOAA management discouraging use of the term, and ordering Simpson to cease use of any further Government resources in conducting research on the phenomenon. From then on, the term "Sub-tropical Cyclone" was used for all such systems. However, the term entered into several dictionaries, including the AMS Glossary of Meteorology (which misidentifies them as "large"), and has been used in the scientific literature. References Bull. Amer. Met. Soc., Feb. 1973, Vo. 54 No. 2, p. 153 Glossary of Meteorology 2nd edition, 2000, (AMS, Boston), p. 522 Weatherwise, Sept.-Oct. 2005, Vo. 58 No. 5, p. 60 Revised May 21, 2007 Subject: A19) What does ATCF stand for and how are tropical cyclones numbered? Contributed by Neal Dorst The Automated Tropical Cyclone Forecast (ATCF) system was developed for the Joint Typhoon Warning Center in 1988. This is a software package used to plot tropical cyclone information and assist in the generation of forecast messages. In order to distinguish different tropical cyclones that might be occurring simultaneously, a distinct alphanumeric code is assigned to each cyclone as it develops. This code system was adopted by other warning centers in order to facilitate the passing of storm information and reduce confusion. The code designation consists of two letters designating the oceanic basin ("AL" for Atlantic, "EP" for Eastern Pacific, "CP" for Central Pacific and "WP" for Western Pacific), a two-digit number designating the sequential number of that particular cyclone for that basin in the year, and lastly a four-digit year number. So the first depression to form in the Atlantic for 2001 would be AL012001, the third depression for the Central Pacific in 1999 would be CP031999. A cyclone retains its ATCF code designation as long as it remains a distinct tropical vortex. Even if it becomes a named Tropical Storm or Hurricane the software will still track it by its ATCF code. References Miller, R.J., J. Schrader, C.R. Sampson, and T.L. Tsui, 1990, "The Automated Tropical Cyclone Forecasting Sytem (ATCF)",Weather and Forecasting, Vol. 5, (Dec. 1990), p. 653-660 Revised August 14, 2009 Subject: A20) What does AL90, AL91, or 92L refer to in the tropical discussions? Contributed by Neal Dorst Oftentimes, hurricane specialists become curious about disturbances in the tropics long before they form into tropical depressions and are given a tropical cyclone number (see A19). In order to alert forecasting centers that they are investigating such a disturbance and that they wish to have it tracked by the various forecast models, the specialist will attach a 9-series number to it. The first such disturbance of the year will be designated 90, the next 91, and so on until 99. After that, they restart the sequence with 90 again. The purpose of these numbers is to clarify which disturbance they are tracking as there are often more than one happening at the same time. To further clarify matters, each number is accompanied by a two-letter code designating which tropical cyclone basin the disturbance is in. "AL" is used for the Atlantic basin (including the Caribbean Sea and Gulf of Mexico), "EP" for the Eastern Pacific, "CP" for Central Pacific, and "WP" for the Western Pacific. Many times in discussions these designations will be shortened to 90L, 91L, and so forth. Also they may be referred to as 'Invest 90L'. However, once a disturbance is designated a tropical depression this 9-series number will be dropped and an ATCF code number (see A19) will be assigned in its place. You may also occasionally see an 8-series number, such as AL82. This means that this is a test investigation. There is no particular disturbance that the specialists are interested in, they're just running a test of the system to make sure communications and software are running properly. Revised August 14, 2009 Subject: A21) What is a "post-tropical cyclone" ? Contributed by Dan Brown (NHC) A former tropical cylone, that no longer possesses sufficient tropical characteristics to be considered a tropical cyclone. Post-tropical cyclones can continue carrying heavy rains and high winds. Note that former tropical cyclones that have become fully extratropical, subtropical, or remnant lows, are three classes of post-tropical cyclones. Revised May 10, 2013