Subject: A1) What is a hurricane, typhoon, or tropical cyclone?
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)
	"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
 A sub-tropical cyclone is a low-pressure system existing in the tropical 
or subtropical latitudes (anywhere from the equator to about 50N) 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 OAR issued 
forecasts and warnings similar to those for tropical cyclones. Now they 
are given names from the tropical cyclone list.
For more information see Penn State University's write up on the 
Subtropical Cyclones.
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 tidal surge ?
Contributed by Neal Dorst
Storm surge is the onshore rush of sea or lake water caused by the high 
winds associated with a landfalling cyclone and secondarily by the low 
pressure of the storm.
Tidal surge is often misused to describe storm surge, but storm surge is 
independent of the usual tidal ebb and flow. In some inlets, such as the 
Bay of Fundy, rapid changes in sea level due to the tides will cause a 
tidal bore or surge to move in to or out of the inlet. This surge occurs 
independent of the present weather.
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 10C [18F] 
warmer or more at an altitude of 12 km [8 mi.] than the surrounding 
environment, but only 0-2C [0-3F] 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 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 August 13, 2004
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 80F (26.5C) 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 80F (26.5C) 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 80F 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 ?
Hurricane Erin and the SAL in 2001
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 Atlantic Ocean. The SAL usually 
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 tropical sounding), and strong winds (~25-55 mph 
or ~10-25 m/s).
The SAL has been shown to have significant negative impact on tropical 
cyclone intensity. Its dry air can act to weaken a tropical cyclone by 
inhibiting updrafts in 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 studies have suggested that it 
too may have a negative impact on intensification.
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 
here.
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 September 21, 2004
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