FAQ: Basic Hurricane Definitions
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).
What is" Cape Verde" hurricane?
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.
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."
What is an easterly wave and what causes them?
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).
What is a tropical disturbance, tropical depression,
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 nonfrontal 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
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 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 National
Hurricane Center 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 subtropical cyclones.
What is an extratropical 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. Occasionally, 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
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.
What is a"CDO"?
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, 74mph), 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.
What is a TUTT?
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).
What is the"eye"? How is it formed and maintained ?
What is the"eyewall" ? What are"spiral bands"?
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 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.
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.
What is UTC, GMT, Z, or Zulu Time? 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 |
Baghdad 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
How do I convert from mph to knots (or m/s), from
inches of mercury to millibars (or hPa), or from 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.
How do tropical cyclones form?
Contributed by Chris Landsea
To undergo tropical cyclogenesis, there are several favorable precursor
environmental conditions that must be in place (Gray 1968,1979) :
1. Warm ocean waters (of at least 26.5°C [80°F]) 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. 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.
Why do tropical cyclones require at least 80°F 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).
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
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 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
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 National Hurricane Center. '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