- A widespread body of air, the properties of which can be
identified as (a)
having been established while that air was situated over a particular region,
undergoing specific modifications while in transit away from the source
Anticyclone (or High-Pressure System, High-Pressure
Cell, or simply High) - An area
within which the pressure is high relative to the surroundings.
The wind circulation is
clockwise around an anticyclone in the Northern Hemisphere,
with the wind crossing the
isobars at an angle and blowing from higher toward lower pressure.
The wind circulation
is counterclockwise around an anticyclone in the Southern Hemisphere,
with the wind
crossing the isobars at an angle and blowing from higher toward lower pressure.
Backing - Refers to the shifting of the wind direction
of a tropical cyclone observed by the
observer from northwest through
southwest by way of west (also known as First Wind).
means that the storm is passing
north of the locality (over the northern hemisphere).
Best Track - A subjectively smoothed
path, versus a precise and very erratic fix-to-fix
used to represent tropical cyclone movement, and based on an assessment
of all available
Binary Interaction - A mutual cyclonic orbit
of two tropical cyclones around their
centroid. Lander and Holland (1993) showed that the behavior of most
cyclones consists of an approach, sudden capture, then a period of steady
followed by a sudden escape or (less frequently) a merger.
Buys Ballot's Law - This is a law describing
the relationship of the horizontal wind
direction in the atmosphere to the pressure distribution. "If one
stands with his back to the
wind in the Northern Hemisphere, lower pressure will always
be to the left, and higher
pressure to the right." "In the Southern Hemisphere,
if one stands with his back to the
wind, lower pressure will always be to the right, and higher pressure to
the left." This law
was first formulated in 1857 by the Dutch meteorologist, Buys Ballot, and
bears his name.
It is also known as the Baric Wind Law.
- The vertical axis or core of a tropical cyclone. Usually
determined by cloud vorticity
wind and/or pressure distribution.
- The location of the center of a tropical or subtropical
cyclone obtained by means
other than reconnaissance
Eye Wall Cycles -
Naturally occur in intense tropical cyclones (wind > 185 kph or
As tropical cyclone reach this threshold of intensity, they usually --
but not always --
have an eyewall
and radius of maximum winds that contracts to a very small size, around
At this point, some of the outer rainbands may organize into an outer ring
that slowly moves inward and robs the inner
eyewall of its needed moisture and
During this phase, the tropical cyclone is
weakening (i.e., the maximum winds die
off a bit and
the central pressure goes up). Eventually,
the outer eyewall replaces the inner one
and the storm can be the same intensity
as it was previously or, in some cases,
A concentric eyewall cycle occured
in Hurricane Andrew (1992) before landfall
a strong intensity was reached,
an outer eyewall formed, this contracted in
weakening of the storm, and as the outer eyewall completely
original one (inner eyewall), the hurricane
re-intensified. Another example can be
on Hurricane David's radar composite
- The contraction of a vector field. In meteorology, the
term is used in a
sense to include the "coming together," or meeting, of winds (air particles)
or areas (horizontally or vertically). Mathematically, convergence
(or Low-Pressure System, or simply Low or Depression) -
An area within which
is low relative to the surroundings. An atmospheric "closed circulation"
counterclockwise around cyclones in the Northern Hemisphere,
in the Southern Hemisphere. The winds cross the isobars at an angle
water and 30 degrees over land), and blow from higher toward lower pressure.
Divergence - The expansion, or spreading out, of a
vector field. In meteorology, the term is
used in a broad sense to include the "spreading apart," or drawing apart,
of winds (air parti-
cles) either horizontally or vertically.
Easterly Wave -
See Tropical Wave.
Position of a body (satellite) in space as a function of time; used for
satellite imagery. Since ephemeris gridding is based solely on the
predicted position of the
satellite, it is susceptible to errors from vehicle wobble, orbital eccentricity,
the oblateness of the
Earth, and variation in vehicle speed.
Explosive Deepening - A decrease in the minimum
sea-level pressure of a tropical cyclone
of 2.5 mb/hr for at least 12 hours or 5 mb/hr for at least
six (6) hours (Dunnavan 1981).
Extratropical - A
term used to indicate that a cyclone has lost its "tropical" characteristics.
The term implies
both poleward displacement from the tropics and the conversion of the
energy source from the release of latent heat of condensation to baroclinic
It is important to note that cyclones can become extratropical and still
winds of typhoon
or storm force.
- The central area of a tropical cyclone when it is more
than half surrounded by wall
Also known as the relatively calm center of a tropical cyclone where the
sky may be
clear or just
- See Wall Cloud.
- The interface, or transition zone, between two air masses
of different density. Since
distribution is the most important "regulator" of atmospheric density,
air masses of different temperature. Fronts are drawn as "lines"
masses on weather maps. Following are the four (4) types of fronts:
- The interaction between two, three, or four tropical cyclones
nied by cumulus-type low clouds and heavy showers, sometimes thundershowers
- A front along which colder air replaces warmer air. Usually
with violent squalls.
ded by gradually lowering and thickening stratus-type clouds, a steady
type of rain or
Warm Front -
A front along which warmer air replaces colder air. Usually
drizzle, and falling pressure.
warm air is "lifted" off the earth's surface (forced aloft). Weather
and clouds prece-
Occluded Front -
A front resulting when a cold front overtakes a warm front and the
ding warm fronts. There is no warm air at the earth's surface, however,
as the front
mass. The front is not moving.
Stationary Front -
A front along which one air mass does not replace another air
km of each other.
These tropical cyclones either attract or repel each other.
- An unusually strong wind. In storm warning terminology,
a wind of 52-102 kph (28-
- Stands for Greenwhich Mean Time (Greenwhich, London) or
Universal Time. Example,
0000 GMT (+ 8)
= 8:00 am Philippine/Hong Kong Time.
- Stands for Geostationary Meteorological Satellite
and is owned by Japan. This
most of Asia in observing weather patterns and disturbances.
- An American weather satellite which stands for Geostationary
Satellite. It covers most of the continental U. S. plus some areas
parts of the Pacific, etc.
- Also known as gustiness - the peak wind speed or
the sudden, temporary increase
in wind speed.
Maximum sustained wind speeds of 56 kph may have superimposed gusts
Area/Cell - See Anticyclone.
Season - That portion of the year having a relatively
high incidence of hurricanes.
In the Atlantic,
Caribbean, and Gulf of Mexico, this is the period from June through November;
in the eastern
Pacific, June through November 15; and in the central Pacific, the period
- The maximum sustained one (1) minute mean (10 min. in other
speed, typically within one degree of the center of a tropical cyclone.
Convergence Zone (ITCZ) aka. Monsoon Trough- An almost-continuous trough
or belt of low
the globe in the equatorial regions. It is where the NE trade winds (of
and the SE trades (of the Southern Hemisphere) converge, or come
The ITCZ is usually characterized by strong, ascending air currents, large
and frequent heavy showers and thunderstorms. It is also known
the Equatorial Trough, the Equatorial Front, the Intertropical
Convergence Zone, etc.
- stands for Infrared. Used mostly on weather
satellites to observed areas even during
- A line of equal or constant pressure. On weather
maps, isobars are lines drawn
through all points
of equal atmospheric pressure along a given reference surface (such as
on surface weather maps). Isobars are usually drawn at four-millibar
- A stormy, rain-bringing wind from the SW or SSW in Hawaii.
It occurs about five times
per year on the
southwest slopes, which are in the lee of the prevailing NE trade winds.
word meaning "leeward."
Area / System (LPA) - See Cyclone.
Maximum Sustained Wind(s) - The highest surface wind
speed averaged over a 1-minute
period of time. Some weather bureaus (eg. PAGASA, Japan Meteorological
averaged it over a 10-minute period of time. (Peak gusts over water
average 20 to 25 percent
higher than sustained winds).
Mei-yu Front - The term "mei-yu" is the Chinese
expression for "plum rains." The mei-yu front
is a persistant east-west zone of disturbed weather during spring which
is quasi-stationary and
stretches from the east China coast, across Taiwan, and eastward into the
Pacific south of
METEOSAT - A European meterorological satellite.
Monsoon - A seasonal wind produced by the alternate
heating and cooling of land and sea
masses. The monsoon is more common in Asia and there two
Monsoon Depression - A tropical cyclonic vortex
characterized by: 1) its large size, the
Southwest or Summer
Monsoon - This monsoon blows steadily from the SW,
from about May to September. These strong winds are accompanied
by heavy squalls and thunderstorms. Rainfall is very much
heavier - sometimes torrential - than during the winter monsoon.
As the season advances, rainfall and squalls become less frequent.
In other places, the wind becomes light and unsteady. In other places,
it continues reasonably steady. Regions affected includes: Most of
Asia (particularly South and Southeast Asia) and Bangladesh.
In the Philippines, the summer monsoon becomes stronger whenever
a LPA or tropical cyclone is present.
Northeast or Winter
Monsoon - Blows from the NE, from about November to
April. It is a rather steady wind, frequently attaining a speed of
65 kph (35 mph). Except for the windward slopes of mountainous or
hilly areas, skies are generally clear to partly cloudy during the winter
monsoon, and there is relatively little rain (except for the windward slopes
of mountains). However, some low cloudiness and/or light fog (yes,
even at low latitudes) may occur along coastal areas. Regions affected
includes: Asia. In Indonesia, winter monsoon is also known
as the Northwest Monsoon - where the winds shifts from NE to NW
after crossing the equator.
outer-most closed isobar may have a diameter on the order of 1000 km (600
nm); 2) a loosely
organized cluster of deep convective elements; 3) a low-level wind distribution
which features a
200-km (100-nm) diameter light-wind core which may be partially surrounded
by a band of
gales; and, 4) a lack of a distinct cloud system center. Note: most
monsoon depressions which
form in the western North Pacific eventually acquire persistent central
accelerated core winds marking its transition into a conventional tropical
Monsoon Gyre - A mode of the summer monsoon circulation
of the western North Pacific
characterized by: 1) a very large nearly circular low-level cyclonic vortex
that has an outer-most
closed isobar with diameter on the order of 2500 km (1350 nm); 2) a cloud
band rimming the
southern through eastern periphery of the vortex/surface low; 3) a relatively
long (two week) life
span - initially, a subsident regime exists in its core and western and
with light winds and scattered low cumulus clouds; later, the area within
the outer closed isobar
may fill with deep convective cloud and become a monsoon depression or
the large vortex cannot be the result of the expanding wind field of a
or tropical cyclone. Note: a series of small or very small tropical
from the "head" or leading edge of the peripheral cloud band of a monsoon
(JTWC 1993; Lander 1994a).
PAR (Philippine Area of Responsibility) - An area bounded
by rhumb lines on the
Philippine Tropical Cyclone Tracking Chart/Map or imaginary lines on the
surface of the earth
that makes equal
oblique angles with all meridians joining the following points: 25°N
5°N 135°E, 5°N
115°E, 15°N 115°E, and 21°N 120°E. Tropical cyclone
bulletins are issued by
Atmospheric, Geophysical and Astronomical Services Administration (PAGASA)
every six or twelve hours for all tropical cyclones within this area.
Present Movement - the best estimate of the movement
of the center of a tropical cyclone at
a given time and at a given position. This estimate does not reflect
small-scale oscillations of the cyclone's center.
Pressure (Atmopsheric) - The earth's atmosphere
has weight, which is manifested by a
downward pressure. A column of air extending from the earth's surface
to the top of the
atmosphere exerts a pressure on the earth's surface equivalent to a column
of mercury 29.92
inches, or 76 centimeters, or 760 millimeters high, or a column of water
34 feet high. This
pressure is 1013.2 millibars or hectopascals (mb/hPa) (units normally used
on weather maps).
The atmosphere exerts a pressure on the earth's surface amounting to about
14.7 pound per
square inch, or about one ton per square foot.
Public Storm Warning Signals - It is the tropical
cyclone warning signals used by the
Philippine weather bureau. Click here to
Quadrant (in Storm Warnings)
- The 90-degree sector of the storm centered on a
point of the compass. An eight-point compass rose is used when referring
Example: The north quadrant refers to the sector of the storm from 315°
360° to 045°.
Rapid Deepening - A decrease in the minimum sea-level
pressure (mslp) of a tropical
1.75 mb/hr or 42 mb for 24 hours (Holliday and Thompson 1979).
Recurvature - The turning of a tropical cyclone
from an initial path toward the west and
poleward to east and poleward, after moving poleward of the mid-tropospheric
Relocated - A term used in an advisory/bulletin/warning
to indicate that a vector drawn from
the preceding advisory/bulletin/warning position to the latest known position
is not necessarily a
reasonable representation of the cyclone's movement.
Reverse-Oriented Monsoon Trough - The distinguishing
characteristics of a
reverse-oriented monsoon trough in the western North Pacific are a SW-NE
orientation of the trough axis, and the penetration of the trough axis
into subtropical areas
normally the province of easterly flow also known as easterlies.
Sea Swells - Decaying waves (originating from a strong
tropical cyclone), which may persist
for a long, long time and travel great distances.
Semicircle (in Storm Warnings) -
The 180-degree sector of the storm centered on the
designated cardinal point of the compass. A four-point compass rose
is used when referring to
a semicircle. Example: The south semicircle refers to the segment
of the storm from 090°
through 180° to 270°.
Severe Weather Bulletin - It is the warning issued
by the Philippine weather bureau,
PAGASA in disseminating tropical cyclone information. Severe weather
bulletin is used
whenever a tropical cyclone is within the Philippine area of responsibility
or PAR. There are
Significant Tropical Cyclone - A tropical cyclone
becomes "significant" with the issuance of
A type of severe weather bulletin issued every twelve (12) hours or twice
a day (11am and 11pm). It is used whenever a tropical cyclone develops
or enters the PAR but has no direct effect to the country.
- A type of severe weather bulletin issued every six (6)
hours or four times a day (4am, 11am, 4pm, and 11pm). It is used
or upgraded from alert status whenever a tropical cyclone threatens or
is starting to affect the country. At this level, storm warning
signal(s) is(are) now hoisted in areas affected by the tropical cyclone.
the first numbered warning by the responsible warning agency.
Significant Tropical Weather Advisory - A daily
message describing significant tropical
activity and the Joint Typhoon Warning Center (JTWC) - Guam's evaluation
of its potential for
development into a significant tropical cyclone.
Size - The areal extent of a tropical cyclone,
usually measured radially outward from the center
to the outer-most closed isobar. Based on an average radius of the
outer-most closed isobar,
size categories in degrees of latitude follow: <2° = very small,
2° to 3° = small, 3° to 6° =
6° to 8° = large, and 8° or greater = very large (Brand 1972
modification of Merrill
Standard Gust Factor - 1.20 to 1.25 over water
(example: a 120 kts. gust = 120 kts.
sustained winds). 1.60 over land (example: a 120 kt. gust = 75 kts.
sustained winds) (Atkinson
Steering Ridge - is the elongated area of High Pressure which steers a tropical cyclone.
Storm - Any disturbed state of the earth's atmosphere,
especially as affecting the earth's
surface, and strongly implying destructive or otherwise unpleasant weather
thunderstorms, tropical cyclones, extratropical cyclones, blizzards, ice
storms, sandstorms, dust
storms, etc.). Winds of 89 kph (48 kts.) or greater, when not associated
with tropical weather
Storm Surge - A rapid rise in the water level
produced by onshore typhoon and hurricane
winds and falling atmospheric pressure. The storm surge normally
occurs just to the right (in the
Northern Hemisphere) and just to the left (in the Southern Hemisphere)
of the cyclone's center,
and either shortly precedes or accompanies the arrival of the center (or
Storm Tide - An abnormal rise of the sea as a
result of storm winds. The storm tide may
basins not normally affected by tide. It may also flood normally
dry lowlands in coastal
Strength - The average wind speed of the surrounding
low-level wind flow, usually measured
within a one to three degree annulus of the center of a tropical cyclone
(Weatherford and Gray
Subsidence - A descending motion of air in the atmosphere,
usually with the implication that
the condition extends over a rather broad area. As the air descends,
it is heated adiabatically
(no transfer of heat or mass; the air is warmed by compression as it descends),
more dry. Good weather is usually associated with subsidence.
Subtropical Cyclone - A low pressure system that
forms over the ocean in the subtropics and
has some characteristics of a tropical circulation, but not a central dense
overcast. Although of
upper cold low or low-level baroclinic origins, the system can transition
to a tropical cyclone.
Subtropics - The indefinite "belts" in each hemisphere
between the tropics and temperate
regions. The polar boundaries are considered to be roughly 35°
- 40°N and S latitudes, but vary
greatly according to continental influence. They are farther poleward
on the west coasts of
continents and farther equatorward on the east coasts.
Super Typhoon (STY) - A typhoon with
maximum sustained 1-minute mean surface winds of
242 kph (130 kt) or greater. * It is coined and
used only by the Joint Typhoon Warning Center
Suspect Area - An area suspected of containing
a developing or existing tropical cyclone.
Tropical Cyclone - A non-frontal, migratory low-pressure
system, usually of synoptic scale,
originating over tropical or subtropical waters and having a definite organized
center is normally warmer than the surroundings. Globally, tropical
cyclone is categorized into
three types(with the exception of the fourth):
Formation Alert (TCFA) - A message advising of
the possible or
(TD) - A tropical cyclone with maximum sustained 1-minute
mean surface winds of 61 kph (33 kt.) or less.
(TS) - A tropical cyclone with maximum 1-minute mean
sustained surface winds in the range of 63 to 117 kph (34 to 63 kt.), inclusive.
Typhoon / Hurricane
(TY/HURR) - A tropical cyclone with maximum sustained
1-minute mean surface winds of 119 to 239 kph (64 to 129 kt.). West
of 180° E longitude they are called typhoons and east of 180°
E longitude -- hurricanes.
Super Typhoon (STY) -
A typhoon with maximum sustained 1-minute mean surface winds of 242 kph
(130 kt.) or greater.
of a tropical cyclone. It is issued only by the Joint Typhoon Warning
Cyclone Warning (TCW) - A message issued by responsible
details of a tropical cyclone location, intensity, size, and movement.
Disturbance - A discrete system of apparently
organized convection, generally 100
to 300 nm (185
to 555 km) in diameter, originating in the tropics or subtropics, having
migratory character and having maintained its identity for 12 to 24 hours.
system may or
may not be associated with a detectable perturbation of the low-level wind
It is the basic generic designation which, in successive stages of development,
may be classified
as a tropical depression, tropical storm, typhoon, or super typhoon.
Upper-Tropospheric Trough (TUTT) - A dominant climatological
system and a
synoptic feature of the summer season, over the tropical North Atlantic,
Pacific and South
Pacific Oceans (Sadler 1979). Cold core lows in the TUTT are
referred to as
cells, or TUTT
Wave (or Easterly Wave) - A trough or cyclonic curvature
maximum in the
The wave may reach maximum amplitude in the lower middle
or may be the reflection of an upper-troposhere cold-low or equatorward
of a middle-latitude
- The zone of earth's surface which lies between the Tropic
of Cancer (approx. 23°
27'N lat.) and the Tropic of Capricorn (approx. 23°
27'S lat.). Same as the Torrid Zone.
Season - There is no true typhoon season. Typhoons
in the western Pacific
in every month of the year. However, 90 percent of the typhoons occur
June and late December. A maximum (22.6 percent) of the total occurs
August, and a
minimum (0.6 percent) in February.
Veering - Refers to the shifting of the wind direction
of a tropical cyclone observed by the
observer from northeast through
southeast by way of east (also known as First Wind).
means that the storm is passing
south of the locality (in the Northern Hemisphere).
Vortex Fix - The location of the surface and/or flight
level center of a tropical cyclone or
subtropical cyclone obtained by reconnaissance aircraft penetration.
Wall Cloud (or Eye Wall) - An organized band
of deep cumuliform clouds that immediately
surrounds the area of a tropical cyclone. The wall cloud may entirely
enclose or partially
surround the center. This is the area where the strongest wind and
rain of a tropical cyclone can
be found. Wall cloud and eye wall are used synonymously.
Westerly Wind Burst (WWB) - A short-duration
low-level westerly wind event along and near
the equator in the western Pacific Ocean (and sometimes in the Indian Ocean)
(Luther et al.
1983). Typically, a westerly wind burst (WWB) lasts several days
and has westerly winds of at
least 19 kph (10 kt) (Keen 1988). Most WWBs occur during the
monsoon transition months of
April-May, and November-December. They show some relationship to
the El Nino Southern
Oscillation (ENSO) phenomenon (Luther et al. 1983; Ramage 1986).
Some WWBs are even
more energetic, with wind speeds of 59 kph (30 kt) observed during well-developed
These intense WWBs are associated with a large cluster of deep-convective
clouds along the
equator. An intense WWB is a necessary precursor to the formation
of tropical cyclone twins
symmetrical with respect to the equator (Keen 1982; Lander 1990).
Wind (and balance of forces) -
Air in motion relative to the surface of the earth. Wind is a
balance of three forces (if we neglect curvature): (1) the coriolis
(or horizontal deflecting) force,
(2) the frictional force, and (3) the pressure force. This balance
results in the wind blowing
across the isobars at angles 15° over water (because friction is less)
and about 30° over land
(where friction is greater), in the direction from higher toward lower
Wind, First -
The initial tropical cyclone winds as observed on the surface from northwest
west (if the storm will pass north of the locality) or from
northeast thru east (if the storm will pass
south of the locality). This means that the tropical cyclone
is approaching. The term is
synonymously used as backing or veering.
Wind, Second - The final tropical cyclone winds
as observed on the surface from southwest
thru south (if the storm just passed north of the locality)
or from southeast thru south (if the
passed south of the locality). This means that the tropical
cyclone is now leaving or
just starting to leave the area. The term is synonymously used as
backing or veering.
Typhoon Warning Center (JTWC), 1996: Appendix A (Definitions).
1996 Annual Tropical Cyclone
Report, JTWC, pp 320-322.
Pacific Meteorology and Oceanography Center West (NPMOCW)/JTWC, 1998:
A Synopsis of
Super Typhoon Paka, NPMOCW/JTWC, http://www.npmocw.navy.mil/npmocw/prods/paka.html
W.J. and R. Henderson, 1984: Weather Definitions, Displays, and
Warnings / Tropical Cyclones.
Heavy Weather Guide, Naval Institute, Anapolis, Maryland, pp 2-15 / pp
C.W., 1998: Why Don't We Try To Destroy Tropical Cyclones?
(FAQ): Hurricanes, Typhoons and Tropical Cyclones, NOAA AOML, Hurricane
R.R., 1990: A Primer on Terms Used By PAGASA. Philippine
Almanac: Book of Facts 1990, Aurora
Publications, pp 968-969.