A
 

      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.

B

     Best Track  -  A subjectively smoothed path, versus a precise and very erratic fix-to-fix  path,
     used to represent tropical cyclone movement, and based on an assessment of all available
     data.

     Binary Interaction  -  A mutual cyclonic orbit of two tropical cyclones around their
     centroid.  Lander and Holland (1993) showed that the behavior of most binary tropical
     cyclones consists of an approach, sudden capture, then a period of steady cyclonic orbit
     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.

 
C

     Center  -  The vertical axis or core of a tropical cyclone. Usually determined by cloud vorticity
     patterns, wind and/or pressure distribution.

     Center Fix  -  The location of the center of a tropical or subtropical cyclone obtained by means
     other than reconnaissance aircraft penetration.

     Concentric Eye Wall Cycles  -  Naturally occur in intense tropical cyclones (wind > 185 kph or
     100 kt).  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 10 to
     25 km.  At this point, some of the outer rainbands may organize into an outer ring of
     thunderstorms that slowly moves inward and robs the inner eyewall of its needed moisture and
     momentum. 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
     completely and the storm can be the same intensity as it was previously or, in some cases,
     even stronger. A concentric eyewall cycle occured in Hurricane Andrew (1992) before landfall
     near Miami:  a strong intensity was reached,  an outer eyewall formed,  this contracted in
     concert with a pronounced weakening of the storm, and as the outer eyewall completely
     replaced the original one (inner eyewall), the hurricane re-intensified.  Another example can be
     seen here on Hurricane David's radar composite photo.
 
     Convergence  -  The contraction of a vector field. In meteorology, the term is used in a
     broad sense to include the "coming together," or meeting, of winds (air particles) from dif-
     ferent directions or areas (horizontally or vertically).  Mathematically, convergence is nega-
     tive divergence.

     Cyclone (or Low-Pressure System, or simply Low or Depression)  -  An area within which
     the pressure is low relative to the surroundings.  An atmospheric "closed circulation" with the
     winds blowing counterclockwise around cyclones in the Northern Hemisphere, and clockwise
     around cyclones in the Southern Hemisphere.  The winds cross the isobars at an angle (15
     degrees over water and 30 degrees over land), and blow from higher toward lower pressure.

D

E
 

     Ephemeris  -  Position of a body (satellite) in space as a function of time; used for gridding
     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
     cyclone's primary energy source from the release of latent heat of condensation to baroclinic
     processes.  It is important to note that cyclones can become extratropical and still maintain
     winds of typhoon or storm force.

     Eye  -  The central area of a tropical cyclone when it is more than half surrounded by wall
     cloud.  Also known as the relatively calm center of a tropical cyclone where the sky may be
     clear or just partly cloudy.

     Eye Wall  -  See Wall Cloud.

F
 

 
•  Cold Front  -  A front along which colder air replaces warmer air.  Usually accom-
           nied by cumulus-type low clouds and heavy showers, sometimes  thundershowers
           with violent squalls.
•  Warm Front  -  A front along which warmer air replaces colder air.  Usually  prece-
           ded by gradually lowering and thickening stratus-type clouds, a steady type of rain or
           drizzle, and falling pressure.
•  Occluded Front  -  A front resulting when a cold front overtakes a warm front and  the
           warm air is "lifted" off the earth's surface (forced aloft).  Weather and clouds prece-
           ding warm fronts.  There is no warm air at the earth's surface, however, as the front
           passes.
•  Stationary Front  -  A front along which one air mass does not replace another air
           mass.  The front is not moving.
 

G
 

     GMT  -  Stands for Greenwhich Mean Time (Greenwhich, London) or Universal Time.  Example,
     0000 GMT (+ 8) = 8:00 am Philippine/Hong Kong Time.

     GMS  -  Stands for Geostationary Meteorological Satellite and is owned by Japan.  This
     satellite covers most of Asia in observing weather patterns and disturbances.

     GOES  - An American weather satellite which stands for Geostationary Operational
     Environmental Satellite. It covers most of the continental U. S. plus some areas like Canada,
     South America, parts of the Pacific, etc.

     Gust(s)  -  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 of
     93 kph.
 

H
 

     Hurricane 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 is from
     June through October.
 

I
 

     Intertropical Convergence Zone (ITCZ)  aka. Monsoon Trough-  An almost-continuous trough or belt of low
     pressure around the globe in the equatorial regions. It is where the NE trade winds (of the
     Northern Hemisphere) and the SE trades (of the Southern Hemisphere) converge, or come
     together.  The ITCZ is usually characterized by strong, ascending air currents, large areas of
     overcast skies, and frequent heavy showers and thunderstorms.  It  is also known by many
     different names:  the Equatorial Trough, the Equatorial Front, the Intertropical Front, the
     Equatorial Convergence Zone, etc.

     IR  -  stands for Infrared.  Used mostly on weather satellites to observed areas even during
     nightime.

     Isobar  -  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
     mean-sea-level on surface weather maps).  Isobars are usually drawn at four-millibar intervals.
 

 J
 

K
 

L

M

     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
     Japan.

     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 kinds:
 

• 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.
  
  

     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 northwestern quadrants
     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 tropical cyclone;
     and,  4) the large vortex cannot be the result of the expanding wind field of a pre-existing
     monsoon depression or tropical cyclone.  Note: a series of small or very small tropical cyclones
     may emerge from the "head" or leading edge of the peripheral cloud band of a monsoon gyre.
     (JTWC 1993; Lander 1994a).

N, O
 

P

     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 the short-period,
     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 view. 

Q

R

     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 subtropical
     ridge axis.

     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 (i.e., reverse)
     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. 

S

     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
     two types of bulletins:
 

• Alert  -  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.
• Warning  -  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.
  
  

     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° =
     medium (average), 6° to 8° = large, and 8° or greater = very large (Brand 1972 and a
     modification of Merrill 1982).

     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
     1974).

     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 (tornadoes,
     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
     systems.

     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 "eye").
 
     Storm Tide  -  An abnormal rise of the sea as a result of storm winds.  The storm tide may
     occur in basins not normally affected by tide.  It may also flood normally dry lowlands in coastal
     areas.

     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
     1985).

     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), and becomes
     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
     (JTWC).

     Suspect Area  -  An area suspected of containing a developing or existing tropical cyclone.
 

T

• Tropical Depression (TD)  -  A tropical cyclone with maximum sustained 1-minute mean surface winds of 61 kph (33 kt.) or less.
• Tropical Storm (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.
  
  

     Tropical Cyclone Warning (TCW)  -  A message issued by responsible forecast activities
     which provides details of a tropical cyclone location, intensity, size, and movement.

     Tropical 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 a
     non-frontal, migratory character and having maintained its identity for 12 to 24 hours.  The
     system may or may not be associated with a detectable perturbation of the low-level wind or
     pressure field. 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.

     Tropical Upper-Tropospheric Trough (TUTT)  -  A dominant climatological system and a
     daily upper-level synoptic feature of the summer season, over the tropical North Atlantic, North
     Pacific and South Pacific Oceans (Sadler 1979).  Cold core lows in the TUTT are referred to as
     cells, or TUTT cells.

     Tropical Wave (or Easterly Wave)  -  A trough or cyclonic curvature maximum in the
     trade-wind easterlies.  The wave may reach maximum amplitude in the lower middle
     troposphere, or may be the reflection of an upper-troposhere cold-low or equatorward extension
     of a middle-latitude trough.

     Tropics  -  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.

     Typhoon Season  -  There is no true typhoon season.  Typhoons in the western Pacific
     can--and do--occur in every month of the year.  However, 90 percent of the typhoons occur
     between early June and late December.  A maximum (22.6 percent) of the total occurs in
     August, and a minimum (0.6 percent) in February. 

U

V
 

     Vortex Fix  -  The location of the surface and/or flight level center of a tropical cyclone or
     subtropical cyclone obtained by reconnaissance aircraft penetration.

W
 

     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 systems.
     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 pressure.

     Wind, First  -  The initial tropical cyclone winds as observed on the surface from northwest thru
     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
     storm just 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.

X, Y, & Z
 

• REFERENCES:

Joint  Typhoon Warning Center (JTWC),  1996:  Appendix A (Definitions).  1996 Annual Tropical Cyclone
     Report, JTWC, pp 320-322.

Naval 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

Kotsch, W.J. and R. Henderson, 1984:  Weather Definitions, Displays, and Warnings / Tropical Cyclones.
     Heavy Weather Guide, Naval Institute, Anapolis, Maryland, pp 2-15 / pp 75-113.

Landsea, C.W., 1998:  Why Don't We Try To Destroy Tropical Cyclones?  Frequently Asked-Questions
     (FAQ): Hurricanes, Typhoons and Tropical Cyclones, NOAA AOML, Hurricane Research Division,
     http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqC.html#C4

Pardo, R.R., 1990:  A Primer on Terms Used By PAGASA.  Philippine Almanac: Book of Facts 1990, Aurora
     Publications, pp 968-969.