Subj : Weather Glossary (P)
To : All
From : Daryl Stout
Date : Thu Jan 17 2019 12:04 am
This weather glossary contains information on more than 2000 terms,
phrases and abbreviations used by the National Weather Service (NWS)...
the government agency who makes weather forecasts, and issues weather
advisories, watches, and warnings, for the United States, and its
territories.
Many of these terms and abbreviations are used by NWS forecasters to
communicate between each other and have been in use for many years and
before many NWS products were directly available to the public. It is the
purpose of this glossary to aid you in better understanding NWS products.
***
PAC
Pacific
Pacific Decadal Oscillation
(Abbrev. PDO) - a recently described pattern of climate variation similar
to ENSO though on a timescale of decades and not seasons. It is
characterized by SST anomalies of one sign in the north-central Pacific
and SST anomalies of another sign to the north and east near the
Aleutians and the Gulf of Alaska. It primarily affects weather patterns
and sea surface temperatures in the Pacific Northwest, Alaska, and
northern Pacific Islands.
Palmer Drought Severity Index
(Abbrev. PDSI) - an index used to gage the severity of drought conditions
by using a water balance equation to track water supply and demand. This
index is calculated weekly by the National Weather Service.
Pan Pan
A headline within National Weather Service high seas forecasts
transmitted via the GMDSS to indicate that a hurricane or hurricane
force winds are forecast.
Pancake Ice
In hydrologic terms, circular flat pieces of ice with a raised rim; the
shape and rim are due to repeated collisions.
Panhandle Hook
Low pressure systems that originate in the panhandle region of Texas and
Oklahoma which initially move east and then "hook" or recurve more
northeast toward the upper Midwest or Great Lakes region. In winter,
these systems usually deposit heavy snows north of their surface track.
Thunderstorms may be found south of the track.
Parameter
A subset of the group of evaluations that constitute each element of an
observation.
Parapet Wall
In hydrologic terms, a solid wall built along the top of the dam for
ornament, safety, or to prevent overtopping.
Parcel
A volume of air small enough to contain uniform distribution of its
meteorological properties and large enough to remain relatively
self-contained and respond to all meteorological processes.
Parhelion
The scientific name for sun dogs. Either of two colored luminous spots
that appear at roughly 22 degrees on both sides of the sun at the same
elevation. They are caused by the refraction of sunlight passing
through ice crystals. They are most commonly seen during winter in the
middle latitudes and are exclusively associated with cirriform clouds.
They are also known as mock suns.
Partial Beam Filling
A limitation of the rainfall estimation techniques used by NEXRAD. At
far ranges from the radar, a storm may occupy only a portion of the
radar beam (which may be several miles across). However, the radiation
received by the radar antenna consists of the average reflectivity
across the entire beam, so the reflectivity and associated rainfall
rates are underestimated.
Partial-Duration Flood Series
In hydrologic terms, a list of all flood peaks that exceed a chosen
base stage or discharge, regardless of the number of peaks occurring
in a year.
Particle Trajectory Model
A computer sub-model that tracks the trajectories of multiple particles
that are released into an atmospheric flow model.
Partly Cloudy
When the predominant/average sky condition is covered 3/8 to 4/8 with
opaque (not transparent) clouds. Same as Partly Sunny.
Partly Sunny
When the predominant/average sky condition is covered 3/8 to 4/8 with
opaque (not transparent) clouds. Same as Partly Cloudy.
Pascal
The unit of pressure produced when one newton acts on one square meter
(1 N/m2). It is abbreviated Pa.
PAT
Pattern
PBL
Probable
PC-GRIDDS
PC-Gridded Interactive Display and Diagnostic System - Allows the
forecaster to view fields of gridded model output in contour or vector
format. By doing this, the forecaster can extract relevant information
from the numerical model grid-point data.
PCPN
Precipitation
PCT
precent
PD
Period
PDI
Palmer Drought Index
PDMT
Predominant
PDO
Pacific Decadal Oscillation - a recently described pattern of climate
variation similar to ENSO though on a timescale of decades and not
seasons. It is characterized by SST anomalies of one sign in the
north-central Pacific and SST anomalies of another sign to the north
and east near the Aleutians and the Gulf of Alaska. It primarily
affects weather patterns and sea surface temperatures in the Pacific
Northwest, Alaska, and northern Pacific Islands. Two main characteristics
distinguish PDO from El Ni�o/Southern Oscillation (ENSO): first,
20th century PDO "events" persisted for 20-to-30 years, while typical
ENSO events persisted for 6 to 18 months; second, the climatic
fingerprints of the PDO are most visible in the North Pacific/North
American sector, while secondary signatures exist in the tropics-
the opposite is true for ENSO. Several independent studies found
evidence of just two full PDO cycles in the past century: cool" PDO
regimes prevailed from 1890-1924 and again from 1947-1976, while
"warm" PDO regimes dominated from 1925-1946 and from 1977 through
(at least) the mid-1990's. Causes for the PDO are not currently known.
Likewise, the potential predictability for this climate oscillation are
not known.
PDS
Particularly Dangerous Situation (PDS) wording is used in rare
situations when long-lived, strong and violent tornadoes are possible.
This enhanced wording may also accompany severe thunderstorm watches
for intense convective wind storms.
PDS Watch
The Particularly Dangerous Situation (PDS) wording is used in rare
situations when long-lived, strong and violent tornadoes are possible.
This enhanced wording may also accompany severe thunderstorm watches
for intense convective wind storms.
PDSI
Palmer Drought Severity Index - an index used to gage the severity of
drought conditions by using a water balance equation to track water
supply and demand. This index is calculated weekly by the National
Weather Service.
PDT
Pacific Daylight Time
Peak Discharge
In hydrologic terms, the rate of discharge of a volume of water passing
a given location.
Peak Gust
The highest instantaneous wind speed observed or recorded.
Peak Pulse
The amount of power transmitted by a radar during a given pulse. Note
that because these pulses are widely spaced, the average power will be
much smaller.
Peak Wind Speed
The maximum instantaneous wind speed since the last observation that
exceeded 25 knots.
Pendant Echo
Radar signature generally similar to a hook echo, except that the hook
shape is not as well defined.
Penetrating Top
Same as Overshooting Top; a dome-like protrusion above a thunderstorm
anvil, representing a very strong updraft and hence a higher potential
for severe weather with that storm. A persistent and/or large
overshooting top (anvil dome) often is present on a supercell. A
short-lived overshooting top, or one that forms and dissipates in
cycles, may indicate the presence of a pulse storm.
Penumbra
In solar-terrestrial terms, the sunspot area that may surround the
darker umbra or umbrae. It consists of linear bright and dark elements
radial from the sunspot umbra.
Perched Groundwater
In hydrologic terms, local saturated zones above the water table which
exist above an impervious layer of limited extent.
Percolation
In hydrologic terms, the movement of water, under hydrostatic pressure,
through the interstices of a rock or soil, except the movement through
large openings such as caves.
Percolation Path
In hydrologic terms, the course followed by water moving or percolating
through any other permeable material, or under a dam which rests upon a
permeable foundation.
Perennial Stream
In hydrologic terms, a stream that flows all year round.
Perigee
The closest distance between moon and earth or the earth and sun.
Perihelion
The point on the annual orbit of a body (about the sun) that is closest
to the sun; at present, the earth reaches this point on about 5 January.
Opposite of aphelion.
Permafrost
A layer of soil at varying depths below the surface in which the
temperature has remained below freezing continuously from a few to
several thousands of years.
Permeability
In hydrologic terms, the ability of a material to transmit fluid through
its pores when subjected to a difference in head.
Permeability Coefficient
In hydrologic terms, the rate of flow of a fluid through a cross section
of a porous mass under a unit hydraulic gradient, at a temperature of 60
degrees Fahrenheit.
Permeameter
In hydrologic terms, a laboratory instrument for determining permeability
by measuring the discharge through a sample of the material when a known
hydraulic head is applied.
Persistence
Continuation of existing conditions. When a physical parameter varies
slowly, the best prediction is often persistence.
Persistence Forecast
A forecast that the current weather condition will persist and that
future weather will be the same as the present (e.g., if it is raining
today, a forecast predicting rain tonight).
Perturbation Model
A computer model used to calculate air pollution concentrations. A
perturbation model produces a wind field from solutions to a simplified
set of equations that describe atmospheric motions.
Pervious Zone
In hydrologic terms, a part of the cross section of an embankment dam
comprising material of high permeability.
PFD
QPF Discussion (issued by the HPC)
Phenomenological Model
A computer model used to calculate air pollution concentrations. A
phenomenological model focuses on an individual phenomenon, such as
plume impingement or fumigation.
Phiezometer
In hydrologic terms, an instrument used to measure pressure head in a
conduit, tank, soil, etc. They are used in dams to measure the level of
saturation.
Photochemical Smog
Air pollution containing ozone and other reactive chemical compounds
formed by the reaction of nitrogen oxides and hydrocarbons in the
presence of sunlight.
Photosphere
The intensely bright portion of the sun visible to the unaided eye;
the "surface" of the sun. Reaching temperatures estimated at about
11,000 degrees Fahrenheit, it is the portion of the sun's atmosphere
which emits continuous electromagnetic radiation.
Phreatic water
In hydrologic terms, water within the earth that supplies wells and
springs; water in the zone of saturation where all openings in rocks
and soil are filled, the upper surface of which forms the water table.
Also termed Groundwater.
PIBAL
Pilot balloon. A small helium-filled meteorological balloon that is
tracked as it rises through the atmosphere to determine how wind speed
and direction change with altitude.
Pilot Balloon
(Abbrev. PIBAL)- A small helium-filled meteorological balloon that is
tracked as it rises through the atmosphere to determine how wind speed
and direction change with altitude.
Pilot Report
(Abbrev. PIREP)- A report of inflight weather by an aircraft pilot or
crew member. A complete coded report includes the following information
in this order: location and/or extent of reported weather phenomenon:
type of aircraft (only with reports turbulence or icing).
Pingo
A large frost mound of more than one-year duration.
PIREP
Pilot Report. A report of inflight weather by an aircraft pilot or crew
member. A complete coded report includes the following information in
this order: location and/or extent of reported weather phenomenon: type
of aircraft (only with reports turbulence or icing).
Pitot Tube
In hydrologic terms, a device for measuring the velocity of flowing water
using the velocity head of the stream as an index of velocity. It
consists essentially of an orifice held to a point upstream in the water,
connected with a tube in which the rise of water due to velocity head
may be observed and measured. It also may be constructed with an
upstream and downstream orifice, with two water columns, in which case
the difference in height of water column in the tubes is the index of
velocity.
PIX
picture
PK
Peak
PL
Ice pellets (sleet)
Plage
In solar-terrestrial terms, an extended emission feature of an active
region that exists from the emergence of the first magnetic flux until
the widely scattered remnant magnetic fields merge with the background.
Plage Corridor
In solar-terrestrial terms, a space in chromospheric plage lacking plage
intensity, coinciding with polarity inversion line.
Plagenil
In solar-terrestrial terms, spotless disc free of calcium plage.
Plan Position Indicator
An acronym for Plan Position Indicator. A PPI displays radar data
horizontally using a map projection. In PPI mode, the radar makes a
360-degree sweep with the antenna at a specific elevation angle. A PPI
display is the familiar radar display shown on the television weather
programs.
Planetary Boundary Layer
The layer within the atmosphere between 1 km and the earth's surface
where friction affects wind speed and wind direction.
Plasma
Any ionized gas; that is, any gas containing ions and electrons.
Platform
A generic radar term, often used to encompass the pedestal and antenna
assembly; sometimes including the radar control, display and analysis
hardware and software as well.
Plow Wind
A term used in the midwestern United States to describe strong,
straight-line winds associated with the downdrafts spreading out in
advance of squall lines and thunderstorms. Resulting damage is
usually confined to narrow zones like that caused by tornadoes;
however, the winds are all in one direction (straight-line winds).
Plume Blight
Visibility impairment caused by air pollution plumes aggregated from
individual sources.
Plume Impingement
The collision of a plume with topography that rises above the plume
altitude; often a temporary condition that occurs as the plume sweeps
by the face of a hill as the wind shifts.
Plume-dominated Fire
A fire whose behavior is governed primarily by the local wind
circulation produced in response to the strong convection above the
fire rather than by the general wind.
Pluvial
In hydrology, anything that is brought about directly by precipitation.
PMD
Prognostic Discussion
PMO
Port Meteorological Officer.
PNA
Pacific North American teleconnection
PNHDL
Panhandle
PNS
Public Information Statement - a narrative statement issued by a
National Weather Service Forecast Office that can be used for:
1) A current or expected nonhazardous event of general interest to the
public that can usually be covered with a single message (e.g.,
unusual atmospheric phenomena such as sun dogs, halos, rainbows, aurora
borealis, lenticular clouds, and stories about a
long-term dry/cold/wet/warm spell).
2) Public educational information and activities, such as storm safety
rules, awareness activities, storm drills, etc.
3) Information regarding service changes, service limitations,
interruptions due to reduced or lost power or equipment outages, or
special information clarifying interpretation of NWS data. For example,
this product may be used to inform users of radar equipment outages or
special information clarifying interpretation of radar data originating
from an unusual source which may be mistaken for precipitation (such as
chaff drops, smoke plumes, etc., that produces echoes on the radar
display.
POH
Probability of Hail - a product from the NEXRAD hail detection algorithm
that estimates the likelihood that hail is present in a storm.
Point Precipitation
Precipitation at a particular site, in contrast to the mean precipitation
over an area.
Point Source
A pollutant source that can be treated in a dispersion model as though
pollutants were emitted from a single point that is fixed in space.
Example: the mouth of a smokestack. Compare area source and line source.
Polar Cap Absorption (PCA)
In solar-terrestrial terms, an anomalous condition of the polar
ionosphere whereby HF and VHF (3 - 300 MHz) radiowaves are absorbed,
and LF and VLF (3 - 300 kHz) radiowaves are reflected at lower altitudes
than normal. In practice, the absorption is inferred from the proton flux
at energies greater than 10 MeV, so that PCAs and proton events are
simultaneous. Transpolar radio paths may still be disturbed for days, up
to weeks, following the end of a proton event.
Polar Front
A semipermanent, semicontinuous front that separates tropical air masses
from polar air masses.
Polar Jet
Marked by a concentration of isotherms and strong vertical shear, this
jet is the boundary between the polar air and the subtropical air. It
often divides into two branches, the north and the south, and marks the
high speed core of the prevailing westerlies. It is associated with the
location and motion of the high and low pressure areas of the middle
latitudes, and therefore, is variable in position, elevation, and wind
speed. Its position tends to migrate south in the Northern Hemispheric
winter and north in the summer, and its core winds increase during the
winter and become less strong in the summer.
Polar Jet Stream
Used interchangably with Polar Jet; a jet stream marked by a concentration
of isotherms and strong vertical shear, this jet is the boundary between
the polar air and the subtropical air. It often divides into two branches,
the north and the south, and marks the high speed core of the prevailing
westerlies. It is associated with the location and motion of the high and
low pressure areas of the middle latitudes, and therefore, is variable in
position, elevation, and wind speed. Its position tends to migrate south
in the Northern Hemispheric winter and north in the summer, and its core
winds increase during the winter and become less strong in the summer.
Polar Orbiting Satellite
A weather satellite which travels over both poles each time it orbits the
Earth. It orbits about 530 miles (850 km) above the Earth's surface. A
satellite with an orbit nearly parallel to the earth's meridian lines
which crosses the polar regions on each orbit.
Polarization Radar
A radar which takes advantage of ways in which the transmitted waves'
polarization affect the backscattering. Such radars may alternately
transmit horizontal and vertically polarized beams, and measure
differential reflectivity.
Pollutant
Particles, gases, or liquid aerosols in the atmosphere which have an
undesirable effect on humans or their surroundings. Something unfavorable
to health and life that has been added to the environment.
Pondage
In hydrologic terms,
(1) The holding back of water for later release for power development
above the dam of a hydroelectric plant to
(a) equalize daily or weekly fluctuations of streamflow or
(b) to permit irregular hourly use of water by the wheels to care for
fluctuations in the load demand.
(2) In general the holding back of water for later releases.
(3) The storage capacity available for the use of such water.
Ponding
In hydrologic terms, in flat areas, runoff collects, or ponds in
depression and cannot drain out. Flood waters must infiltrate slowly
into the soil, evaporate, or be pumped out.
Pool
The elevation of the surface of a body of water such as a lake.
Specifically, the pool at a lock and dam or a reservoir is the
elevation of the water surface immediately upstream from the dam.
Pool Height
In hydrologic terms, the height of the water behind a dam. (Various
datums may be used and various pool height may be used, e.g.,
conservation pool, flood control pool, etc.).
POP
Probability of Precipitation
Popcorn Convection
Slang for showers and thunderstorms that form on a scattered basis
with little or no apparent organization, usually during the afternoon
in response to diurnal heating. Individual thunderstorms typically
are of the type sometimes referred to as air-mass thunderstorms: they
are small, short-lived, very rarely severe, and they almost always
dissipate near or just after sunset.
POPS
Probability of Precipitation
Porosity
In hydrologic terms,
(1) The ratio of pore volume to total volume of the formation. Sandy
soils have large pores and a higher porosity than clays and other
fine-grained soils.
(2) An index of the void characteristics of a soil or stream as
pertaining to percolation; degree of previousness.
POS
Positive
Positive Area
The area on a sounding representing the layer in which a lifted parcel
would be warmer than the environment; thus, the area between the
environmental temperature profile and the path of the lifted parcel.
Positive area is a measure of the energy available for convection;
see CAPE.
Positive Cloud to Ground Lightning
A CG flash that delivers positive charge to the ground, as opposed to
the more common negative charge. Positive CGs have been found to occur
more frequently in some severe thunderstorms. Their occurrence is
detectable by most lightning detection networks, but visually it is
not considered possible to distinguish between a positive CG and a
negative CG. (Some claim to have observed a relationship between
staccato lightning and positive CGs, but this relationship is as yet
unproven.)
Positive Vorticity Advection
(Abbrev. PVA) - Advection of higher values of vorticity into an area,
which often is associated with upward motion (lifting) of the air. PVA
typically is found in advance of disturbances aloft (i.e., shortwaves),
and is a property which often enhances the potential for thunderstorm
development.
Positive-tilt Trough
An upper level system which is tilted to the east with increasing
latitude (i.e., from southwest to northeast). A positive-tilt trough
often is a sign of a weakening weather system, and generally is less
likely to result in severe weather than a negative-tilt trough if all
other factors are equal.
Post-Flare Loops
In solar-terrestrial terms, a loop prominence system often seen after
a major two-ribbon flare, which bridges the ribbons.
Post-storm Report
A report issued by a local National Weather Service office summarizing
the impact of a tropical cyclone on it's forecast area. These reports
include information on observed winds, pressures, storm surges,
rainfall, tornadoes, damage and casualties.
Potential Temperature
The temperature a parcel of dry air would have if brought adiabatically
(i.e., without transfer of heat or mass) to a standard pressure level
of 1000 mb.
Potential Vorticity
This plays an important role in the generation of vorticity in
cyclogenesis, especially along the polar front. It is also very useful
in tracing intrusions of stratospheric air deep into the troposphere in
the vicinity of jet streaks.
Powder Snow
Dry, loose, unconsolidated snow.
Power
A radar equation to describe the amount of power that a radar emits.
P = I * V (or)
P = V2 / R (or)
P = I2 / R
where I is current (amps), V is voltage (volts), R is resistance (ohms),
P is power (watts).
PPI
Plan Position Indicator
PPINE
Plan Position Indicates No Echoes, referring to the fact that a radar
detects no precipitation within its range. An intensity-modulated
display on which echo signals are shown in plain view with range and
azimuth angle displayed in polar coordinates, forming a map-like display.
Each PPI is taken at a single, fixed elevation angle, and thus forms a
cone of coverage in space. PPIs may be run in sequence, creating a
"volume scan".
PQPF
Probabilistic QPF; a form of QPF (see below) that includes an assigned
probability of occurrence for each numerical value in the forecast
product.
PRBLTY
Probability
PRBLY
probably
PRCP
Precipitation
Pre-Frontal Squall Line
A line of thunderstorms that precedes an advancing cold front.
Pre-Frontal Trough
An elongated area of relatively low pressure preceding a cold front that
is usually associated with a shift in wind direction.
Pre-Hurricane Squall Line
It is often the first serious indication that a hurricane is
approaching. It is a generally a straight line and resembles a
squall-line that occurs with a mid-latitude cold front. It is as much
as 50 miles or even more before the first ragged rain echoes of the
hurricane's bands and is usually about 100 to 200 miles ahead of the
eye, but it has been observed to be as much as 500 miles ahead of the
eye in the largest hurricanes.
PRECD
Precede
Precipitable Water
Measure of the depth of liquid water at the surface that would result
after precipitating all of the water vapor in a vertical column over a
given location, usually extending from the surface to 300 mb.
Precipitation
The process where water vapor condenses in the atmosphere to form water
droplets that fall to the Earth as rain, sleet, snow, hail, etc.
Precipitation Attenuation
The loss of energy that radar beam experiences as it passes through an
area of precipitation.
Precipitation Mode
The standard, or default, operational mode of the WSR-88D. The radar
automatically switches into precipitation mode from clear-air mode if
the measured reflectivity exceeds a specific threshold value. The
precipitation mode of NEXRAD is more sensitive than previous weather
radars. The minimum detectable reflectivity in NEXRAD's precipitation
mode is 5 dBZ, compared to 28 dBZ with the old WSR-57.
Precipitation Processing System
The WSR-88D system that generates 1-hour running, 3-hourly, and running
storm total precipitation accumulations. Five functional steps are
performed to calculate the best estimate of precipitation:
1) development of a sectorized hybrid scan,
2) conversion to precipitation rate,
3) precipitation accumulation,
4) adjustment using rain gages,
5) product update.
Precision
The accuracy with which a number can be represented, i.e., the number of
digits used to represent a number.
Predominant Wind
The wind that prevails and generates the local component of the
significant sea conditions across the forecast area. This is the wind
included in all marine forecast products and is defined as a
10-meter wind, except over the nearshore marine zones where it is
defined to be the wind at a 3-meter height.
Preliminary Report
Now known as the "Tropical Cyclone Report". A report summarizing the
life history and effects of an Atlantic or eastern Pacific tropical
cyclone. It contains a summary of the cyclone life cycle and pertinent
meteorological data, including the post-analysis best track (six-hourly
positions and intensities) and other meteorological statistics. It also
contains a description of damage and casualties the system produced, as
well as information on forecasts and warnings associated with the
cyclone. NHC writes a report on every tropical cyclone in its area of
responsibility.
PRES
Pressure
Prescribed Fire
A management ignited or natural wildland fire that burns under
specified conditions where the fire is confined to a predetermined area
and produces the fire behavior and fire characteristics required to
attain planned fire treatment and resource management objectives.
Present Movement
The best estimate of the movement of the center of a tropical cyclone
at a given time and given position. This estimate does not reflect the
short-period, small scale oscillations of the cyclone center.
Present Weather
The type of weather observed at the reporting time. These conditions
may include types and intensity of precipitation such as light rain or
heavy snow, as well as the condition of the air environment such as
foggy, hazy or blowing dust.
Pressure
The exertion of force upon a surface by a fluid (e.g., the atmosphere)
in contact with it.
Pressure Altimeter
An aneroid barometer calibrated to indicate altitude in feet instead of
units of pressure. It is read accurately only in a standard atmosphere
and when the correct altimeter setting is used.
Pressure Altitude
The altitude in standard atmosphere at which a given pressure will be
observed. It is the indicated altitude of a pressure altimeter at an
altitude setting of 29.92 inches of mercury, and is therefore the
indicated altitude above the 29.92 constant pressure surface.
Pressure Change
The net difference between the barometric pressure at the beginning and
ending of a specified interval of time, usually the three hour period
preceding an observation.
Pressure Characteristic
The pattern of the pressure change during the specified period of time,
usually the three hour period preceding an observation. This is recorded
in three categories: falling, rising, or steady.
Pressure Couplet
It is an area where you have a high pressure area located adjacent to a
low pressure area.
Pressure Falling Rapidly
A decrease in station pressure at a rate of 0.06 inch of mercury or more
per hour which totals 0.02 inch or more.
Pressure Gage
A device for registering the pressure of solids, liquids, or gases. It
may be graduated to register pressure in any units desired.
Pressure Gradient
The amount of pressure change occurring over a given distance.
Pressure Gradient Force
A three-dimensional force vector operating in the atmosphere that
accelerates air parcels away from regions of high pressure and toward
regions of low pressure in response to an air pressure gradient. Usually
resolved into vertical and horizontal components.
Pressure Head
Energy contained by fluid because of its pressure, usually expressed in
feet of fluid (foot pounds per pound).
Pressure Ice
Floating sea, river, or lake ice that has been deformed, altered, or
forced upward in pressure ridges by the lateral stresses of any
combination of wind, water currents, tides, waves, and surf.
Pressure Induced Wave
A rare type of wave that does not develop from wind or seismic activity.
Instead, these waves develop as a pressure perturbation moves over the
water surface. The water surface adjusts to account for the atmospheric
pressure change. As atmospheric pressure decreases, the force exerted
upward by the water increases, creating a pressure induced wave.
Pressure Jump
A sudden, sharp increase in atmospheric pressure, typically occurring
along an active front and preceding a storm.
Pressure Rising Rapidly
An increase in station pressure at a rate of 0.06 inch of mercury or
more per hour which totals 0.02 inch or more.
Pressure Tendency
The character and amount of atmospheric pressure change during a
specified period of time, usually 3-hour period preceding an
observation.
Pressure Unsteady
A pressure that fluctuates by 0.03 inch of mercury or more from the
mean pressure during the period of measurement.
Pressure-driven Channeling
Channeling of wind in a valley by synoptic-scale pressure gradients
superimposed along the valley's axis. Compare forced channeling.
Prevailing Visibility
The visibility that is considered representative of conditions at the
station; the greatest distance that can be seen throughout at least
half the horizon circle, not necessarily continuous.
Prevailing Westerlies
The westerly winds that dominant in middle latitudes.
Prevailing Winds
A wind that consistently blows from one direction more than from any
other.
Prevention of Significant Deterioration
A program, specified in the Clean Air Act, whose goal is to prevent air
quality from deteriorating significantly in areas of the country that
are presently meeting the ambient air quality standards.
PRIM
Primary
Primary Ambient Air Quality Standards
Air quality standards designed to protect human health.
Primary Control Tide Station
A tide station where continuous observations have been made for a
minimum of 19 years. Its purpose is to provide data for computing
accepted values essential to tide predictions and for determining
tidal datums for coastal and marine boundaries. The data series from
primary control tide stations serves as a primary control for the
reduction of tidal datum for subordinate tide stations with a shorter
period of record. The 19 year period is the official tidal epoch for
calculating tidal datums.
Primary Pollutant
Substances that are pollutants immediately on entering the atmosphere.
Compare secondary pollutant.
Primary Swell Direction
Prevailing direction of swell propagation.
PRIN
Principal
Probability
A chance, or likelihood, that a certain event might happen.
Probability Forecast
A forecast of the probability that one or more of a mutually exclusive
set of weather conditions will occur.
Probability of Hail
(Abbrev. POH) - a product from the NEXRAD hail detection algorithm that
estimates the likelihood that hail is present in a storm.
Probability of Precipitation
(Abbrev. PoP)- The probability that precipitation will be reported at a
certain location during a specified period of time.
Probability of Thunderstorms
The probability based on climatology that a thunderstorm will be reported
at that location during a specified period of time.
Probability of Tropical Cyclone Conditions
The probability, in percent, that the cyclone center will pass within 50
miles to the right or 75 miles to the left of the listed location within
the indicated time period when looking at the coast in the direction of
the cyclone's movement.
Product Resolution
The smallest spatial increment or data element that is distinguishable in
a given Doppler radar product.
Profiler
An instrument designed to measure horizontal winds directly above its
location, and thus measure the vertical wind profile. Profilers operate
on the same principles as Doppler radar.
PROG
Forecast (prognostication)
PROGGED
Forecasted
Prognostic Discussion
This Hydrometeorological Prediction Center (HPC) discussion may include
analysis of numerical and statistical models, meteorological
circulation patterns and trends, and confidence factors. Reference is
usually made to the manually produced 6 to 10-day Northern Hemisphere
prognoses for mean 500 millibar heights and mean 500 millibar height
anomalies. Discussions may also refer to the method of operational
ensemble predictions.
Progressive Derecho
Derecho characterized by a short curved squall line oriented nearly
perpendicular to the mean wind direction with a bulge in the general
direction of the mean flow. Downburst activity occurs along the bulging
portion of the line. This type of derecho typically occurs in the warm
season (May through August) and is most frequent in a zone extending
from eastern South Dakota to the upper Ohio Valley. The severe wind
storms typically begin during the afternoon and continue into the
evening hours. Several hours typically pass between initial convection
and the first wind damage report.
Prominence
A term identifying cloud-like features in the solar atmosphere. The
features appear as bright structures in the CORONA above the solar LIMB
and as dark FILAMENTs when seen projected against the solar DISK.
PROPA
Propagation (movement)
Propagation
1. The movement of an atmospheric phenomenon. This term is frequently
applied to the motion of thunderstorms into regions favorable for their
continued development (into a maritime tropical airmass).
2. The transmission of electromagnetic energy as waves through or along
a medium.
Property Protection
Measures that are undertaken usually by property owners in order to
prevent, or reduce flood damage. Property protection measures are often
inexpensive for the community because they are implemented by or
cost-shared with property owners. In many cases the buildings'
appearance or use is unaffected, so these measurements are particularly
appropriate for historical sites and landmarks. These measures include
relocation and acquisition, flood proofing, and buying flood insurance.
Proton
Solar activity levels with at least one high energy event (Class X
Flares).
Proton Event
In solar-terrestrial terms, the measurement of at least 10
protons/cm2/sec/steradian at energies greater than 10 MeV.
Proton Flare
In solar-terrestrial terms, any flare producing significant fluxes of
greater-than-10 MeV protons in the vicinity of the earth.
PRST
Persist
PRVD
Provide
PSBL
Possible
PSBLY
possibly
Pseudo-Cold Front
A boundary between a supercell's inflow region and the rear-flank
downdraft (or RFD). It extends outward from the mesocyclone center,
usually toward the south or southwest (but occasionally bows outward
to the east or southeast in the case of an occluded mesocyclone), and
is characterized by advancing of the downdraft air toward the inflow
region. It is a particular form of gust front.
Pseudo-Warm Front
A boundary between a supercell's inflow region and the forward-flank
downdraft (or FFD). It extends outward from at or near the mesocyclone
center, usually toward the east or southeast, and normally is either
nearly stationary or moves northward or northeastward ahead of the
mesocyclone.
PSG
Passage
PST
Pacific Standard Time
Psychrometer
An instrument used to measure the water vapor content of the air; a
hygrometer consisting essentially of two similar thermometers with the
bulb of one being kept wet so that the cooling that results from
evaporation makes it register a lower temperature than the dry one and
with the difference between the readings constituting a measure of the
dryness of the atmosphere.
PTCHY
Patchy
PTCLDY
Partly Cloudy
PTDY
On a buoy report, Pressure Tendency is the sign (plus or minus) and the
amount of pressure change (hPa) for a three hour period ending at the
time of observation.
PTLY
Partly
PTN
Portion
PTTN
pattern
PTWC
(Pacific Tsunami Warning Center) - The Pacific Tsunami Warning Center in
Ewa Beach (pronounced Eva Beach), HI has an international warning
responsibility for the entire Pacific and a regional warning
responsibility for the State of Hawaii. See also WC/ATWC.
PTYPE
precipitation type
Public Information Statement
A narrative statement issued by a National Weather Service Forecast
Office that can be used for:
1) A current or expected nonhazardous event of general interest to the
public that can usually be covered with a single message (e.g.,
unusual atmospheric phenomena such as sun dogs, halos, rainbows, aurora
borealis, lenticular clouds, and stories about a long-term
dry/cold/wet/warm spell).
2) Public educational information and activities, such as storm safety
rules, awareness activities, storm drills, etc.
3) Information regarding service changes, service limitations,
interruptions due to reduced or lost power or equipment outages, or
special information clarifying interpretation of NWS data. For example,
this product may be used to inform users of radar equipment outages or
special information clarifying interpretation of radar data originating
from an unusual source which may be mistaken for precipitation (such as
chaff drops, smoke plumes, etc., that produces echoes on the radar
display.
Public Severe Weather Outlook
These are issued when the Storm Prediction Center (SPC) in Norman,
Oklahoma anticipates an especially significant and/or widespread outbreak
of severe weather. This outlook will stress the seriousness of the
situation, defines the threat area, and provides information on the
timing of the outbreak. The lead time on this outlook is normally less
than 36 hours prior to the severe weather event.
Puddle
In hydrologic terms,
(1) The act of compacting earth, soil clay, etc., by mixing them with
water and rolling or tamping the mixture.
(2) A compact mass of earth, soil, clay, or a mixture of material,
which has been compacted through the addition of water, rolling and
tamping. This makes the material less permeable.
(3) A small pool of water, usually a few inches in depth and from several
inches to several feet in it greatest dimension.
Puget Sound Convergence Zone
A situation where wind forced around the Olympic Mountains converges over
the Puget Sound. Causes extreme variability in weather conditions around
Seattle, Washington with some areas of sunshine and others in clouds and
rain.
Pulse
A short burst of electromagnetic energy that a radar sends out in a
straight line to detect a precipitation target. The straight line that
this pulse travels along is called a radar beam.
Pulse Duration
The time over which a radar pulse lasts. The pulse duration can be
multiplied by the speed of light to determine the pulse length or pulse
width.
Pulse Length
The linear distance in range occupied by an individual pulse from a
radar. h = c * t , where t is the duration of the transmitted pulse,
c is the speed of light, h is the length of the pulse in space.
Note, in the radar equation, the length h/2 is actually used for
calculating pulse volume because we are only interested in signals
that arrive back at the radar simultaneously. This is also called a
pulse width.
Pulse Radar
A type of radar, designed to facilitate range (distance) measurements,
in which are transmitted energy emitted in periodic, brief transmission.
Pulse Repetition Frequency (PRF)
The amount of time between successive pulses, or bursts, of
electromagnetic energy that is transmitted by a radar. The PRF
determines the maximum range at which echoes can be detected and also
the maximum radial velocity that can be detected by a Doppler radar.
Pulse Repetition Time (PRT)
The time elapsed between pulses by the radar. This is also called the
pulse interval.
Pulse Resolution Volume
A discrete radar sampling volume, of dimensions (horizontal beamwidth *
vertical beamwidth * 1 range gate).
Pulse Severe Thunderstorms
Single cell thunderstorms which produce brief periods of severe weather
(3/4 inch hail, wind gusts in the excess of 58 miles an hour, or a
tornado).
Pulse Storm
A thunderstorm within which a brief period (pulse) of strong updraft
occurs, during and immediately after which the storm produces a short
episode of severe weather. These storms generally are not tornado
producers, but often produce large hail and/or damaging winds. See also
overshooting top.
Pulse Width
Same as Pulse Length; the linear distance in range occupied by an
individual pulse from a radar. h = c * t , where t is the duration of
the transmitted pulse, c is the speed of light, h is the length of the
pulse in space. Note, in the radar equation, the length h/2 is actually
used for calculating pulse volume because we are only interested in
signals that arrive back at the radar simultaneously.
Pulse-Pair Processing
Nickname for the technique of mean velocity estimation by calculation
of the signal complex covariance argument. The calculation requires two
consecutive pulses, hence "pulse-pair".
PVA
Positive Vorticity Advection - the advection of higher values of
vorticity into an area.
PVL
Prevail
PW
Precipitable Water - measure of the depth of liquid water at the surface
that would result after precipitating all of the water vapor in a
vertical column over a given location, usually extending from the
surface to 300 mb.
PWAT
Precipitable Water - measure of the depth of liquid water at the surface
that would result after precipitating all of the water vapor in a
vertical column over a given location, usually extending from the
surface to 300 mb.
PWO
Public Severe Weather Outlook
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