PALMER DROUGHT INDEX
A long-term meteorological
drought severity index produced by the NOAA/USDA (Department of Agriculture)
Joint Agricultural Weather Facility. The index depicts prolonged times,
as in months or years, of abnormal dryness or wetness. It responds slowly,
changing little from week to week, and reflects long-term moisture
runoff, recharge, and deep percolation, as well as evapotranspiration.
PALOUSER
A strong, dangerous, katabatic wind that descends
from the mountains into the Palouse River valley in northern Idaho and
eastern Washington. May be called a Cow-Killer.
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 OBSCURATION
Denotes that 1/8th or more of the sky, but not all of the sky,
is hidden by any surface-based phenomena in the atmosphere,
excluding precipitation. It often reduces horizontal visibility
but not the vertical. It is reported as "X" in an observation and on
the METAR. Related term: obscuration
PARTLY CLOUDY
The state
of the weather when the clouds are conspicuously present, but do not
completely dull the sky or the day at any moment. The National
Weather Service does not have an amount of sky cover for this condition.
Related terms: clear, few, scattered, broken, and overcast
PASCAL
The unit of pressure
produced when one newton acts on about one square meter.
PASCAL'S LAW
When an external pressure is applied
to any confined fluid at rest, the pressure is increased at every point
in the fluid by the amount of external pressure applied. It means that
the pressureof the atmosphere is exerted not only downward on the surface of an object, but also in
all directions against a surface which is exposed to the atmosphere.
Formulated by Blaise Pascal (1623-1662), a French mathematician, theologian,
and physicist.
PEAK GUST
The highest instantaneous wind speed observed or recorded.
PERIGEE
The point nearest the earth on the moon's orbit. This term can be applied to any other
body orbiting the earth, such as satellites. It is the opposite of apogee.
PERIHELION
The point of the earth's orbit that is nearest to the sun. Although the position
is part of a 21,000 year cycle, currently it occurs around January,
when the earth is about 3 million miles closer to the sun than at aphelion.
This term can be applied to any other celestial body in orbit around
the sun. It is the opposite of aphelion.
PHOTOMETER
Any of a number of atmospheric phenomena
which appear as luminous patterns in the sky. They do not directly cause
adverse weather. They include halos,
coronas. rainbows, and fogbows.
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 F, it is
the portion of the sun's atmosphere which emits continuous electromagnetic
radiation.
PILOT BALLOON
A small balloon whose ascent is used
to determine the direction and speed of low level atmospheric winds.
Also known as a pibal.
PILOT REPORT
A report of in-flight weather by an
aircraft pilot or crew member. Often referred to as a PIREP.
PLAN POSITION INDICATOR
Also known as a PPI Scope,
it is a radar indicator scope displaying range and azimuth of targets in polar coordinates.
PLOW/PLOUGH WIND
The spreading downdraft and strong
straight-line winds preceding a thunderstorm.
So named in the American Midwest because of its ability to flatten tall
grasses as it passes. Related term: first
gust
POLES/POLAR
The poles are the geographic point at 90 degrees latitude
North and South on the earth's surface. They are equal distance from
the equator. The polar region is considered to be that area between 60 degrees and 90 degrees latitude, both North
and South.
POLAR AIR MASS
An air mass that forms over a high latitude region. Continental polar air
(cP) is formed over cold surface regions and is typically very stable
with low moisture. Maritime polar air (mP), produced over warmer waters,
is less stable with high moisture.
POLAR FRONT
A semi-continuous, semi-permanent boundary between polar air
masses and tropical air masses. An integral part of an early meteorological
theory known as the Polar Front Theory.
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-ORBITING SATELLITE
A satellite
whose orbit passes over both of the earth's between poles. Related term:
geostationary satellite: View WALTER Satellite Tutorial
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.
POSITIVE VORTICITY ADVECTION
The advection
of higher values of vorticity into an area. It is also known as cyclonic
vorticity. related term: negative vorticity advection
POUNDS PER SQUARE INCH (PSI)
A unit for measuring
pressure. One PSI equals the pressure resulting from a force of one pound force acting over an area of one
square inch.
PRECIPITATION
Any and all forms of water, liquid or solid, that falls from clouds and reaches
the ground. This includes drizzle, freezing drizzle, freezing rain,
hail, ice crystals, ice pellets, rain, snow, snow pellets, and snow
grains. The amount of fall is usually expressed in inches of liquid
water depth of the substance that has fallen at a given point over a
specified time period.
PRE-FRONTAL SQUALL LINE
A line of thunderstorms
that precedes an advancing cold front. Related term:
squall line
PRE-FRONTAL TROUGH
An elongated area of relatively
low pressure preceding a cold front that is usually associated with a shift in wind direction. Related term:
trough
PRESSURE
The force per unit area exerted by the weight of the atmosphere
above a point on or above the earth's surface. Related terms: atmospheric
pressure and barometric pressure View Current Pressure at MNSU
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 altimetersetting 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 GRADIENT
The amount of pressure change that occurs over a fixed distance at
a fixed altitude.
PRESSURE JUMP
A sudden increase in the observed atmospheric
pressureor station pressure.
PRESSURE TENDENCY
The pressure characteristic and
amount of pressure change during a specified time period, usually the three hour period preceding
the observation.
PREVAILING WIND
A wind that blows from one direction
more frequently than any other during a given period, such as a day,
month, season, or year.
PREVAILING VISIBILITY
It is considered representative
of visibility conditions at the observation station. It is the greatest distance that can be seen
throughout at least half the horizon circle, but not necessarily continuous.
PROFILER
A type of Doppler radar that typically measures both wind
speed and direction from the surface to 55,000 feet in the atmosphere.
PROGNOSTIC CHART
A chart of forecast predictions
that may include pressure, fronts. precipitation, temperature,
and other meteorological elements. Also known as a prog.
PSYCHROMETER
An instrument used to measure water vapor content of the atmosphere. It consists of
two thermometers, a wet bulb and dry bulb. May also be referred
to as a sling psychrometer.
PULSE
A very short duration of time. In regard to a radar, it is a brief burst of a electromagnetic
radiation emitted by the radar.
