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PACIFIC FRONTS
A variety of cold fronts and storm systems move onto the west coast during the winter months. Coming on shore with plenty of moisture, the mountains of California, Oregon and Washington squeeze out much of that water. Fortunately for our ski areas, many systems retain enough moisture to dump heavy snow on the mountains of Colorado. By the time these storms get past our mountains, they are relatively dry. As the saying goes..."Pacific front, mountains bear the brunt. Southeast low, Denver gets snow."

PALMER DIVIDE
A ridge of land that extends from the Front Range of the Rockies in central Colorado, eastward toward the city of Limon on the eastern plains. It sits right in between the cities of Denver and Colorado Springs. The elevation of the Palmer Divide runs roughly between 6,000 and 7500 feet, peaking at Monument Hill. This terrain feature is the cause of several small scale, or microscale weather patterns, and can make a world of difference in the weather in Denver versus Colorado Springs.

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.

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.

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 pressure of 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.

PDS WATCH
A tornado watch with enhanced wording (Particularly Dangerous Situation).

PEAK GUST
The highest instantaneous wind speed observed or recorded.

PENDANT ECHO
Radar signature generally similar to a hook echo, except that the hook shape is not as well defined.

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°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.

POGONIP
A type of fog consisting of ice crystals suspended in the air. The name "pogonip" is a Shoshone word meaning "white death". Pogonip only forms under the right conditions, the humidity has to be near 100% as the air temperature drops to below 0° C, allowing ice crystals to form in the air. The ice crystals will then settle onto surfaces. The word pogonip was first coined by Native Americans to describe the very rarely occurring frozen fog composed of fine ice needles instead of water droplets that forms in the mountain valleys of the western United States during winter. Breathing pogonip can damage your lungs thus the term "white death".

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° and 90° 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.

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.

POPCORN CONVECTION
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.

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 CG
A CG flash that delivers positive charge to the ground, as opposed to the more ommon 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-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.

POSITIVE VORTICITY ADVECTION
The advection of higher values of vorticity into an area. It is also known as cyclonic vorticity.

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.

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.

PRE-FRONTAL TROUGH
An elongated area of relatively low pressure preceding a cold front that is usually associated with a shift in wind direction.

PRESSURE
The force per unit area exerted by the weight of the atmosphere above a point on or above the earth's surface.

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 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 pressure or 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.

PSEUDO- COLD FRONT
A boundary between a supercell's inflow region and the rear-flank downdraft (or RFD). It extends outward from the meso-cyclone center, usually toward the south or southwest (but occasionally bows outward to the east or southeast in the case of an occluded meso-cyclone), and is characterized by advancing of the downdraft air toward the inflow region. It is a particular form of gust front. See also pseudo-warm 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 meso-cyclone center, usually toward the east or southeast, and normally is either nearly stationary or moves northward or northeastward ahead of the meso-cyclone. See pseudo-cold front and beaver tail.

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.

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 overshooting top, cyclic storm.

PVA
Positive Vorticity Advection. 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.

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