Science / Weather Glossary

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), pr . . . View Full Definition

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 th . . . View Full Definition

Polar-Orbiting Satellite: A satellite whose orbit passes over both of the earth's between poles.

Poles or 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 bet . . . View Full Definition

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 e . . . View Full Definition

Positive Vorticity Advection: The advection of higher values of vorticity into an area. It is also known as cyclonic vorticity.

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.

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.

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, sno . . . View Full Definition

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 . . . View Full Definition

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.

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