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Pacific high

A semipermanent anticyclone located in the Eastern North Pacific.

Paleosol

Soil horizon from the geologic past.

Pareto Optimum

Situation in which it is impossible to make any individual better off without making someone else worse off, where better off means more preferred and worse off means less preferred. Every competitive market equilibrium is a Pareto optimum and every Pareto optimum is a competitive equilibrium if a set of assumptions (e.g. perfect information, absence of externalities, etc.) holds true.

Partial-duration flood series

A list of all flood peaks that exceed a chosen base stage or discharge, regardless of the number of peaks occurring in a year. (Also called basic-stage flood series, or floods above a base.)

Particulate matter (PM)

Solid particles or liquid droplets suspended or carried in the air (e.g., soot, dust, fumes, mist). See aerosol, sulfate aerosols.

Particulates

The Environmental Protection Agency has set National Ambient Air Quality Standards for particulates. One, in effect since 1987, regulates particles smaller than 10 microns in diameter (PM10); the other, promulgated in 1997, would regulate particles smaller than 2.5 microns in diameter (PM2.5) — but court challenges are delaying its implementation. These are of interest to agriculture because dust from tillage and smoke from burning field residues may contribute to pollutant levels. Whether controls might be imposed on agricultural activities depends largely on how each state chooses to meet the standards, however.

Partly Cloudy

Sky condition when between 3/10 and 7/10 of the sky is covered.

Parts per billion (ppb)

Number of parts of a chemical found in one billion parts of a particular gas, liquid, or solid mixture. See concentration.

Parts per million (ppm)

Number of parts of a chemical found in one million parts of a particular gas, liquid, or solid. See concentration.

Pastureland

Land used primarily for the production of domesticated forage plants for livestock (in contrast to rangeland, where vegetation is naturally-occurring and is dominated by grasses and perhaps shrubs). Rotation pasture or cropland under winter cover crops is not included in this definition. The 1992 national resources inventory recorded 126 million acres of pastureland, 9% of all nonfederal rural lands.

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