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Mauna Kea Weather Center Meteorology Glossary/Dictionary

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  • PAGES - (Acronym for Past Global Changes.) A core project of the International Geosphere-Biosphere Program of the International Council of Scientific Unions that is focused on two temporal streams¾[—]the last 2000 years studied at interannual to interdecadal resolution, and the longer timescale of the cycles of glaciation/deglaciation.

  • PAN - Common abbreviation for peroxyacetyl nitrate.

  • PAR - Abbreviation for photosynthetically active radiation.

  • parameter - 1. In general, any quantity of a problem that is not an independent variable. More specifically, the term is often used to distinguish, from dependent variables, quantities that may be more or less arbitrarily assigned values for purposes of the problem at hand. 2. In statistical terminology, any numerical constant derived from a population or a probability distribution. Specifically, it is an arbitrary constant in the mathematical expression of a probability distribution. For example, in the distribution given by f(x) = a[&agr;]e-a[&agr;]x, the constant a[&agr;] is a parameter.

  • parameterization - The representation, in a dynamic model, of physical effects in terms of admittedly oversimplified parameters, rather than realistically requiring such effects to be consequences of the dynamics of the system. See Mellor-Yamada parameterization, subgrid-scale process, convective adjustment.

  • parcel - An imaginary volume of fluid to which may be assigned various thermodynamic and kinematic quantities. The size of a parcel is arbitrary but is generally much smaller than the characteristic scale of variability of its environment.

  • particle - An aggregation of sufficiently many atoms or molecules that it can be assigned macroscopic properties such as volume, density, pressure, and temperature. But sometimes by particle, without qualification, is meant a subatomic particle such as the proton or neutron (which themselves are composed of other "elementary particles") or the electron. See also particles.

  • particles - Components of the atmosphere composed of solid or liquid matter. Particles may be both released from the earth's surface, such as dust or smoke, or formed in the atmosphere, as in rain or ice particles or sulfate aerosol. The particles in the atmosphere are usually defined in terms of their size, or diameter. Particles less than 100 m[&mgr;]m in diameter are referred to as aerosol particles. These are aerodynamically stable and settle out only slowly (strictly speaking, the term aerosol refers to the gas-particle colloidal system, not just the particulate phase). The aerosol is usually divided into three modes: the Aitken mode (diameter less than 0.5 m[&mgr;]m), the accumulation mode (0.5-2.0 m[&mgr;]m), and the coarse mode (greater than 2 m[&mgr;]m). The Aitken and accumulation modes are collectively referred to as fine particles. The larger particles of the coarse mode compose clouds and hydrometeors such as rain and sleet, which precipitate out. See also particle.

  • path - Same as trajectory.

  • PE - In radar, abbreviation for permanent echo.

  • period - Any function of time f(t) is periodic with period t if f(t) = f(t + t[&tgr;]) for all times t, where t[&tgr;] is the smallest number for which this equality holds. Without qualification, period often means temporal period, but could mean spatial period (wavelength), the repeat distance of a spatially periodic function. See frequency.

  • persistence - 1. The previous value in a time series. Thus, if x(t) denotes the present value, the value of persistence would be x(t - 1), whence the latter value is regarded as "persisting." It is used as an objective standard in the verification of weather forecasts. Sometimes it is extended to mean x(t - h) where h is arbitrary. 2. (Also called constancy, steadiness.) With respect to the long-term nature of the wind at a given location, the ratio of the magnitude of the main wind vector to the average speed of the wind without regard to direction. 3. (Also called inertial forecast.) In general, the tendency for the occurrence of a specific event to be more probable, at a given time, if that same event has occurred in the immediately preceding time period.

  • persistence forecast - In meteorology, a forecast that the future weather condition will be the same as the present condition. The persistence forecast is often used as a standard of comparison in measuring the degree of skill of forecasts prepared by other methods, especially for very short projections. See persistence; compare random forecast, probability forecast.

  • pH - In aqueous solution, the logarithm (base 10) of the inverse of the concentration of hydrogen ions expressed in moles per liter; a measure of solution acidity. Thus, pH = -log[H+]. The pH of neutral water is 7, since it contains 10-7 moles hydrogen ions and 10-7 moles hydroxyl ions per liter. Lower pH values correspond to more acidic solution. The pH of natural waters in the atmosphere is 5.5 or less, due to the dissolution of acidic gases such as carbon dioxide and sulfur dioxide. In polluted continental areas the pH can drop to as low as 3.0, a condition known as acid rain, due to the production of sulfuric and nitric acids from anthropogenic activity.

  • phase - 1. For any type of periodic motion (e.g., rotation, oscillation) a point or stage in the period to which the motion has advanced with respect to a given initial point. Specifically, the phase or phase angle is the angular measure along a simple harmonic wave, the linear distance of one wavelength being 360° of phase measure. This is often generalized by equating one cycle of any oscillation to 360°. See delay, interference, surface of constant phase. 2. The state of aggregation of a substance, for example, solid, liquid, or gas.

  • point - 1. Position or time of occurrence, as in boiling point, freezing point, compass point, point rainfall, etc. 2. In Australia, a unit of precipitation amount; equal to one one-hundredth of an inch.

  • PORTS - Abbreviation for Physical Oceanographic Real-Time System.

  • potential - 1. A function of space, the gradient of which is equal to a force. In symbols, F = -f[&phgr;], where F is the force, the del operator, and f[&phgr;] the potential. A force that may be so expressed is said to be "conservative," and the work done against it in motion from one given equipotential surface to another is independent of the path of the motion. In meteorology, the force of gravity has a potential, the geopotential, which, if the acceleration of gravity g is taken as constant, may be written f[&phgr;] = gZ, where Z is the height coordinate. The pressure force has in general no potential, nor do the Coriolis or viscous forces. By extension and analogy, the velocity potential, acceleration potential, and Gibbs function (thermodynamic potential) are defined. 2. Applied to the value that an atmospheric thermodynamic variable would attain if processed adiabatically from its initial pressure to a standard pressure, typically 100 kPa. See potential density, potential temperature.

  • potential energy - The energy a system has by virtue of its position; the negative of the work done in taking a system from a reference configuration, where the potential energy is assigned the value zero, to a given configuration, with no change in kinetic energy of the system. An example of potential energy is the gravitational potential energy of a point mass m at a distance r from the center of a spherically symmetric body with mass M (e.g., a planet): where G is the universal gravitational constant and the reference potential energy is taken as zero at infinity. At distances z above the surface of the body that are small compared with its radius, the potential energy is approximately mgz, where g is the acceleration due to gravity at the surface and the zero of potential energy is taken at the surface (z = 0). Molecular potential energies, arising from short-range forces much stronger than gravitation, are involved in all chemical reactions, are responsible for the cohesiveness of liquids and solids, and influence a host of processes such as evaporation and condensation.

  • power - The rate, often expressed in watts, at which energy is exchanged or transmitted. In radar it usually refers to the rate at which electromagnetic energy is radiated from or received at the antenna.

  • PPI - Abbreviation for plan position indicator.

  • precipitable water - (Or precipitable water vapor.) The total atmospheric water vapor contained in a vertical column of unit cross-sectional area extending between any two specified levels, commonly expressed in terms of the height to which that water substance would stand if completely condensed and collected in a vessel of the same unit cross section. The total precipitable water is that contained in a column of unit cross section extending all of the way from the earth's surface to the "top" of the atmosphere. Mathematically, if x(p) is the mixing ratio at the pressure level, p, then the precipitable water vapor, W, contained in a layer bounded by pressures p1 and p2 is given by where g is the acceleration of gravity. In actual rainstorms, particularly thunderstorms, amounts of rain very often exceed the total precipitable water vapor of the overlying atmosphere. This results from the action of convergence that brings into the rainstorm the water vapor from a surrounding area that is often quite large. Nevertheless, there is general correlation between precipitation amounts in given storms and the precipitable water vapor of the air masses involved in those storms.

  • precipitation - 1. All liquid or solid phase aqueous particles that originate in the atmosphere and fall to the earth's surface. 2. The amount, usually expressed in millimeters or inches of liquid water depth, of the water substance that has fallen at a given point over a specified period of time. As this is usually measured in a fixed rain gauge, small amounts of dew, frost, rime, etc., may be included in the total. The more common term rainfall is also used in this total sense to include not only amounts of rain, but also the water equivalents of frozen precipitation. For obvious reasons, precipitation is the preferred general term.

  • prediction - See numerical weather prediction, climate prediction; compare weather forecast.

  • pressure - 1. A type of stress characterized by uniformity in all directions. As a measurable on a surface, the net force per unit area normal to that surface exerted by molecules rebounding from it. In dynamics, it is that part of the stress tensor that is independent of viscosity and depends only upon the molecular motion appropriate to the local temperature and density. It is the negative of the mean of the three normal stresses. The concept of pressure as employed in thermodynamics is based upon an equilibrium system, where tangential forces vanish and normal forces are equal. 2. In meteorology, commonly used for atmospheric pressure. 3. In mechanics, same as stress. 4. See radiation pressure.

  • pressure gradient - (In meteorology, also called barometric gradient.) The rate of decrease (gradient) of pressure in space at a fixed time. The term is sometimes loosely used to denote simply the magnitude of the gradient of the pressure field.

  • pressure ridge - 1. See ridge. 2. A ridge of ice, up to 35 m (100 ft) high and sometimes several kilometers long, in pressure ice.

  • pressure system - An individual cyclonic-scale feature of atmospheric circulation, commonly used to denote either a high or a low, less frequently a ridge or a trough.

  • probability - The chance that a prescribed event will occur, represented as a pure number p in the range 0 £[≤] p £[≤] 1. The probability of an impossible event is zero and that of an inevitable event is unity. Probability is estimated empirically by relative frequency, that is, the number of times the particular event occurs divided by the total count of all events in the class considered. See probability theory.

  • profile - In meteorology, a graph of the value of a scalar quantity versus a horizontal, vertical, or timescale. It usually refers to a vertical representation. Compare contour, cross section, time section.

  • prog - Common contraction for prognostic chart.

  • projection - The correspondence between a domain of the earth's surface and a plane surface (map) such that each point on one corresponds to one and only one point on the other. Typical projections used on weather charts include stereographic, Lambert conic, and Mercator.

  • pulse - In radar, sodar, or lidar a single short-duration transmission (or burst) of energy. A pulse is characterized by its radio frequency, pulse repetition frequency, pulse duration, and peak power.

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