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Mauna Kea Weather Center Meteorology Glossary/Dictionary

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  • IAS - Abbreviation for indicated airspeed.

  • IBL - Abbreviation for internal boundary layer.

  • ice - The solid, crystalline form of water substance; it is found in the atmosphere as snow crystals, hail, ice pellets, etc., and on the earth's surface in forms such as hoarfrost, rime, glaze, sea ice, glacier ice, ground ice, frazil, anchor ice, etc. This form of water is, strictly speaking, called ice I, the only one of the several known modifications of solid water substance that is stable at commonly occurring temperatures and pressures. (Some of the other forms have very unusual properties, ice VII, for example, being stable only at pressures above 22 400 kg cm-2, but then existing at temperatures up to about 100°C.) Ice has an open structure because the water molecules bond to their neighbors covalently only in four directions; it therefore floats on higher density water, where broken molecular bonds permit closer packing. All commonly occurring forms of ice are crystalline, although large single crystals are relatively rare except in glaciers. The ice crystal lattice possesses hexagonal symmetry that manifests itself in the gross forms of such single crystals as are sometimes found in snow. At an air pressure of one atmosphere, ice melts at 0°C by definition of the Celsius temperature scale. (Strictly at equilibrium among water, ice, and vapor occurs at +0.01°C, the triple point.) On the other hand, ice does not invariably form in liquid water cooled below this temperature; it has a tendency to supercool, more so in the absence of ice nuclei. See Bernal-Fowler rules, ice crystal.

  • IF - Abbreviation for intermediate frequency.

  • IFF - (Abbreviation for identification: friend or foe.) A system of transponders carried by an aircraft that makes it possible to locate and identify the aircraft by radar. Although the system was originally developed for military uses, it has also been used in nonmilitary applications for tracking aircraft on radar displays. When a radar signal is detected by the transponder on the aircraft, the transponder sends back a coded message that is detected by the radar. The radar then plots on its display (usually a PPI) an indication of both the location and the identity (coded) of the aircraft.

  • ig - Same as hig.

  • ILS - Abbreviation for instrument landing system.

  • image - A two-dimensional array of data with values at each element of the array related to an intensity or a color. An image is typically defined as the result of some type of image collection system; however, it could be the representation in two dimensions of any data by intensity or color.

  • independence - See statistical independence, independent variable.

  • index - 1. The indicating part of an instrument; for example, the hand of a watch or the meniscus of a mercury column. 2. See circulation index. 3. See zonal index.

  • initial condition - The state of a time-dependent dynamical system, for instance, an NWP model, at a given time used to start a forecast of the future state of the system. See analysis.

  • initialization - Any method that modifies observed atmospheric initial conditions so that high-frequency oscillations are removed from a subsequent forecast with an NWP model. See dynamic initialization, normal mode initialization.

  • INS - (Abbreviation for inertial navigation system.) A type of dead-reckoning navigational system, used on aircraft and other vehicles, which is based on the measurement of accelerations. Accelerations are measured by devices such as gyroscopes, stabilized with respect to inertial space. Navigational information such as vehicle velocity, orientation, and position is determined from these measurements by computers or other instrumentation.

  • instability - A property of the steady state of a system such that certain disturbances or perturbations introduced into the steady state will increase in magnitude, the maximum perturbation amplitude always remaining larger than the initial amplitude. The method of small perturbations, assuming permanent waves, is the usual method of testing for instability; unstable perturbations then usually increase exponentially with time. An unstable nonlinear system may or may not approach another steady state; the method of small perturbations is incapable of making this prediction. The small perturbation may be a wave or a parcel displacement. The parcel method assumes that the environment is unaffected by the displacement of the parcel. The slice method has occasionally been used as a modification of the parcel method to gain a little information about the interaction of parcel and environment. Stability as defined above is an asymptotic concept; other definitions are possible. Precision is required of the user, and caution of the reader. The concept of instability is employed in many sciences. In meteorology the reference is usually to one of the following. 1)Static instability (or hydrostatic instability) of vertical displacements of a parcel in a fluid in hydrostatic equilibrium. (See conditional instability, absolute instability, convective instability, buoyant instability.) 2)Hydrodynamic instability (or dynamic instability) of parcel displacements or, more usually, of waves in a moving fluid system governed by the fundamental equations of hydrodynamics, to which the quasi-hydrostatic approximation may or may not apply. (See Helmholtz instability, inertial instability, shearing instability, baroclinic instability, barotropic instability, rotational instability.) The space scale of unstable waves is important in meteorology: Thus Helmholtz, baroclinic, and barotropic instability give, in general, unstable waves of increasing wavelength. The timescale is also important: A perturbation that grows for two days before dying out is effectively unstable for many meteorological purposes, but this is an initial-value problem and one cannot assume the existence of permanent waves. These meteorological types of hydrodynamic instability must not be confused with the phenomenon often referred to by mathematicians and physicists by the same term. A great deal of study has been devoted to the problem of the onset of turbulence in simple flows under laboratory conditions, and here viscosity is a source of instability. See computational instability.

  • insulation - A nonconducting material designed to reduce the transfer of energy or electric current between two materials.

  • intensity - 1. In general, expresses the rate of transfer per unit area of some condition or physical quantity, such as rainfall, electromagnetic energy, sound, etc. 2. (Or radiant intensity.) Radiant power per unit solid angle; in SI units, W sr-1. 3. In synoptic meteorology, the general strength of flow around an individual cyclone or anticyclone (most often applied to the former). This concept is commonly used in terms of a process, "intensification," or descriptively, as an "intense low."

  • inversion - In meteorology, a departure from the usual decrease or increase with altitude of the value of an atmospheric property; also, the layer through which this departure occurs (the "inversion layer"), or the lowest altitude at which the departure is found (the "base of the inversion").

  • ion - 1. In chemistry, atoms or specific groupings of atoms that have gained or lost one or more electrons, as the "chloride ion" or "ammonium ion." Ions are most familiar in aqueous solutions and in crystal structures, but they also exist in the gas phase at all altitudes in the atmosphere. They are most abundant, and have the greatest importance, in the ionosphere, between about 70 and 300 km in altitude. 2. In atmospheric electricity, any of several types of electrically charged submicroscopic particles normally found in the atmosphere. Atmospheric ions are of two principal types, small ions and large ions, although a class of intermediate ions has occasionally been reported. The ionization process that forms small ions depends upon two distinct agencies, cosmic rays and radioactive emanations. Each of these consists of very energetic particles that ionize neutral air molecules by knocking out one or more planetary electrons. The resulting free electron and positively charged molecule (or atom) very quickly attach themselves to one or, at most, a small number of neutral air molecules, thereby forming new small ions. In the presence of Aitken nuclei, some of the small ions will in turn attach themselves to these nuclei, thereby creating new large ions. The two main classes of ions differ widely in mobility. Only the highly mobile small ions contribute significantly to the electrical conductivity of the air under most conditions. The intermediate ions and large ions are important in certain space charge effects, but are too sluggish to contribute much to conductivity. The processes of formation of ions are offset by certain processes of destruction of the ions. See recombination, ion mobility.

  • IPS - Abbreviation for International Pyrheliometric Scale.

  • IR - Abbreviation for infrared, or infrared radiation.

  • ITCZ - Abbreviation for intertropical convergence zone.

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