Aeronautical Terms beginning with I
Ice crystals
A type of precipitation composed of unbranched crystals in the form of needles, columns, or plates; usually having a very slight downward motion, may fall from a cloudless sky.
Ice Crystals
Ice crystals, which are often in high concentrations near convective weather systems and lower concentrations in stratus or cirrus clouds, can accrete within turbine engines and cause power loss when in high concentrations. Ice crystals are not typically detected by either conventional ice detectors or airborne radar, and typically do not accrete on external airframe surfaces.
Ice fog
A type of fog composed of minute suspended particles of ice; occurs at very low temperatures and may cause halo phenomena.
Ice needles
A form of ice crystals.
Ice pellets
Small, transparent or translucent, round or irregularly shaped pellets of ice. They may be (1) hard grains that rebound on striking a hard surface or (2) pellets of snow encased in ice.
Icing
In general, any deposit of ice forming on an object. Types include clear icing, rime icing, and glaze.
Icing Envelopes
Icing envelopes used for the certification of aircraft for flight in icing conditions specify atmospheric icing conditions in terms of altitude, temperature, Liquid Water Content (LWC), and drop size represented by the Median Volume Diameter (MVD). The envelopes use the term mean effective diameter, but this equates to the MVD for the instrumentation and assumptions current at the time the envelopes were established.) There are two classes of icing envelopes: continuous maximum and intermittent maximum. The continuous maximum is for stratus-type clouds, and the intermittent maximum is for cumulus-type clouds.
Impingement
The striking and adherence of a water droplet on an aircraft surface. The impingement rate is the rate at which droplets of a given size collect on a particular surface. In general, impingement rates are higher for larger drops and smaller components, such as a very high frequency (VHF) or a Global Positioning System (GPS) antenna.
Indefinite ceiling
A ceiling classification denoting vertical visibility into a surface based obscuration.
Insolation
Incoming solar radiation falling upon the earth and its atmosphere.
Instability
A general term to indicate various states of the atmosphere in which spontaneous convection will occur when prescribed criteria are met; indicative of turbulence. Types include: absolute instability, conditionally unstable air, convective instability.
Intertropical convergence zone
The boundary zone between the trade wind system of the Northern and Southern Hemispheres; it is characterized in maritime climates by showery precipitation with cumulonimbus clouds sometimes extending to great heights.
Inversion
An increase in temperature with height-a reversal of the normal decrease with height in the troposphere; may also be applied to other meteorological properties.
Iso echo
In radar circuitry, a circuit that reverses signal strength above a specified intensity level, thus causing a void on the scope in the most intense portion of an echo when maximum intensity is greater than the specified level.
Isobar
A line of equal or constant barometric pressure.
Isobars
Lines which connect points of equal barometric pressure.
Isoheight
On a weather chart, a line of equal height; same as contour (1).
Isoline
A line of equal value of a variable quantity, i.e., an isoline of temperature is an isotherm, etc. Isobars (A line of equal or constant barometric pressure.) and isotachs (A line of equal or constant wind speed.) are examples.
Isoshear
A line of equal wind shear.
Isotach
A line of equal or constant wind speed.
Isotherm
A line of equal or constant temperature.
Isothermal
Of equal or constant temperature, with respect to either space or time; more commonly, temperature with height; a zero lapse rate.
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