Aeronautical Terms beginning with I

Indicated altitude

The altitude above mean sea level indicated on a pressure altimeter set at current local altimeter setting.

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 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.

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.

Isobar

A line of equal or constant barometric pressure.

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.

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.