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IC

(abbr) Ice crystals (also known as diamond dust); used in METAR/aviation reports.

ICE

When used in aviation weather reports/forecasts, implies aircraft superstructure icing.

Ice day

A period of 24hr (conventionally beginning 0900UTC), during which the air temperature is less than 0 degC.

Ice fog

Visibility reduced to less than 1000 m by suspension of minute collection of ice crystals in high concentration. The crystals will glitter and may give rise to optical phenomena. (NB: this is NOT the same as freezing fog, which is composed of water droplets - see definition elsewhere.)

Initialisation

The process whereby a model 'analysis' is produced by utilising model fields from an earlier run, and integrating synoptic, and asynoptic observations to produce the 'initial state' at t=0. The model analysis may not be the same (in detail), as a hand-drawn analysis, and intervention (q.v.) is sometimes needed as a result to preserve some small scale features which can influence the forecast run.

Insolation

Radiant energy received from the sun on any particular surface. Often used when discussing receipt of infra-red radiation on the surface of the earth.

Instant Occlusion

[ or pseudo-occlusion ] The name that has been coined to label the cloud mass associated with an active trough in the cold air, that comes close to, and interacts with a pre-existing baroclinic zone, forming a pattern that looks superficially as if it was part of a traditional occlusion process.

Intertropical Convergence Zone

(usually abbr. ITCZ) A zone (often rather broad, but sometimes quite narrow), which separates the 'air-masses' brought together by the low-level outflow from the sub-tropical high pressure belts north and south of the equator. Over the oceans, the zone can be well marked; over land, sensible heating usually leads to 'breaks' or other anomalies, and the regional-scale monsoon circulations also distort, or swamp the idealised structure of the ITCZ. Cloudiness (and hence precipitation activity) can vary sharply over a period of 24hr. Day-to-day change of position is often small, but the zone migrates north & south through the course of a year, roughly in sympathy with the changing position of the sun.

Intervention

A process where forecasters force acceptance of a report rejected in the model initialisation routine ('supporting'), or use 'bogus' observations to input a conceptual model observed in imagery.

INTSF

(abbr) Intensifying (as used in SIGMETs for a phenomenon becoming more intense or extensive).

Inversion (of temperature)

A layer in the atmosphere (usually very shallow < 0.4 km), where temperature rises with increasing height. Two of the best-known in operational meteorology are the nocturnal inversion (formed due to strong cooling of land surfaces after sunset), and the subsidence inversion (due to descent & adiabatic warming of air associated with anticyclones). Another near-surface type is that formed when warm air travels over a cold surface (e.g. cold seas or ice/snow).

IPV

(abbr) Isentropic Potential Vorticity - the product of the absolute vorticity of an air parcel, and its static stability, calculated along a constant surface of 'theta' (potential temperature), hence the 'isentropic'. Anomalies in IPV around the level of the tropopause (and hence in the region of the driving jet stream) can be related to developments through the troposphere, leading to cyclogenesis. Because IPV is a highly conservative property for any sample of air, it is found to be particularly useful for tracking the path that stratospheric air (high IPV values) will take as it enters the upper troposphere during rapid cyclogenesis events. NWP models can be programmed to output the height of a particular IPV value - defined such that it 'samples' air in the model stratosphere. These patterns are then overlaid on water vapour imagery, and any mis-match between model and reality are quickly seen and allowed for. (See also Potential Vorticity) and also this article on Water Vapour Imagery.

IR

(abbr) Infra-red (used in connection with satellite imagery) See "What are various types of satellite imagery available?"

ISA

(abbr) International Standard Atmosphere. A standard reference for temperature, pressure, and relative density structure in the troposphere and lower stratosphere, used for the calibration of (pressure) altimeters.

Isobar

A line on a synoptic chart joining points of equal atmospheric pressure.

ISOL

Isolated (as in ISOL CB etc.)

Isotherm

A line connecting points of equal temperature.

ITCZ

Intertropical Convergence Zone (Sometimes seen as ICZ, or perhaps, erroneously, ITF [ intertropical front ]).