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Ice crystals – ice crystals are tiny sprinkles that sparkle in the sunshine like diamond dust and hang in the air.

Ice fog – a suspension of numerous minute ice crystals in the air, or water droplets at temperatures below zero degrees Celsius, based at the Earth's surface, which reduces horizontal visibility. Usually occurs at -20 degrees Fahrenheit and below.

Ice pellets – precipitation of transparent or translucent pellets of ice, which are round or irregular, rarely conical, and which have a diameter of 0.2 inch (five millimeters), or less. There are two main types. Hard grains of ice consisting of frozen raindrops and pellets of snow encased in a thin layer of ice.

Ice storm – liquid rain falling and freezing on contact with cold objects creating ice build-ups of 1/4th inch or more that can cause severe damage.

Ice storm warning – older terminology replaced by winter storm warning for severe icing. Issued when 1/2 inch or more of accretion of freezing rain is expected. This may lead to dangerous walking or driving conditions and the pulling down of power lines and trees. A warning is used for winter weather conditions posing a threat to life and property.

Indian summer – an unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.

Inflow bands (or feeder bands) – bands of low clouds, arranged parallel to the low-level winds and moving into or toward a thunderstorm.

Insolation – incoming solar radiation. Solar heating; sunshine.

Instability – a state of the atmosphere in which convection takes place spontaneously, leading to cloud formation and precipitation.

Intermittent rain – intermittent rain stops and starts repeatedly, although not as abruptly or as frequently as showers.

Intertropical Convergence Zone (ITCZ) – the region where the northeasterly and southeasterly trade winds converge, forming an often continuous band of clouds or thunderstorms near the equator.

Inversion – an increase in temperature with height. The reverse of the normal cooling with height in the atmosphere. Temperature inversions trap atmospheric pollutants in the lower troposphere, resulting in higher concentrations of pollutants at ground levels than would usually be experienced.

Iridescence – brilliant patches of green or pink sometimes seen near the edges of high- or medium-level clouds.

Isentropic lift – lifting of air that is traveling along an upward-sloping isentropic surface. Situations involving isentropic lift often are characterized by widespread stratiform clouds and precipitation.