Ice - frozen water; the solid form of water substance

Ice Age - a time of widespread glaciation

Ice Crystal - any one of a number of crystalline forms of ice which are visible to the naked eye

Ideal Gas - a gas that obeys Boyle's law and Charles's law and satisfies the equation of state, has internal energy that depends on temperature only, and has specific heat that is independent of temperature

Indian Summer - a period of abnormally warm or mild weather in late autumn or early winter

Inflow Notch - a distinct feature on the radar characterized by an indentation in the reflectivity pattern on the inflow side of the storm. The indentation often is V-shaped, but this term should not be confused with V-notch.

Infrared Radiation - radiation which is less energetic than visible radiation and more energetic than microwave radiation; the radiation emitted by the earth's surface or atmosphere

Insolation - incoming solar radiation; sunshine

Instability - the tendency for an object, if moved, to accelerate in the direction of initial movement; in particular for meteorologists, the tendency for air parcels to accelerate upward after being lifted

Internal Energy - a measure of the molecular activity of a substance or the sum of the total energies of all molecules in a specific mass. In an ideal gas, internal energy is directly proportional to the temperature of the substance.

Interpolate - to estimate a value within an interval between two known values

Intertropical Convergence Zone (ITCZ) - the axis dividing the southeast trades from the northeast trades, toward which the surface winds tend to converge

Inversion - in meteorology, a reversal of the normal atmospheric temperature gradient with height. An inversion is present in the lower part of a cap.

Irradiance - the total incident radiant flux, in watts per square meter, received on a unit area of a surface

Isobar - a line connecting points of equal pressure

Isodrosotherm - a line connecting points of equal dewpoint temperature

Isohyet - a line connecting points of equal precipitation amounts

Isopleth - a general term for a line connecting points of equal value of some quantity. Isobars, isotherms, etc. are all examples of isopleths.

Isotach - a line connecting points of equal wind speed

Isotherm - a line connecting points of equal temperature

Isothermal - having equal or constant temperature with respect to either space or time