Impulse - See upper level system.

Inflow Bands (or Feeder Bands) - Bands of low clouds, arranged parallel to the low-level winds and moving into or toward a thunderstorm. They may indicate the strength of the inflow of moist air into the storm, and, hence, its potential severity. Spotters should be especially wary of inflow bands that are curved in a manner suggesting Inflow Jets - Local jets of air near the ground flowing inward toward the base of a tornado.

Inflow Notch - A radar signature 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. Supercell thunderstorms often exhibit inflow notches, usually in the right quadrant of a classic supercell, but sometimes in the eastern part of an HP storm or in the rear part of a storm (rear inflow notch).
Inflow Stinger - A beaver tail cloud with a stinger-like shape.

Insolation - Incoming solar radiation. Solar heating; sunshine.

Instability - The tendency for air parcels to accelerate when they are displaced from their original position; especially, the tendency to accelerate upward after being lifted. Instability is a prerequisite for severe weather - the greater the instability, the greater the potential for severe thunderstorms.

Inversion - Generally, a departure from the usual increase or decrease in an atmospheric property with altitude. Specifically it almost always refers to a temperature inversion, i.e., an increase in temperature with height, or to the layer within which such an increase occurs. An inversion is present in the lower part of a cap.

Isentropic Lift - Lifting of air that is travelling along an upward-sloping isentropic surface.

Isentropic lift often is referred to erroneously as overrunning, but more accurately describes the physical process by which the lifting occurs. Situations involving isentropic lift often are characterized by widespread stratiform clouds and precipitation, but may include elevated convection in the form of embedded thunderstorms.

Isentropic Surface - A two-dimensional surface containing points of equal potential temperature.

Isobar - A line connecting points of equal pressure.

Isodrosotherm - A line connecting points of equal dew point temperature.

Isohyet - A line connecting points of equal precipitation amounts.

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

Isotach - A line connecting points of equal wind speed.

Isotherm - A line connecting points of equal temperature.