Glossary
Ice Crystal
any one of a number of crystalline forms of ice that may be barely visible to the naked eye
Ice Nuclei
small particles whose molecular structure has a structure similar to frozen water (ice), allowing the formation of an ice crystal. These particles help liquid water freeze at temperatures above -40°C (-40°F). Minerals, soot, organic matter, and sulfates can act as ice nuclei.
Ice Pellets (PL)
also known as sleet. Pellets of ice composed of frozen or mostly frozen raindrops or refrozen partially melted snowflakes. These pellets usually bounce after hitting the ground or other hard surfaces.
Ice Storm
when damaging accumulations of ice occur during freezing rain situations. Significant ice accumulations are usually 1/4 inch or greater.
Ideal Gas
a gas that obeys Boyle’s law and Charles’ law and satisfies the Equation of State, has internal energy that depends on temperature only, and has specific heat that is independent of temperature
Impulse (IMPL)
another name for an upper level system or shortwave; a general term for any large-scale or mesoscale disturbance that can produce lift.
Inflow Band
a band of low clouds, arranged parallel to the low-level winds and moving into or toward a thunderstorm. Inflow bands that are curved cyclonically (counter-clockwise) could suggest the presence of a mesocyclone.
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 (IR)
radiation that is less energetic than visible radiation and more energetic than microwave radiation; the radiation emitted by the earth’s surface or atmosphere. This is also a type of satellite imagery.
Instability (INSTAB or INSTBY)
the tendency for an object, if moved, to accelerate in the direction of initial movement; in particular for meteorology, the tendency for air parcels that are warmer than their environment to accelerate upward after being lifted
Intergovernmental Panel on Climate Change (IPCC)
a scientific intergovernmental body set up by the World Meteorological Organization (WMO) and by the United Nations Environment Programme (UNEP). It provides the decision-makers and others interested in climate change with an objective source of information about climate change. The IPCC does not conduct any research nor does it monitor climate related data or parameters. Its role is to assess on a comprehensive, objective, open and transparent basis the latest scientific, technical and socio-economic literature produced worldwide that is relevant to the understanding of the risk of human-induced climate change, its observed and projected impacts, and options for adaptation and mitigation.
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.
International System of Units (SI units)
also known as the metric system. The base units are meters (length), kilograms (weight), Kelvin (temperature), and seconds (time). It is the world's most widely used system of units--even the United States uses it, mainly in science.
Intertropical Convergence Zone (ITCZ)
the axis dividing the southeast trades from the northeast trades, toward which the surface winds tend to converge
Inversion (INVRN)
in meteorology, a reversal of the normal atmospheric temperature gradient with height (normal is temperature getting cooler with increasing height). An inversion is present in the lower part of a cap.
Iridescence
brilliant spots or borders of colors in clouds, usually red and green, caused by diffraction of light by small cloud particles. It is usually observed in thin cirrus clouds within about 30° of the sun and is characterized by bands of color in the cloud that contour the cloud edges.
Irradiance
the total incident radiant flux, in watts per square meter, received on a unit area of a surface
Isallobar
a line of equal change in atmospheric pressure during a specific time period. For example, one isallobar might be a pressure change of 5 mb in 24 hours.
Isentrope
a line of constant potential temperature (also known as a line that connects points of equal potential temperature, a line of constant entropy).
Isentropic Lift
lifting of air that is traveling along an upward-sloping isentropic surface. Isentropic lift often is referred to erroneously as overrunning. Situations involving isentropic lift are often characterized by widespread stratiform clouds and precipitation.
Isentropic Surface
a 2D surface containing points of equal potential temperature (constant entropy).
Isolated (ISOL or ISOLD)
an NWS convective precipitation descriptor for a 10 percent chance of measurable precipitation (0.01 inch). It is used interchangeably with few.
