Science / Weather Glossary

Ice Fog: Fog that is composed of minute ice particles. It occurs in very low temperatures under clear, calm conditions in the polar latitudes and may produce a halo around the sun or moon.

Ice Jam: An accumulation of broken river ice caught in a narrow channel, frequently producing local flooding. Primarily occurs during a thaw in the late winter or early spring.

Ice Pellets: Precipitation in the form of transparent or translucent pellets of ice, which are round or irregular in shape. They have a diameter of 0.2 inches (5 mm) or less. They are classified into two . . . View Full Definition

Ice Storm: A severe weather condition characterized by falling freezing precipitation. Such a storm forms a glaze on objects, creating hazardous travel conditions and utility problems.

Icelandic Low: A semi-permanent, subpolar area of low pressure in the North Atlantic Ocean. Because of its broad area and range of central pressure, it is an area where migratory lows tend to slow down and . . . View Full Definition

Icicle: Ice that forms in the shape of a narrow cone hanging point down. It usually forms when liquid water from a sheltered or heated source comes in contact with below-freezing air and freezes mor . . . View Full Definition

Icing: The forming or depositing of ice on an object.

Inches Of Mercury (Hg): The name comes from the use of mercurial barometers which equate the height of a column of mercury with air pressure. One inch of mercury is equivalent to 33.86 millibars or 25.40 millimeter . . . View Full Definition

Indian Summer: A period of abnormally warm weather in mid to late autumn with clear skies and cool nights. A first frost normally precedes this warm spell.

Infrared: The long wave, electromagnetic radiation of radiant heat emitted by all hot objects. On the electromagnetic spectrum, it can be found between microwave radiation and visible light. Water vap . . . View Full Definition

Insolation: Solar radiation or heating received at the earth's surface. The name is derived from INcoming SOLar radiATION.

Instability: The state of equilibrium in which a parcel of air when displaced has a tendency to move further away from its original position. It is the condition of the atmosphere when spontaneous convec . . . View Full Definition

Instrument Flight Rules (IFR): Refers to the general weather conditions pilots can expect at the surface and applies to the weather situations at an airport during which a pilot must use instruments to assist take off and . . . View Full Definition

Instrument Shelter: A boxlike structure designed to protect temperature measuring instruments from exposure to direct sunshine, precipitation, and condensation, while at the same time time providing adequate ve . . . View Full Definition

Intermountain High: An area of high pressure that occurs during the winter between the Rocky Mountains and the Sierra-Cascade ranges. It blocks the eastward movement of Pacific cyclones. Also called Plateau Hig . . . View Full Definition

International Date Line: The line of longitude located at 180° East or West (with a few local deviations) where the date changes by a day. West of the line it is one day later than east of the line.

Intertropical Convergence Zone (ITCZ): An area where the Northern and Southern Hemispheric trade winds converge, usually located between 10 degrees North and South of the equator. It is a broad area of low pressure where both the . . . View Full Definition

Inversion: A departure from the usual increase or decrease of an atmospheric property with altitude. It usually refers to an increase in temperature with increasing altitude, which is a departure from . . . View Full Definition

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