Weather Words - I

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Ice: (1) Ice, the solid phase of water, has a volume that is about 10 percent greater than the liquid water into which it melts. Consequently, ice floats at the surface of water with about 10 percent of its volume above the water-air surface and 90 percent below it.

Ice: (2) Water substance in the solid phase. Its density is 0.917 that of liquid water, and so ice floats in water with 8.3 percent of its volume above the water surface and 91.7 percent below it.

Ice age: A geologic period during which ice sheets cover large portions of the continents. The present-day ice sheets of Greenland and Antarctica are relics of the most recent ice age, when ice covered much of North America and Europe.

Ice Bowl: The NFL Championship game on Sunday, Dec. 31, 1967, when the temperature was -13 degrees F at game time in Green Bay, Wisc. The Packers defeated the Dallas Cowboys 21-17 in front of 50,861 hardy fans. The Packers went on to beat the Oakland Raiders in Super Bowl II.

Ice cap: A permanent cover of ice and snow over an extensive portion of the Earth's surface.

Ice flower: Irregular ribbons of ice, up to several inches in length, formed when freezing water expands inside a plant, splitting the stem and pushing ice through the cracks.

Ice fog: A type of fog composed of suspended particles of ice. It occurs at very low temperatures and usually in clear, calm weather in high latitudes. Ice fog is rare at temperatures warmer than -20 degrees Fahrenheit.

Ice jam: An accumulation of broken river ice caught in a channel, usually at a shallow, narrow or curved portion, frequently producing local floods during the spring breakup.

Ice pellets: Sleet; precipitation in the form of spherical or irregularly shaped particles of ice five millimeters (0.2 inch) or less in diameter.

Ice sheet: (1) A large area of land ice almost completely covering the land on which it rests. The largest ice sheets at present are those of Antarctica and Greenland.

Ice sheet: (2) A large area of land ice, with a dome-shaped and almost level surface, almost completely covering the land on which it rests, although mountain peaks may penetrate to the surface.

Ice shelf: The portion of a land-based ice sheet that has spread out over water. The world's largest ice shelves are found at the periphery of Antarctica.

Iceberg: A mass of ice formed on land but now floating in the sea. Glacier bergs are greenish in color and irregularly shaped. Tabular bergs originate from barrier ice fields such as those around Antarctica.

Icebox of the nation: Communities claiming to be the "nation's icebox" include Pellston, Mich., International Falls, Minn., Fraser, Colo., Truckee, Calif., Stanley, Idaho, and West Yellowstone, Mont.

Icicle: Ice in the shape of a narrow cone, hanging point downward from a roof, fence, cliffside, etc, forming when liquid water from a sheltered or heated source comes into contact with below-freezing air and freezes as it flows; in sizes ranging from those of finger length to some three feet in diameter and 25 feet long.

IFR weather: In aviation terminology, weather conditions in which the bases of clouds covering six tenths of the sky or more are below 1,000 feet and/or horizontal visibility is three miles or less.

Igloo: (1) A dome-shaped hut built of blocks of hard snow; also an excavation made by a seal in the snow over its breathing hole in the ice.

Igloo: (2) The word is derived from the Inuit word "igdlu," meaning "house."

Illinois tornado season: (1) The tornado season in Illinois is the four-month period of March, April, May and June. More tornadoes occur in the state in April than in any other month. However, tornadoes can and do occur in all months of the year. 03/29/05

Illinois tornado season: (2) The four-month period from March through June. More tornadoes occur in Illinois in April than in any other month, but the state's (and the nation's) worst tornado -- the Great Tri-State Tornado -- was a March event (March 18, 1925), claiming 695 lives.

Illinois' and Indiana's lowest temperatures: -36 degrees F in both states. Illinois: at Congerville (15 miles southeast of Peoria) on Jan. 5, 1999; Indiana: at New Whiteland (15 miles south of Indianapolis) on Jan. 19, 1994.

Illinois' highest temperature: 117 degrees F on July 14, 1954, at East St. Louis. East St. Louis, just across the Mississippi River from St. Louis, is located about 250 miles southwest of Chicago.

Illinois' lowest temperature: (1) On January 5, 1999, the temperature at Congerville, Illinois, (a cooperative observing site for the National Weather Service) plunged to -36 degrees F, establishing an all-time record low temperature for the state. The Congerville station is located 15 miles northwest of Bloomington and Normal.

Illinois' lowest temperature:(2) -36 degrees F at Congerville on Jan. 5, 1999. The Congerville station is located along the Mackinaw River in southern Woodford County 15 miles northwest of Bloomington.

Illinois' temperature extremes: A range of 153 degrees from 117 degrees F at East St. Louis on July 14, 1954, to -36 degrees F at Congerville (15 miles northwest of Bloomington and Normal) on January 5, 1999.

Impact winter: Extreme cooling of the Earth's climate following the impact of a massive asteroid or comet that throws enough debris into the atmosphere to greatly reduce the amount of sunlight that reaches the Earth's surface.

In Irons: (nautical) For a sailing boat, having turned into the wind or lost the wind; stuck and unable to make headway.

Inauguration day temperature extremes: Hottest: 92 degrees F on July 10, 1850, when Vice President Millard Fillmore became president after Zachary Taylor died. Coldest: 9 degrees on January 21, 1985, when Ronald Reagan's inaugural parade was cancelled.

Inches of mercury: A unit of air pressure; the height (about 30 inches) of a column of mercury whose weight creates a pressure equal to the pressure exerted by the weight of the Earth's atmosphere.

Incus: The upper, flattened, anvil-shaped, ice-crystal portion of a cumulonimbus (thunderhead) cloud formed when the storm's updraft encounters a layer of stable air and spreads horizontally.

Indian summer: (1) A warm, tranquil spell of weather in the autumn, especially after a frost or period of abnormally cold weather. The term originated in the United States and first came into use in the late 1700s.

Indian summer: (2) The term is used most often in the Midwest and New England, but its usage extends throughout English-speaking countries.

Indiana's greatest 24-hour rainfall: 10.50 inches on August 6, 1905, at Princeton. Princeton is located in the south tip of Indiana.

Indiana's lowest temperature: -36 degrees F at New Whiteland on Jan. 19, 1994. New Whiteland is located in central Indiana, about 15 miles south-southeast of downtown Indianapolis. On that same day, Chicago registered a temperature of -19 degrees.

Indoor relative humidity: Indoor relative humidity is acceptable when it is half the indoor air temperature. That is, an indoor temperature of 70 degrees should feel comfortable with a relative humidity of 35 percent.

Industrial meteorology: A field of meteorology that involves the application of meteorological information and techniques to industrial, business or commercial problems.

Infiltration: The movement of rainwater and meltwater from the surface of the ground down into the earth through voids, cracks and joints. It provides water for plants and, in general, recharges groundwater supplies.

Insolation: INcoming SOLar radiATION, the radiated energy that the Earth receives from the sun.

Instrument shelter: A box-like structure designed to protect weather instruments from exposure to precipitation, condensation and direct sunshine, while at the same time providing adequate ventilation.

Interception: The process by which precipitation is caught and retained on vegetation or structures and subsequently evaporated or drained away without reaching the ground. Also, the loss of sunshine that is blocked by mountains, hills, trees or tall buildings.

International Ice Patrol: Established in 1914 in response to the Titanic disaster on April 15, 1912. Now operated by the Coast Guard, the Patrol locates and issues warnings for North Atlantic icebergs.

Inversion: An atmospheric condition in which air temperature increases with increasing height. It occurs when warmer air overlies cooler air. Ordinarily, air temperature decreases with increasing height.

Ionization: In atmospheric electricity, the process by which neutral atmospheric molecules or other particles suspended in the air are rendered electrically charged.

Iridescent clouds: Clouds (always ice-crystal clouds) that exhibit brilliant spots or borders of colors, usually red and green. The phenomenon is known as iridescence; the colors are called irisation.

Irrigation: The artificial application of water to land to promote the growth of crops. Originally, the intent was to supply water in rain-deficient areas but now it is often to obtain maximum crop yield.

Iso-: A prefix meaning "equal". It is extensively used in meteorology in conjunction with another word to denote lines on a weather map along which values of a given element, such as temperature (isotherm), are constant.

Isobar: On a weather map, a line along which the air pressure is the same; that is, a line drawn through all points of equal air pressure. Isobars form closed loops, and cannot branch or end. Isobaric patterns identify areas of relative high and low air pressure.

Isodrosotherm: On a weather map, a line drawn through all points of equal dew point temperatures.

Isohyet: On a map, a line drawn through points having the same amount of precipitation during a specified period. (Iso- is a prefix meaning "same".)

Isolated showers: Showers expected to affect 10 percent of the area.

Isotherm: On weather maps, a line joining places along which the temperature of the air is the same; On oceanic maps, a line of equal water temperature. Introduced and first used in 1817 by German naturalist Alexander Baron von Humboldt.