Weather Words - O | Weather | WGNTV.com | WGN TV

Weather Words - O

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Oak Lawn tornado: On April 21, 1967, an F4 tornado (winds 207-260 m.p.h.) slashed across Oak Lawn, Hometown, Evergreen Park and Chicago's south side, killing 33 and injuring 500; Chicago area's worst tornado.

Oakland Hills, Calif., firestorm: Driven by hot dry "diablo winds," the worst firestorm in modern U.S. history raged through this San Francisco area city on Oct. 20, 1991. The toll: 25 deaths, 3,000 homes destroyed.

Oasis: A fertile or green spot in a desert or wasteland, made so by the presence of water.

Obscuration: The state of the sky said to exist when the sky is completely hidden by surface-based phenomena such as fog or precipitation. An obscured sky is usually an even gray in appearance.

Observational day: Any 24-hour period selected as the basis for climatological or hydrologic observations of the weather. Where observations are recorded by automatic weather instruments, the observational day is taken as the calendar day.

Occluded front: A composite of two fronts that is formed when a cold front (the leading edge of an advancing mass of cold air) overtakes a warm front.

Ocean: (Or sea) In general, the intercommunicating body of salt water occupying the depressions of the earth's surface. Specifically, the five subdivisions (Atlantic, Pacific, Indian, Arctic, Antarctic) of the larger world ocean.

Oceanic climate: A climate dominated by proximity to the sea -- small daily, day-to-day and annual temperature changes, typical of oceanic islands (like the Hawaiian Islands) and coastal cities (like Miami).

Oceanicity: A measure of the extent to which the climate of a place is influenced by the oceans; the opposite of "continentality". The influence of oceanicity is greatest on temperatures.

Oceanography: The study of the sea, embracing and integrating all knowledge pertaining to the sea's physical boundaries, the chemistry and physics of sea water, and marine biology.

Oceans of the world: Geographers recognize three oceans: the Pacific, Atlantic and Indian. The Arctic is considered an extension of the Atlantic; the Antarctic, surrounding Antarctica, consists of the extreme southern portions of the three major world oceans.

October: (1) The tenth month of the year and the middle month of meteorological autumn. It is Chicago's sixth warmest and third driest month. (2) October has brought a trace or more of snow to Chicago in 37 of 74 years (or exactly 50 percent of the months). Most October snowfall: 4.4 inches in 1967. (Midway Airport data, 1928-2001).

October 6: In 1963, Chicago's temperature soars to 94 degrees, the city's latest 90+ temperature and a record high for the date.

October temperature extremes: U.S. temperature extremes for October span a range of 149 degrees, from 116 degrees at Sentinel, Ariz., on Oct. 5, 1917, to -33 degrees at Soda Butte, Wyo., on Oct. 29, 1917. October's extremes occurred in the same year -- the only month for which this is true.

Offshore wind: Wind blowing from land to water.

Oklahoma City, Okla: The most tornado-prone city in the United States and possibly the world, having been struck by at least 112 tornadoes (as of 2004) since the late 1800s.

Ombrophobia: The irrational or morbid fear of rain.

Ombroscope: An instrument consisting of a heated, water-sensitive surface that electronically registers the occurrence of precipitation. Its output may be arranged to trip an alarm, to record on a time chart, to raise the top of a convertible, etc.

Onshore wind: Wind blowing from water to land.

100-year storm: A storm (or other event, such as a flood) that has an annual probability of occurrence of 1/100. When such a storm takes place in a given year, its likelihood of happening again in the next year is still 1/100.

Opacus: An extensive patch, sheet or layer of clouds, the greater part of which is dense enough to hide the position of the sun during the day or the moon at night; the opposite of translucidus.

Opaque clouds: Clouds which are sufficiently dense to completely block the view of all that might be above them. During the day sunlight which penetrates an opaque overcast will not cast shadows.

Optical meteor: Any of the optical phenomena that occur in the atmosphere, such as a halo, rainbow, corona or mirage.

Orographic: Related to, or caused by, physical geography (such as mountains or sloping terrain). Air forced upward by terrain can cause or aid in the development of fog, clouds and precipitation.

Orographic clouds: Clouds resulting from the interference of rising land in the path of moisture-laden winds. If air is forced far enough upslope, dynamic cooling leads to clouds and sometimes to precipitation.

Orographic precipitation: Precipitation resulting from the interference of rising land in the path of moisture-laden winds. If air is forced far enough upslope, dynamic cooling leads to clouds and precipitation.

Orographic storm: A rain or snow event that results from, or is significantly enhanced by, lifting of an air mass over a mountain barrier.

Outburst flood: Catastrophic flooding that results from the sudden release of water in cavities inside a glacier or at the margins of glaciers or from rivers and lakes that are dammed by flowing glaciers.

Outlook: A weather forecast intended to provide, in general terms, an indication of the trend of the weather for a few days beyond the current weather forecast.

Overcast: The state of the sky when a widespread layer of clouds covers all of the sky. There may be thin or bright spots in the cloud layer, but no openings.

Overdraft: The quantity of water pumped from a ground water basin (such as a reservoir or well) that exceeds recharge over an extended period of time.

Overpressure: The destructive "wall" of compressed air created in the immediate aftermath of a nuclear explosion. Overpressure surges outward from the center of the explosion.

Overrunning: A weather situation in which one layer of air flows over the top of another layer, as when warm air ascends over colder air.

Overrunning cloudiness: Widespread cloudiness (hundreds of miles, or more) that results from lifting and saturation of a warm air mass as it moves over the top of a cooler layer beneath it.

Overrunning precipitation: Widespread, steady precipitation which results from the lifting and saturation of a warm, moist air mass as it moves over the top of a cooler layer beneath it.

Overturning: In a lake or sea, the sinking of surface water and rise of bottom water that results from changes in water temperature, usually in the spring and autumn; one complete cycle of top-to-bottom mixing.

Oxygen: A colorless, tasteless, odorless gaseous element; the second-most abundant gas in the atmosphere, comprising 20.9 percent of dry air by volume. It is necessary for almost all forms of animal life.

Ozone: (1) (O3): A form of oxygen; has a weak chlorine odor. The highest natural concentration (12 parts per million) is found at a height of 15 miles. Ozone heats the upper atmosphere by absorbing ultraviolet from sunlight. (2) A nearly colorless form of oxygen. Its highest natural concentration (12 parts per million) is found at a height of 15 miles. If all the normal content of atmospheric ozone were compressed to sea level pressure, it would form a layer only about one inch thick. (3) A form of oxygen; its highest natural concentration occurs at a height of about 15 miles. Ozone heats the upper atmosphere (12-18 miles up) by absorbing ultraviolet from sunlight.

Ozone amounts: If all the natural atmospheric ozone were compressed to sea level pressure, it would form a layer only about one inch thick. The highest concentration (12 parts per million) occurs 15 miles up.

Ozone depletion: The thinning of the Earth's protective stratospheric ozone layer (10-15 miles up). It occurs when ozone formed by natural processes is destroyed at a rate faster than it is being created.

Ozone hole: The seasonal severe depletion (by as much as 50 percent) of stratospheric ozone that occurs each spring over Antarctica. It is linked to the spring return of sunshine to the Antarctic atmosphere.

Ozone layer: (1) An atmospheric layer about 9.5-15 miles above the surface in which a high portion of oxygen exists as ozone. It is important because ozone strongly absorbs incoming ultraviolet radiation. (2) An atmospheric layer about 10-15 miles above the Earth's surface in which ozone (a form of oxygen) is formed from the conversion of oxygen molecules by solar radiation.

Ozone shield: Protection from ultraviolet radiation in sunlight given to the Earth's surface by the stratospheric ozone layer. Ozone strongly absorbs the ultraviolet component of sunlight (the component that causes sunburn).

Ozonosphere: (also called the ozone layer) An atmospheric layer 9.5-15 miles above the surface in which a high portion of oxygen exists in the form of ozone. It is important because ozone strongly absorbs incoming ultraviolet radiation.