Weather Words - D

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Dam: Any artificial barrier that impounds or diverts water, generally considered hydrologically significant if it is 1.25 feet or more in height from the natural bed of the stream.

Damming: The piling up or complete blocking of cold air approaching the slopes of a mountain barrier. It is a frequent occurrence when cold Canadian air moves south across the Great Plains but is unable to push over the Rockies to the west.

Damp air: A colloquial, non-technical term used to describe moist air of moderate temperature.

Dana, Charles: (1819-1897) Editor of the New York City Sun who, according to a widely circulated urban myth, grew weary of hearing Chicagoans boast of the huge success of Chicago's 1893 World's Columbian Exposition and dubbed Chicago the "Windy City." Research has shown that the phrase is erroneously attributed to Dana.

Dancing devil: A small, vigorous, short-duration whirlwind, made visible by dust carried aloft, up to 100 feet in diameter, average height 600 feet. Other names: dust devil, dancing dervish, dust whirl.

Dark days: A reference by residents of coastal Oregon, Washington and British Columbia to the period beginning in November and extending through December and January, when the fewest hours of daylight coincide with the annual maximum of clouds and rain.

Dark December: A nickname earned because December (the first month of meteorological winter) brings the least sunshine of any month. Hours of daylight are at an annual minimum and cloudiness is at an annual maximum in Chicago, across the Great Lakes Region and many other areas of the United States as well.

Dawn: The morning period between complete darkness and sunrise (the instant when the top of the rising sun just appears above the sea-level horizon); technically, also the comparable evening period.

Daytime visibility: The greatest distance at which it is just possible to identify with the unaided eye a prominent dark object against the sky at the horizon.

Dead zone: Oxygen-depleted water in the Gulf of Mexico off the Louisiana coast formed from nitrogen-rich Mississippi River water that empties into the Gulf, fueling rapid growth of oxygen-consuming algae.

Death Valley: A depressed basin in Inyo County, California, and the hottest and lowest location in the United States. Approximately 50 miles long and 20 miles wide, it lies 276 feet below sea level. The highest U.S. temperature, 134 degrees F, was recorded there in July 10, 1913, at the Greenland Ranch.

Death Valley heat wave: On this date (Aug. 17) in 1917, the Death Valley, California, weather observation station concluded a consecutive string of 43 days on each of which the maximum temperature rose to 120 degrees or higher; the consecutive period was Jul. 6 - Aug. 17, 1917.

Deaths from snow: Snow kills hundreds of people in the United States each year. The primary snow-related deaths are from traffic accidents, overexertion, and exposure, but deaths from avalanches have been steadily increasing.

Debris clouds: (1) The clouds lingering a few hours after thunderstorms die away. Because debris clouds exist at high levels (8,000-60,000 feet) where winds can be strong, they often move hundreds of miles downstream after the thunderstorms have dissipated.

Debris clouds: (2) Rotating clouds of dust and debris surrounding the base of a tornado at or near the ground; also the clouds lingering for a few hours after thunderstorms die away.

December: The 12th month of the year and the first month of meteorological winter. With a 77-year average temperature of 28.9 degrees (Midway data), average precipitation of 2.29 inches and average snowfall of 9.2 inches, it is Chicago's third coldest month, third driest (behind Jan. and Feb. in both cases) and second snowiest (behind Jan.).

December 19-25: The period when the hours of daylight are fewest.

December 26, 1776: Because of an exceptionally cold winter, the normally unfrozen Delaware River in New Jersey was frozen solidly, enabling George Washington to cross the river with 2,400 men and surprise and capture much of the British garrison in Trenton.

December temperature extremes: United States: A range of 159 degrees F; highest 100 at La Mesa, Calif. (Dec. 8, 1938); lowest -59 at West Yellowstone, Mont., and Riverside Ranger Station, Wyoming (Dec. 10, 1924). Chicago: A range of 96 degrees; highest: 71 (Dec. 2, 1982, and Dec. 3, 1970); lowest -25 (Dec. 24, 1983).

Declination: The latitude that the sun is directly above at a given time. It is about 23 degrees N at the summer solstice (June 21, the beginning of astronomical summer) and 23 degrees S at the winter solstice.

Decoupling: A process during which one layer of the atmosphere stops interacting with an adjacent layer; a frequent occurrence at night when ground-level winds become calm but continue aloft (say above 1,000 ft.).

Deflation: The removal of loose soil and other material by the wind, leaving bare rock surfaces exposed to the continuous attack of the weather.

Degree Day: Using a base of 65 degrees F., there is one cooling degree day for every degree that a day's average temperature is above 65, or one heating degree day for every degree that it is below 65.

Degrees of frost: (British) The number of degrees Fahrenheit below the freezing point (32 degrees F), as in: "We had ten degrees of frost"; that is, the temperature was 22 degrees.

Dehumidify: To remove moisture from.

Deicer: A compound, such as ethylene glycol, used to prevent the formation of ice, as on a windshield.

Delayed radioactive fallout: Radioactive debris created in a nuclear explosion that rises into the atmosphere and remains there for weeks or months before falling to earth. Atmospheric winds eventually distribute delayed radioactive fallout around the world.

Deliquescence: The characteristic of a substance whereby it absorbs water vapor from the air and eventually forms a solution.

Deluge: An exceptionally heavy rainfall that results in major flooding; also, the actual flood or inundation of water.

Dendrochronology: (1) The dating of past events and variations in the environment and climate (especially temperature and moisture) by studying the annual growth-rings of trees.

Dendrochronology: (2) The analysis of annual growth rings in the trunks of trees in order to determine climates of the recent past. Wide growth rings indicate favorable growing conditions; narrow rings, unfavorable.

Dense fog: Fog that reduces horizontal visibility to 1/4 mile or less.

Density of water: As water is warmed from its freezing point (32 degrees F) to 39.2, it contracts and becomes more dense. With further warming beyond 39.2, water expands. Water is therefore most dense at 39.2.

Denver snowstorm: Because temperatures drop 2-5 degrees for every additional 1,000 feet of elevation, higher elevations can be surprisingly cold. Denver, at 5,280 feet, experienced an unusually early heavy, wet snow that accumulated 21.3 inches on Sept. 26-27, 1936.

Deposition: The accumulation of material dropped because of a lessening of speed of the transporting medium such as air or water; also, the transfer of gases or particles from air to the ground.

Depression: A low pressure area; a part of the atmosphere in which air pressure is lower than in all surrounding areas; isobars (lines of constant air pressure on weather maps) form closed circles or ovals around a depression.

Derecho: A widespread, thunderstorm-induced windstorm with straight-line winds (as compared to the rotary winds of tornadoes). Derechoes usually occur with southeast-moving thunderstorm lines, sometimes producing damage areas hundreds of miles in length.

Des Veaux, Harold: The London, England, physician who coined the word smog in 1905 to describe natural fog contaminated by smoke. Today smog is a synonym for visible air pollution, especially in urban areas.

Desert: A region of extreme aridity and, usually, a region of such high temperature and low rainfall that moisture is insufficient to support appreciable vegetation. One-seventh of the Earth's land surface is covered by deserts, and most of the world's deserts border on or are geographically near the oceans. Hot deserts experience high summer temperatures; cold deserts (also called arctic deserts) are dominated by bare rocks, snow and ice.

Desert climate: A climate that is characterized by insufficient moisture to support appreciable plant life; a climate of extreme aridity.

Desert wind: A wind blowing from a desert to surrounding areas. Local names are: brickfielder (Sydney, Australia); harmattan (West Africa); Santa Ana (southern California); simoom (Sahara area).

Desertification: The formation or increase of desert-like conditions in a region resulting from inappropriate land usage (deforestation, overcropping, etc.), or from climatic change resulting in permanently decreased precipitation.

Desiccate: To dry out thoroughly; to preserve a substance (such as food) by removing its water.

Desiccation: The permanent disappearance of water from an area due mainly to a change of climate; minor causes are deforestation, a failure to maintain irrigation, overcropping, diversion of water.

Dew: Water condensed from water vapor in the air directly onto grass and other objects near the ground when their temperature falls below the dew point temperature. It is not considered to be precipitation. It forms on leafs when their temperature falls below the dew point temperature.

Dew and frost: Dew and frost form, they do not fall; hence they are not considered to be precipitation.

Dew point: The temperature at which a given sample of air will have a relative humidity of 100 percent. Hence, the saturation temperature. The higher the dew point, the greater is the moisture content of the air. The dew point temperature changes only when the moisture content of the air changes.

Dew point temperature: (1) The dew point; the saturation, temperature. It is a direct measure of airborne moisture. The higher the dew point, the greater the moisture content of air. Highest ever in Chicago: 83 degrees F.

Dew point temperature: (2) The saturation temperature. Individual tolerances vary but air feels uncomfortably moist when its dew point rises to 70 degrees F; at 75-77, extreme discomfort is felt; at 80° it's insufferable.

Dewbow: A rainbow formed in the drops of water often found on grass in the early morning. The name incorrectly implies the causative drops are dew, but actually they are drops exuded from the tips of grass blades.

Diablo winds: Hot, dry winds that blow, sometimes with great force, from the east across the San Francisco Bay area. Diablo winds caused the disastrous (3,000 homes destroyed) $1.5 billion Oakland Hills fire on Oct. 20, 1991.

Diamond dust: Tiny ice crystals falling from an apparently clear sky, usually at subzero temperatures. The ice crystals glitter like diamonds in sunlight or moonlight or in artificial light.

Direct hit: The close approach of a tropical cyclone (such as a hurricane) to a particular location -- generally a distance of one or two times the cyclone's radius of maximum winds.

Dirty high: High pressure systems, usually associated with widespread fair weather, will on rare occasions bring extensive, persistent cloudiness. Meteorologists colloquially refer to these as dirty highs.

Disaster area: An area that officially qualifies for emergency governmental aid as a result of a catastrophe such as an earthquake, flood or tornado.

Disphotic zone: The dimly illuminated layer within a large body of water, usually 300-900 feet below the surface, within which there is insufficient sunlight for plant photosynthesis.

Disturbance: A term loosely applied to any agitated state of the atmosphere, large or small: (1) low pressure system, (2) a tropical area of storminess like an easterly wave, (3) any area in which the weather is in a state of cloudiness, precipitation or wind.

Diurnal: Daily; pertaining to cycles which are completed within 24 hours and which recur every 24 hours. The diurnal variation of most atmospheric variables (like temperature) is one of the most striking and consistent features of the weather.

Diurnal cycles: Atmospheric cycles completed within and recurring every 24 hours. The diurnal variation of most atmospheric variables (like temperature) is the most consistent feature of the weather.

Divergence: The flow of the wind resulting in a horizontal outflow of air from a region. Divergence at ground level requires that air immediately above the diverging layer must descend. Divergence is the opposite of convergence.

Divine Wind: A huge Mongol fleet on its way to what would have been a successful invasion of Japan was twice destroyed by typhoons (in 1274 & 1281). The good fortune for Japan gave rise to kamikaze, divine wind.

Dixie Alley: The United States' second tornado alley, extending from Louisiana and Arkansas on the west to Georgia on the east. It features a maximum of tornado activity in the spring (just like the primary Great Plains tornado alley) but, unlike the Plains, a secondary maximum in November and December.

Dizzy heights: (1) By the early 1800s, balloon pioneers were able to ascend high enough that the amount of oxygen in the air was dangerously lacking, giving rise to the phrase, dizzy heights.

Dizzy heights: (2) Balloon pioneers of the 1780s were able to ascend high enough that the amount of oxygen in the air was dangerously lacking, giving rise to the phrase, "dizzy heights".

Dog days: A summer hot spell. Early Egyptians believed the star Sirius, the Dog Star rising with the Sun, added to the sun's heat; dog days (July 3-Aug. 11) combined the sun's heat and that of the Dog Star.

Dog days of summer: A summer hot spell. Early Egyptians believed the star Sirius, the Dog Star rising with the sun, added to the sun's heat; dog days (July 3-Aug. 11) combined the heat of the sun and Dog Star.

Doldrums: The equatorial belt into which northeast trade winds from the northern hemisphere and southeast trades from the southern hemisphere converge. The weather there is warm, humid, showery, with light winds.

Donora, Penn., smog disaster: Industrial air pollutants, trapped under an inversion, created a deadly smog across Donora Oct. 26-31, 1948. Half the city's 14,000 residents became ill, 20 were asphyxiated. The incident resulted in this country's first meaningful air pollution laws, culminating in the federal Clean Air Act of 1970.

Doppler, Christian Johann: (1803-1853) The Austrian physicist who, in 1842, noted in the case of sound waves, the change of frequency with which the sound reaches a receiver when the receiver and source are in motion relative to each other.

Downburst: A strong thunderstorm downdraft that, when it strikes the surface, produces winds affecting an area greater than 2.5 miles across; if less than 2.5 miles across, it is known as a microburst,

Downdraft: A relatively small-scale current of air with marked downward motion. The term is frequently used in describing descending air currents associated with cumulonimbus (thunderhead) clouds.

Draft: A small-scale current of air, especially in an enclosed space like a room or chimney. Drafts often consist of cold air moving across a floor and, as such, are unpleasant.

Dreary: Monotonously gloomy weather occurring most frequently in Chicago in November and December, and characterized by extensive periods of cloudy, gray skies often accompanied by fog or rain.

Drift Ice: Any sea or lake ice that has drifted from its place of origin; ice in an area containing several small pieces of floating ice, but with total water area exceeding the total area of ice.

Drifted snow: Snow that has been moved by the wind and usually, though not necessarily, has collected into snowdrifts. Drifted snow is difficult to shovel or plow because of its great density.

Drifting snow: Fallen snow that has been raised by the wind to a height less than six feet; if it is raised six feet or higher it is known as blowing snow.

Drip line: A line on the ground that marks the outer edge of a tree's branch spread. Rain, sunshine and wind speed are usually less within the area bounded by the drip line of the tree.

Driven snow: Snow that has been moved by wind and collected into snowdrifts. A wind of about 13 m.p.h. will move light surface snow when the temperature is below freezing.

Drizzle: Liquid precipitation composed of very small water droplets that appear almost to float while following air currents. The droplet size is 0.019 to 0.001 inch in diameter. Unlike fog, drizzle falls to the ground. It is erroneous to refer to very light rain as drizzle.

Drizzle, fog, mist: All three are composed of tiny water droplets that appear to float in the air. Drizzle droplets are large enough so that they do fall, and therefore drizzle is precipitation. Fog and mist droplets are smaller and do not fall and are not precipitation.

Drop: A liquid particle (such as a raindrop), with shape maintained by a balance between the drop's surface tension and air drag when the drop is falling through the atmosphere at its maximum fall speed (that is, at its terminal velocity).

Dropsonde: A expendable package of instruments (dropped from an aircraft and floated down by parachute) to measure temperature, humidity and air pressure -- often used in hurricanes.

Drosometer: An instrument used to measure the amount of dew formed on a given surface.

Drought: (1) Extreme dryness due to lack of precipitation over periods that are relatively extensive in area and time. The term is applied regardless of the season, though the effects of drought are most noticeable during the growing season.

Drought: (2) Extreme dryness due to excessive precipitation deficit, relative to the climatic normals for a given area, over periods that are extensive in area and time.

Drought: (3) Three kinds of drought are recognized: permanent drought (insufficient precipitation, as in a desert); contingent drought (occasional failure of precipitation in areas that are otherwise sufficiently well-watered; seasonal drought (areas with distinct wet and dry seasons).

Drought: (4) It is a retrospective phenomenon in the sense that its actuality is not realized until it is well under way and its demise is not recognized until well after heavier precipitation has returned.

Dry: Having little precipitation, as in a dry climate; having little moisture, as in dry air; a period (usually at least 15 days) marked by significantly below-normal precipitation, as in a dry month; a significant deficiency or failure of normal precipitation, as in a dry spell.

Dry front: A cold front (the leading edge of an advancing cold air mass) that is producing no precipitation. About three-fourths of cold frontal passages at Chicago are accompanied by precipitation.

Dry ice: Solid carbon dioxide. It changes directly from solid to gas (without passing through a liquid stage) at -110 degrees F. Carbon dioxide is a trace component of air, at a concentration of about 360 parts per million.

Dry lightning: Lightning ground strikes to locations that receive no rain from the parent thunderstorm. Dry lightning is the leading natural cause of forest fires in the western states.

Dry line: A boundary separating warm, dry air from warm, moist air. Because dry air is more dense than moist air, a dry line behaves like a cold front in that it represents an instability zone that can trigger thunderstorms. It is an important weather feature in the western Great Plains within a few hundred miles of the Rocky Mountains.

Dry season: In climates in which the seasonal variation of precipitation is a major feature, the dry season is the annually recurring period of a few months during which precipitation is absent or nearly so for that region. Chicago does not have a well-defined dry season.

Dry snow: Snow from which a snowball cannot readily be made. In the Chicago area, this is usually snow that falls when the air temperature is in the teens, or lower.

Dry spell: Colloquially, an interval of abnormally dry weather less extensive and less severe than a drought. In the United States, it is used in reference to a rainless period lasting at least 15 days.

Dry thunderstorm: A thunderstorm that produces little or no rain, and mainly a phenomenon of the western United States. Rain falling from the base of such a thunderstorm evaporates before it reaches the ground.

Duff: The litter composed of leaves, twigs, dead logs, etc., that covers the ground in the forest. The quality and moisture content of duff is important in considerations of forest fire hazards, especially in the early spring before vegetation has started growing.

Dune: A ridge or mound of sand piled up by wind, usually in desert regions or near oceans or large lakes. Dunes can grow to several hundred feet in height.

Dusk: The period of time between sunset (the instant when the top of the setting sun just disappears below the sea-level horizon) and complete darkness; technically, also the comparable morning period.

Dust: Solid materials suspended in the air in the form of small, even microscopic, irregular particles. It imparts a tannish or grayish hue to distant objects; the sun's disk is pale or colorless.

Dust Bowl: A name given, early in 1935, to the region in the south-central U.S. afflicted by drought and dust storms at that time. It included parts of Colorado, Kansas, Oklahoma, Texas and New Mexico.

Dust devil: A small, vigorous, short-duration whirlwind, made visible by dust or other debris carried aloft, ranging up to 100 feet in diameter, average height 600 feet. Winds are clockwise or counterclockwise.

Dust wall: The leading edge of a dust storm with the appearance of a knobby or convex wall when viewed from the clear air ahead of the storm; often caused in deserts by the gust front ahead of thunderstorms.

Dust well: A pit in a snow or ice surface produced when small particles on the surface are heated by sunshine and melt down into the snow (or ice).

Dusting of snow: A very light sprinkling of newly fallen snow on a surface, at most only two or three tenths of an inch, and usually at temperatures low enough so that the snow is not wet or slushy.

Dwigh: In Newfoundland, Canada, a sudden and often violent shower or snowstorm.

Dynamics: (Atmospheric dynamics) Any forces that produce motion in the atmosphere. Forecasters often use the term to describe forces that cause storm systems to develop or intensity.