| Section A | A index | 331-339 of 917 terms |
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airborne fraction—The fractional amount of carbon dioxide, CO2, that remains in the gas phase relative to a given increase in the total amount of CO2 (atmosphere and ocean combined). The relatively high solubility of CO2 in water causes it to partition more favorably in the oceans, and so any potential atmospheric increase is buffered.
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airborne particulates—Solid particles suspended in the air. Larger particles (>100 μm approximately) have terminal velocities greater than about 0.5 m s−1 and fall out quickly. These include hail, snow, graupel, insect debris, room dust, soot aggregates, coarse sand, gravel, and sea spray. Medium-size particles (1 to 100 μm approximately) have sedimentation velocities greater than 0.2 m h−1 and settle out slowly. These include fine ice crystals, pollen, hair, large bacteria, windblown dust, fly ash, coal dust, silt, fine sand, and small dust. Small particles (<1 μm, approximately) fall so slowly that they can take days to years to settle out of a quiescent atmosphere. For a turbulent atmosphere they may never fall out; however, they can be washed out by rain in a process called rainout or washout, leading to wet deposition onto the earth's surface. Examples of these particles include viruses, small bacteria, metallurgical fumes, soot, oil smoke, tobacco smoke, clay, and fumes. Oil and tobacco smoke are sticky, and are removed from the atmosphere when they happen to touch and stick to an object such as plant or house furnishings, in a process called dry deposition. See criteria pollutants, precipitation.
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airborne weather radar—Radar equipment mounted on an aircraft for purposes of weather avoidance or weather observation. Airborne weather avoidance radars are usually mounted in the nose of the aircraft, scanning ahead of the aircraft for weather hazards. Airborne research radars are frequently Doppler radars that provide measurements of radial wind or turbulence in addition to radar reflectivity. Such radars are typically side-pointing or helical scanning and may use dual beams to enable dual- Doppler analysis of storm motions. Airborne weather radars ordinarily operate at wavelengths of C-band or shorter to keep the physical size of the equipment small.
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aircraft ceiling—1. After U.S. weather observing practice, the ceiling classification applied when the reported ceiling value has been determined by a pilot while in flight within one and one-half nautical miles of any runway of the airport. Aircraft ceilings may refer to vertical visibility or obscuring phenomena aloft as well as to clouds, and are designated A in aviation weather observations. 2. The maximum altitude at which any given aircraft can be operated safely.
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aircraft electrification—The accumulation of a net electrical charge on the surface of aircraft, or the separation of charge into two concentrations of opposite sign on distinct portions of the aircraft surface. Net charges appear as a result of autogenous electrification when aircraft fly through clouds of ice crystals or dust. Charge separation occurs by induction when aircraft fly through regions of strong atmospheric electric field, as in thunderstorms. Charging may also occur by the engine exhaust carrying away a net charge leaving the aircraft charged. Development of large local charges on aircraft parts may lead to appearance of corona discharge or St. Elmo's fire and is almost always accompanied by poor radio communication due to so-called precipitation static.
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aircraft icing—(Also called airframe icing, aircraft ice accretion.) Formation of ice, rime, or hoarfrost on an aircraft.
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