| Section A | A index | 301-309 of 917 terms |
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air–fuel ratio—The ratio of air in a fuel mixture relative to the exact amount required to convert all of a hydrocarbon fuel to water and carbon dioxide (known as a stoichiometric mixture). If the air content is higher than stoichiometric, the mixture is said to be fuel-lean; if the air content is less, the mixture is fuel-rich.
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air line correction—Depth corrections applied to sounding lines for any portion of the line above the water, when high velocities, great depth, insufficient sounding weight, etc., result in large angles of inclination.
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air-line sounding—A technique to determine the water level in a well, lake, or any water body with a free surface, using the principle of measuring back-pressure in a pressurized air line that extends downward below the water surface to a known elevation. When air under pressure is introduced to the air line, it forces the water in the line out until the air bubbles from the bottom of the tube. At this time, noting the air pressure in the line, the pressure is converted mathematically into a height-of-water volume. Air-line sounding is a very effective means of measuring water levels in very deep wells.
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air mass—1. A widespread body of air, the properties of which can be identified as 1) having been established while that air was situated over a particular region of the earth's surface (airmass source region), and 2) undergoing specific modifications while in transit away from the source region. An air mass is often defined as a widespread body of air that is approximately homogeneous in its horizontal extent, particularly with reference to temperature and moisture distribution; in addition, the vertical temperature and moisture variations are approximately the same over its horizontal extent. The stagnation or long-continued motion of air over a source region permits the vertical temperature and moisture distribution of the air to reach relative equilibrium with the underlying surface. See airmass classification. 2. In radiation, the ratio of the actual path length taken by the direct solar beam to the analogous path when the sun is overhead from the top of the atmosphere to the surface. Extrapolation of surface measurements to zero air mass was the original method for estimating the value of solar irradiance at the top of the atmosphere. 3. See optical air mass.
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air meter—A small, sensitive windmill anemometer having flat vanes. It indicates the number of linear feet (or meters) of air that have passed the instrument during its exposure.
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air parcel—An imaginary volume of air to which may be assigned any or all of the basic dynamic and thermodynamic properties of atmospheric air. A parcel is large enough to contain a very great number of molecules, but small enough so that the properties assigned to it are approximately uniform within it and so that its motions with respect to the surrounding atmosphere do not induce marked compensatory movements. It cannot be given precise numerical definition, but a cubic foot of air might fit well into most contexts where air parcels are discussed, particularly those related to static stability. Any fluid parcel may be defined similarly.
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air pocket—A local downdraft or an abrupt reduction of headwind or increase in tailwind that causes an airplane to drop suddenly.
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