| Section A | A index | 421-429 of 917 terms |
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alpha particle—(Or α-particle; symbol 2He4.) Physically indistinguishable from the nucleus of a helium atom—two protons and two neutrons bound together by nuclear forces—but usually restricted to the product of nuclear reactions. For example, alpha particles with energies of several million electron volts are emitted in the radioactive disintegration of naturally occurring isotopes of uranium and thorium. The alpha particle carries a positive charge twice the magnitude of the electron charge. Alpha particles are important in atmospheric electricity as one of the agents responsible for atmospheric ionization. Compare beta particle, gamma ray.
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alpha ray—(Or alpha radiation.) A stream of alpha particles; can also mean a single alpha particle.
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alpine glacier—Same as mountain glacier.
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alpine tundra—(Also called mountain tundra.) A form of tundra in which the absence of trees is due to high altitude instead of high latitude. It lies roughly between the summer isotherm of 10°C and the snow line.
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Alter shield—A type of rain or snow gauge shield consisting of freely hanging, spaced slats, arranged circularly around the gauge, the height of which is the same as the gauge height. The purpose is to reduce undercatch due to wind by forcing the air toward the surface instead of accelerating over the gauge orifice. Compare Nipher shield.
Alter, J. C., 1937: Shielded storage precipitation gauges. Mon. Wea. Rev., 65, 262–265.
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alternate airport—An airport (aerodrome) to which an aircraft may proceed when it becomes either impossible or inadvisable to proceed to or to land at the airport (aerodrome) of intended landing. Alternate airports (aerodromes) include the following: | Takeoff alternate: An alternate airport (aerodrome) at which an aircraft can land should this become necessary shortly after takeoff and it is not possible to use the airport (aerodrome) of departure. |
| En route alternate: An airport (aerodrome) at which an aircraft would be able to land after experiencing an abnormal or emergency condition en route. |
| Destination alternate: An airport (aerodrome) to which an aircraft may proceed should it become impossible or inadvisable to land at the airport (aerodrome) of intended landing. |
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alternate forecast—Aviation forecast for another airport (aerodrome) to be used by an aircraft if it becomes inadvisable to land at the airport (aerodrome) of intended landing.
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alternating unit tensor—A mathematical function with symbol εijk defined to switch between the discrete values of +1, 0, and -1, depending on the values of the three indices i, j, and k:  It is one of the tools used in Einstein's summation notation to handle operations equivalent to cross products in vector notation. For example: A × B = εijkAiBjδk, where Ai and Bj represent vectors in summation notation, and δk is the unit vector. Subscripts or indices i, j, and k must each take on the values of 1, 2, and 3, representing the Cartesian directions of x, y, and z. The alternating unit tensor is useful to describe the Coriolis term in the equations of motion: ∂Ui/ ∂t = . . . + fεij3Uj + . . ., where f is the Coriolis parameter, Ui and Uj are wind components, and t is time. Summation notation is commonly used in atmospheric turbulence studies.
Stull, R. B., 1988: An Introduction to Boundary Layer Meteorology, 666 pp.
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alti-electrograph—A balloon-borne instrument for recording the value of the atmospheric electric field strength within active thunderstorms. A disk of specially treated paper (pole-finding paper), slowly rotated by an aneroid element as the balloon ascends, lies between two iron electrodes, the upper one being electrically connected to a discharge point attached to the balloon and the lower one being electrically connected to a long trailing wire. Currents passing through the paper under the influence of the potential difference set up in the two electrodes discolor the treated paper in a pattern with a size roughly related to the ambient electric field strength. A thermal element and a humidity element that record in aerograph fashion complete the device.
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