| Section A | A index | 291-299 of 917 terms |
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Agulhas Current—(Also called Agulhas stream.) The major western semi of the subtropical gyre in the southern Indian Ocean and one of the swiftest ocean currents with mean speeds of 1.6 m s−1 and peak speeds exceeding 2.5 m s−1. Its total transport of 70 Sv (70 × 106 m3s−1) near 31°S and up to 135 Sv (135 × 106 m3s−1) near 35°S is also among the largest of all ocean currents. The Agulhas Current is fed mainly from the East Madagascar Current and to a smaller degree from the Mozambique Current. When passing the Agulhas Bank, the current produces significant upwelling. To the south of the Cape of Good Hope, the current flows west to southwestward first but turns sharply eastward when reaching the Agulhas Current retroflexion region near 40°S, 20°E. Eddies spawned in this region continue to move westward and turn northward to join the Benguela Current. The transport of water from the Indian into the Atlantic Ocean through the eddies is an important part of the global ocean conveyor belt.
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air acoustic ranging sensor—A pulsed, acoustic ranging device to determine water levels using the air column in a tube as the acoustic sound path. The fundamental measurement is the time it takes for the acoustic signal to travel from a transmitter to the water surface and then back to the receiver. The distance from a reference point to the water surface is derived from the travel time. A calibration point is set at a fixed distance from the acoustic transducer and is used to correct the measured distance using the calibrated sound velocity in the tube.
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air current—Very generally, any moving stream of air; it has no particular technical connotation. See wind; compare ocean current.
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air–earth conduction current—(Also called fair-weather current.) That part of the air–earth current contributed by the electrical conduction of the atmosphere itself. It is represented as a downward current in storm-free regions all over the world. The conduction current is the largest portion of the air–earth current, far outweighing the contributions made by the precipitation current and convection current, which are zero in storm-free regions. Its magnitude is approximately 3 × 10−12 amperes (A) m−2, or about 1800 A for the entire earth. Such observations of the vertical variation of the conduction current as have been made indicate that it is approximately uniform throughout the depth of the troposphere, a condition that is consistent with the generally accepted view that the conduction current flows from a positively charged conducting region in the lower ionosphere downward to the negatively charged earth. Only in areas of temporarily disturbed weather does the conduction current become replaced by reverse flow. Accumulating evidence points to the conclusion that the conduction current continues to exist only because of the action of thunderstorms scattered at all times over the earth, which supply the positive charge to the upper atmosphere and negative charge to the earth. See supply current.
Gish, O. H., 1951: Compendium of Meteorology, p. 113.
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air–earth current—The transfer of electric charge from the positively charged atmosphere to the negatively charged earth. This current is made up of the air–earth conduction current, a point-discharge current, a precipitation current, a convection current, and miscellaneous smaller contributions. Of these, the air–earth conduction current is by far the largest. This is not just true locally, but throughout the world where there are no thunderstorms occurring, which is estimated to be 80%–90% percent of the earth. The existence of this quasi-steady current in fair weather and the observed maintenance of the earth's net negative charge are both better established than the nature of the supply current, which must replenish the positive charge in the upper atmosphere and the negative charge on the earth.
Gish, O. H., 1951: Compendium of Meteorology, p. 113.
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