| Section A | A index | 671-679 of 917 terms |
|---|
|
Archimedes's principle—The statement that a net upward or buoyant force, equal in magnitude to the weight of the displaced fluid, acts upon a body either partly or wholly submerged in a fluid at rest under the influence of gravity. This force is known as the Archimedean buoyant force (or buoyancy), is independent of the shape of the submerged body, and does not depend upon any special properties of the fluid.
|
|
arcs of Lowitz—A halo in the form of arcs that pass both obliquely and vertically through the 22° parhelia. The arcs of Lowitz are explained by refraction through the 60° prism of hexagonal plate ice crystals, oscillating as they descend. The principal axis of the plate must oscillate about the vertical by less than 30° to produce the pattern customarily called Lowitz arcs; larger oscillations lead to more complex arcs spreading far above and below the 22° parhelia and halo. See parhelion.
|
|
arctic air—A type of air mass with characteristics developed mostly in winter over arctic surfaces of ice and snow. Arctic air is cold aloft and extends to great heights, but the surface temperatures are often higher than those of polar air. For two or three months in summer arctic air masses are shallow and rapidly lose their characteristics as they move southward. See also antarctic air, airmass classification.
|
|
arctic-alpine—Of, or pertaining to, areas above the timber line in mountainous regions, and to the biologic, geologic, etc., characteristics of such areas.
|
|
Arctic Bottom Water—The water mass formed in the Arctic Ocean by a combination of freezing on the arctic shelf and deep winter convection in the Greenland and Norwegian Seas. Freezing increases the salinity under the ice; the dense water sinks to the ocean floor and leaves the arctic basins to enter the Greenland and Norwegian Seas, where it mixes with water that sinks under the influence of surface cooling. The resulting water mass has a salinity of 34.95 psu and a temperature of −0.8° to −0.9°C. It fills the Arctic Ocean at all depths below 800 m, the sill depth to the Atlantic. It enters the Atlantic in bursts, when the passage of atmospheric depressions lifts the thermocline and allows Arctic Bottom Water to flow over the sill. Overflow events in the Denmark Strait and across the Iceland–Faeroe sill contribute some 5 Sv (5 × 106 m3s−1) to the formation of North Atlantic Deep Water.
|
|
Arctic Circle—The line of latitude 66°34′N (often taken as 66 °N). Along this line the sun does not set on the day of the summer solstice, about June 21, and does not rise on the day of the winter solstice, about December 22. From this line the number of twenty- four-hour periods of continuous day or of continuous night increases northward to about six months each at the North Pole.
|
|
