- 行业: Weather
- Number of terms: 60695
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The American Meteorological Society promotes the development and dissemination of information and education on the atmospheric and related oceanic and hydrologic sciences and the advancement of their professional applications. Founded in 1919, AMS has a membership of more than 14,000 professionals, ...
A silent, nonluminous, gaseous electrical discharge from a pointed conductor maintained at a potential that differs from that of the surrounding gas. In the atmosphere, trees and other grounded objects with points and protuberances may, in disturbed weather, be sources of point discharge current. Close to a pointed and grounded conductor that extends above surrounding objects, the local electric field strength may be many times greater than that existing at the same level far from the elevated conductor. When this local field reaches such a value that a free electron, finding itself acted upon by this field, can be accelerated (in one mean free path) to a sufficiently high velocity to ionize neutral air molecules, point discharge will begin. Different structures will yield point discharge under quite different gross field conditions, for geometry is critically important. Point discharge is recognized as a major process of charge transfer between electrified clouds and the earth, and is a leading item in the charge balance of the global electrical circuit. Compare corona discharge, spark discharge.
Industry:Weather
A skiing term for a cover of dry snow that has not been compacted in any way.
Industry:Weather
A situation when a part of the celestial dome is hidden by a weather phenomenon. In U. S. Weather observing procedures, this is the designation for sky cover when part (0. 1– 0. 9) of the sky is completely hidden by surface-based obscuring phenomena. It is encoded “−X” in aviation weather observations; it never, by itself, constitutes a ceiling, for the overhead vertical visibility is not restricted. Compare obscuration, thin.
Industry:Weather
A situation that is changing slowly enough that it can be considered to be constant. For example, atmospheric turbulence has a fast response time, while the atmospheric boundary layer depth that controls the turbulence grows with a slower timescale. Thus, turbulence can be analyzed as if it were unchanging because it quickly reaches equilibrium for any instantaneous boundary layer depth.
Industry:Weather
A situation similar to resonance waves over hills, except that the wavy air is flowing over the tops of convective thermals or cumulus clouds. Some of the kinetic energy of the rising thermals, when they reach and overshoot into a statically stable capping inversion, is converted into wave energy.
Industry:Weather
A simple, cooled cylindrical rod, usually of metal or glass, that is exposed to the airflow in a cloud to collect supercooled cloud droplets for chemical analysis. The collection efficiency of the rod, which is dependent on the size of the drops, can be calculated.
Industry:Weather
A simplifying approximation regarding the horizontal momentum in the mixed layer, based on the assumption that vertical shears across the mixed layer are much smaller than the average shear between the mixed layer and the transition layer below it; hence the mixed layer can be viewed as moving like a slab.
Industry:Weather
A short-period oscillation of pressure such as that associated with the propagation of sound through the atmosphere; a type of longitudinal wave. Pressure waves are usually recorded on sensitive microbarographs capable of measuring pressure changes of amounts down to 10<sup>−4</sup> mb. Typical values for the period and wavelength of pressure waves are ½ to 5 s and 100 to 1500 m, respectively. Pressure waves produced by explosions in the upper atmosphere are of value in determining the high-altitude temperatures and winds. See sound wave, compression wave, microbarm.
Industry:Weather
A short-duration, high-amperage, electric current impulse that may sweep through an electrical network, as a power transmission network, when some portion of it is strongly influenced by the electrical activity of a thunderstorm. This activity may take the form of a direct lightning strike, or it may be simply the release of previously induced charge on the line when a thundercloud overhead suddenly discharges itself. Such current surges flow rapidly through the lines until they find a path to the ground through arc-over at a weak insulator or by entering terminal equipment at the end of the line. Use of grounded guard wires above the power lines of a transmission system reduces the frequency of surge current difficulties, and installation of lightning arresters at sensitive terminal equipment protects the system against damage there.
Industry:Weather
