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Mauna Kea Weather Center Meteorology Glossary/Dictionary

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  • hail - Precipitation in the form of balls or irregular lumps of ice, always produced by convective clouds, nearly always cumulonimbus. An individual unit of hail is called a hailstone. By convention, hail has a diameter of 5 mm or more, while smaller particles of similar origin, formerly called small hail, may be classed as either ice pellets or snow pellets. Thunderstorms that are characterized by strong updrafts, large liquid water contents, large cloud-drop sizes, and great vertical height are favorable to hail formation. The destructive effects of hailstorms upon plant and animal life, buildings and property, and aircraft in flight render them a prime object of weather modification studies. In aviation weather observations, hail is encoded A.

  • hailstorm - Any storm that produces hailstones that fall to the ground; usually used when the amount or size of the hail is considered significant.

  • haze - Particles suspended in air, reducing visibility by scattering light; often a mixture of aerosols and photochemical smog. Many aerosols increase in size with increasing relative humidity due to deliquescence, drastically decreasing visibility. On Köhler curve plots of saturation relative humidity versus aerosol particle radius, equilibrium haze particles are to the left of the peak, while growing cloud droplets are to the right. Many haze formations are caused by the presence of an abundance of condensation nuclei which may grow in size, due to a variety of causes, and become mist, fog, or cloud. Distinction is sometimes drawn between dry haze and damp haze, largely on the basis of differences in optical effects produced by the smaller particles (dry haze) and larger particles (damp haze), which develop from slow condensation upon the hygroscopic haze particles. Dry haze particles, with diameters of the order of 0.1 m[&mgr;]m, are small enough to scatter shorter wavelengths of light preferentially though not according to the inverse fourth-power law of Rayleigh scattering. Such haze particles produce a bluish color when the haze is viewed against a dark background, for dispersion allows only the slightly bluish scattered light to reach the eye. The same type of haze, when viewed against a light background, appears as a yellowish veil, for here the principal effect is the removal of the bluer components from the light originating in the distant light-colored background. Haze may be distinguished by this same effect from mist, which yields only a gray obscuration, since the particle sizes are too large to yield appreciable differential scattering of various wavelengths. See smaze, arctic haze; compare particulates.

  • head - The energy of fluid per weight units; dimensionally expressed as length unit, for example, Newton ´[×] meters/Newton = meters.

  • heading - The direction toward which an aircraft or ocean vessel is oriented. A heading may be with reference to true north or magnetic north. The heading and course may be different, especially in air navigation, because of drift.

  • heat - 1. (Or heat content.) A form of energy transferred between systems, existing only in the process of transfer. 2. Same as enthalpy. Heat, used as a noun, is confusing and controversial in its scientific meaning. The differential of heat is considered imperfect in that its value depends on the process applied. In the thermodynamic definitions in this glossary, heat is avoided as a noun or adjective except where required by established use. The process of heating is, however, defined as the net absorption of internal energy by a system.

  • heavy rain - Rain with a rate of accumulation exceeding a specific value that is geographically dependent.

  • height - See altitude, elevation.

  • helm - See helm wind.

  • HF - Abbreviation for high frequency. See radio frequency band.

  • hig - (Also called ig.) In England, a sharp, short-lived storm of rain or wind.

  • high - In meteorology, an area of high pressure, referring to a maximum of atmospheric pressure in two dimensions (closed isobars) in the synoptic surface chart, or a maximum of height (closed contours) in the constant-pressure chart. Since a high is, on the synoptic chart, always associated with anticyclonic circulation, the term is used interchangeably with anticyclone. Compare low.

  • high cloud - See cloud classification.

  • high clouds - See cloud classification.

  • horizon - One of several lines or planes used as reference for observation and measurement relative to a given location on the surface of the earth, and referred generally to a horizontal direction (i.e., at right angles to the zenith). Considerable contradiction exists between the nomenclatures for the several concepts of horizon. Aside from the distinctly different geological horizons (strata of earth material), it may be said that there are two types of horizons: earth-sky horizons (1, 2, and 3 below) and celestial horizons (4 and 5 below). Meteorology is primarily concerned with the former, astronomy with the latter. Specifically, the following constitute the major variant usages, with suggested nomenclature along with other names that have been applied. 1) Local horizon: the actual lower boundary of the observed sky or the upper outline of terrestrial objects including nearby natural obstructions. 2) Geographic horizon (also called apparent horizon, local horizon, visible horizon): the distant line along which earth and sky appear to meet. In both popular usage and weather observing, this is the usually conception of horizon. Nearby prominences are said to obscure the horizon and are not considered to be a part of it. The minimum desirable horizon distance should be of the order of three miles. 3) Sea level horizon (also called ideal horizon, sensible horizon, sea horizon, visible horizon, apparent horizon): the apparent junction of the sky and the sea level surface of the earth; the horizon as actually observed at sea. This type of horizon is used as the reference for establishing times of sunrise and sunset. 4) Astronomical horizon (also called sensible horizon, real horizon): the plane that passes through the observer's eye and is perpendicular to the zenith at that point; or, the intersection of that plane with the celestial sphere (i.e., a great circle on the celestial sphere equidistant from the observer's zenith and nadir). It is the projection of a horizontal plane in every direction from the point of orientation. 5) Celestial horizon (also called rational horizon, geometrical horizon, true horizon): the plane, through the center of the earth, that is perpendicular to a radius of the earth that passes through the point of observation on the earth's surface; or, the intersection of that plane with the celestial sphere. See also artificial horizon, fog horizon, haze horizon, smoke horizon.

  • humidity - 1. Generally, some measure of the water vapor content of air. The multiplicity of humidity measures is partly due to different methods of measurement and partly because the conservative measures (mixing ratio, specific humidity) cover an extremely wide dynamic range, as a result of the rapid variation of saturation vapor pressure with temperature. 2. Popularly, same as relative humidity.

  • hurricane - (Many regional names.) A tropical cyclone with 1-min average surface (10 m) winds in excess of 32 m s-1 (64 knots) in the Western Hemisphere (North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and in the eastern and central North Pacific east of the date line). The name is derived from "huracan," a Taino and Carib god, or "hunraken," the Mayan storm god. For a more complete discussion, see tropical cyclone.

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