H Hail Precipitation composed of balls or irregular lumps of ice with diameters between 5 and 50 mm.     Hair hygrograph A recording hair hygrometer.     Hair hygrometer A hygrometer in which the sensitive element is a strand or strands of human hair, the length of which is a function of the relative humidity of the air.     Half duplex Operation mode of a communication circuit in which each end can transmit and receive, but not simultaneously.     Hardware Physical equipment used in data processing. Compare to firmware, software.     Haze Fine dust or salt particles dispersed through a portion of the atmosphere; a type of lithometer. The particles are so small they cannot be felt or seen with the naked eye. 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.     Hazemeter Name sometimes given to a transmissometer.      Head wind A wind blowing in a direction opposite to the heading of a moving object, thus opposing the object's intended progress; the opposite of a tailwind.     Heliograph An instrument which records the duration of sunshine and gives a quantitative measure of the amount of sunshine by the action of the sun's rays upon blueprint paper. A type of sunshine recorder.      Heliostat A clock-driven instrument mounting which automatically and continuously points in the direction of the sun. It is used with a pyrheliometer when continuous direct solar radiation measurements are required.      Heliotropic wind A subtle, diurnal component of the wind velocity leading to a diurnal shift of the wind or turning of the wind with the sun, produced by the east-to-west progression of daytime surface heating.      Helium (He) An inert gas. A colorless, monatomic element which is found to occur in dry air to the extent of only 0.000524 percent by volume. Helium is very light, having a molecular weight of only 4.003 and specific gravity referred to air of 0.138. Because helium is non-inflammable and has a lifting power 92 percent of that of hydrogen, it is widely used as the inflation gas for meteorological balloons.     Hexadecimal A numbering system using a base number of 16 and including the ten decimal digits (0 to 9) along with six alpha digits (A to F). Thus, a digit is available to represent each of the possible values of a 4-bit binary digit.     High An area of high barometric pressure, with its attendant system of winds; an anticyclone.      Hi-reference signal The audio-frequency signal transmitted by the Diamond-Hinman radiosonde when the baroswitch pen passes each fifteenth contact of the commutator, up to a number determined by the design of the commutator, and each fifth contact thereafter. This signal is transmitted so that the pressure, temperature, and humidity may be more readily distinguished.     Histogram A graphical representation of a frequency distribution. The range of the variable is divided into class intervals for which the frequency of occurrence is represented by a rectangular column. The height of the column is proportional to the frequency of observations within the interval.     Holosteric barometer Same as aneroid barometer. Holosteric means wholly made of solids, while aneroid means devoid of liquid.     Hook gauge An instrument used to measure changes in the level of the water in an evaporation pan. The gauge is normally placed in a stillwell and adjusted so that the point of the hook just breaks the water surface. The change in water level is read on the attached micrometer.     Hot film anemometer Anemometer which measures wind speed by measuring the degree of cooling of a metal film heated by an electric current. A type of cooling-power anemometer.     Hot wire anemometer Anemometer which measures wind speed by measuring the degree of cooling of a metal wire heated by an electric current. A type of cooling-power anemometer.     Humidity Water vapor content of the air. See absolute humidity, dew point, mixing ratio, relative humidity, specific humidity.     Humidity coefficient A measure, proposed by Angstrom, of the precipitation effectiveness of a region.      Humidity element The transducer of any hygrometer, i.e. that part of a hygrometer that quantitatively "senses" atmospheric water vapor.      Humidity strip The humidity transducing element in a Diamond-Hinman radiosonde. Also called electrolytic strip.     Hurricane balloon See hurricane beacon.     Hurricane beacon An air-launched balloon designed to be released in the eye of a tropical cyclone, float within the eye at predetermined levels, and transmit radio signals for RDF positioning.     Hurricane-force wind Wind with a speed above 64 knots (73 mph); Beaufort scale numbers 12 through 17.     Hurricane wind In general, the severe wind of an intense tropical cyclone (hurricane or typhoon). The term has no further technical connotation, but, unfortunately, is easily confused with the strictly defined hurricane-force wind.     Hydrogen (H) A colorless and odorless gaseous element. The lightest and apparently the most abundant chemical element in the universe. However, it is found only in trace quantities in the observable portion of our atmosphere, only about 0.00005 percent by volume of dry air. Hydrogen has a molecular weight of 2.0160 and specific gravity referred to air of 0.0695. At one time hydrogen was the commonly used inflation gas for meteorological balloons, but because of its dangerous combustibility, it has been largely replaced by helium.     Hydrograph A graphical representation of stage or discharge at a point on a stream as a function of time.       Hydrography The study of waters (including oceans, lakes, and rivers) embracing either: (a) their physical characteristics, from the standpoint of the oceanographer or limnologist; or (b) the elements affecting safe navigation, from the point of view of the mariner. Compare to hydrology.     Hydrologic accounting A systematic summary of the terms (inflow, outflow, and storage) of the storage equation as applied to the computation of soil-moisture changes, ground-water changes, etc. An evaluation of the hydrologic balance of an area. Also called basin accounting, water budget.     Hydrologic balance Generally, the relative states of inflow, outflow, and storage of moisture over a given area of earth's surface.     Hydrologic cycle The succession of stages through which water passes on the ground and in the atmosphere: evaporation from land or bodies of water, condensation to form clouds, precipitation, accumulation in the soil or in bodies of water, and re-evaporation.     Hydrologic year Same as water year.     Hydrology The scientific study of the waters of the earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of water in streams, lakes, and on or below the land surface. In terms of the hydrologic cycle, the scope of hydrology may be defined as that portion of the cycle from precipitation to re-evaporation or return to the water of the seas. Applied hydrology utilizes scientific findings to predict rates and amounts of runoff (river-forecasting), estimate required spillway and reservoir capacities, study soil-water-plant relationships in agriculture, estimate available water supply, and for other applications necessary to the management of water resources. Compare to hydrography.     Hydrometeor A general term for atmospheric water in any of its forms, i.e. clouds, fog, hail, ice crystals, rain.      Hydrometer An instrument used for measuring the specific gravity of a liquid.      Hydrophotometer An instrument for measuring the extinction coefficient in water.     Hydrosphere The water portion of the earth as distinguished from the solid part, called the lithosphere, and from the gaseous outer envelope, called the atmosphere.     Hyetal Of or pertaining to rain.      Hygristor A modification of the dew cell used in radiosonde equipment.      Hygrodeik A form of psychrometer with wet-bulb and dry-bulb thermometers mounted on opposite sides of a specially designed graph of the psychrometric tables. It is so arranged that the intersections of two curves determined by the wet-bulb and dry-bulb readings yield the relative humidity, dew-point, and absolute humidity.      Hygrogram The record made by a hygrograph.     Hygrograph A hygrometer which includes an arrangement for the time recording of atmospheric humidity.     Hygrometer An instrument used to measure the water vapor content of the air.      Hygroscope An instrument that shows changes in humidity.      Hygroscopic Readily taking up and retaining moisture.      Hygrothermograph An instrument resulting from the combination of a thermograph and a hygrograph and furnishing, on the same chart, simultaneous time recording of ambient temperature and humidity.      Hygrothermoscope Apparatus using the combined simultaneous action of a bimetallic thermometer and a hair hygrometer to move a needle in front of a divided scale. Its construction permits dew point variations to be indicated approximately.     Hypsometer An instrument used to determine atmospheric pressure or elevation by observing the boiling point of water or other liquids. The sensitivity of the hypsometer increases with decreasing pressure, making it more useful for high altitude work.      Hysteresis The maximum difference in output for any given input (within the specified range) when the value is approached first with increasing, and then with decreasing, input signals. Caused by energy absorption in the elements of the measuring instrument. Usually expressed as a percentage of full-scale range.      Hythergraph A type of climatic diagram whose coordinates are some form of temperature vs. a form of humidity or precipitation.