HURRICANE SURGE . See HURRICANE WAVE. 

 HURRICANE TIDE . See HURRICANE WAVE. 



HURRICANE WAVE (HURRICANE SURGE , HURRICANE TIDE) . 



As experienced on islands and along a shore, 

 a sudden rise in the level of the sea associated 

 with a hurricane. 



In low latitudes, the hurricane wave appears 

 to occur in the proximity of the storm's center. 

 As the hurricane moves into higher latitudes, 

 however, the maximum wave appears to become asso- 

 ciated more and more with only the dangerous 

 semicircle. (12) 



HUYGENS'S PRINCIPLE . A very general principle ap- 

 plying to all forms of wave motion which states 

 that every point on the instantaneous position of 

 an advancing phase front (wave front) may be re- 

 garded as a source of secondary spherical "wave- 

 lets." The position of the phase front a moment 

 later is then determined as the envelope of all of 

 the secondary wavelets (ad infinitum) . 



This principle, stated by the Dutch physicist 

 Christian Huygens (1629-1695), is extremely useful 

 in understanding effects due to refraction, re- 

 flection, diffraction, and scattering, of all types 

 radiation, including sonic radiation as well as 

 electromagnetic radiation and applying even to 

 ocean wave propagation. (24) 



HYALINE . Glassy or semitransparent . Hyaline ani- 

 mals in the sea, living from the surface to more 

 than 300m, are extremely abundant. (19,13) 



HYDRAULIC CURRENT . A current in a channel that 

 results from a difference in the surface level at 

 the two ends. Such a current may be expected in a 

 strait connecting two bodies of water in which the 

 tides differ in time or range. The current in the 

 East River, New York, connecting Long Island Sound 

 and New York Harbor, is an example. (14) 



HYD RAULIC GRADIE NT. The slope of the profile of 

 the static level for a hydraulic system. In open 

 channel flow the hydraulic gradient is the slope 

 of the water surface taken parallel to the flow; 

 in unconfined ground water flow it is the slope of 

 the water table taken normal to its contours ; and 

 for artesian ground water it is the slope of the 

 PIEZOMETRIC surface taken normal to its contours, 

 (24) 



HYDRAULIC JUMP . In fluid flow, a change in flow 

 conditions accompanied by a stationary, abrupt tur- 

 bulent rise in water level in the direction of 

 flow. A type of STATIONARY WAVE. (11) 



HYDRAULIC RADIUS . The ratio of the area of a cross 

 section of a stream to its wetted perimeter. (14) 



HYDRO . Hydrographic Office. (Now officially 

 designated Navy Oceanographic Office) . 



HYDRODIST . A navigation positioning system which 

 is actually a modification of the TELLUROMETER sys- 

 tem. In Hydrodist, two separate tellurometer mas- 

 ters are mounted side by side on board a vessel 

 and these aimed to their respective remote units. 

 The Hydrodist system is used for hydrographic 

 control. (29) 



HYDROGRAPHIC WIRE SLOPE AND AZIMUTH INDICATOR . 

 This mechanical instrument enables determination 

 of the positioning of hydrographic instruments 

 attached to a hydro wire, by furnishing information 

 concerning the wire slope and set. In this indi- 

 cator, a vertically orienting and magnetic north- 

 seeking unit of practically neutral buoyancy is 

 housed spherically to allow the inner unit to 

 preserve its northward and vertical orientation, 

 while magnetic needles attached to this unit cause 

 it to seek magnetic north. The spherical shape 



of the inner member permits its being locked 

 accurately, relative to the wire orientation. (30) 



HYDROGRAPHY . The science which deals with the 

 measurement of the physical features of the oceans, 

 seas, lakes, rivers, and other waters, and their 

 marginal land areas, with special reference to the 

 elements that affect safe navigation, and the publi- 

 cation of such information in a suitable form for 

 use of navigators. (17) 



HYDROLOGY . The scientific study of the waters of 

 the earth, especially with relation to the effects 

 of precipitation and evaporation upon the occur- 

 rence 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 precipi- 

 tation to re-evaporation or return of the water to 

 the seas . 



Applied hydrology utilizes scientific findings 

 to predict rates and amounts of runoff (river fore- 

 casting), estimate required spillway and reservoir 

 capacities, study soil-water-plant relationships 

 in agriculture, estimate available water supply, 

 and for other applications necessary to the manage- 

 ment of water resources. (12) 



HYDROMETER . An instrument used to determine the 

 density or specific gravity of a liquid. 



HYDROPHONE. The hydrophone is an electro-acoustic 

 transducer that responds to water borne sound 

 waves and delivers essentially equivalent electric 

 waves. The conversion from sound energy to elec- 

 trical energy is achieved through the use of either 

 the piezoelectric or magnetos trie tive effect. The 

 varying potential generated across a piezoelectric 

 material when it is subjected to a varying mechani- 

 cal force can be coupled to an amplifier as an 

 electric signal with the same frequency character- 

 istics as those of the mechanical vibration that 

 excited the material. (30) 



HYDROPHONE LOSS . The hydrophone loss of a sonar 

 transducer, used for the reception of acoustic 

 energy, at a specified frequency, may be defined 

 as the transmission loss measured by the ratio of 

 (1) the source power of the free-field acoustic 

 energy available as plane sinusoidal waves from a 

 water surface having an area of one square centi- 

 meter and lying perpendicular to the direction of 

 the maximum response reference axis of the trans- 

 ducer at the point to be occupied by its effective 

 center to (2) the resulting output power of the 

 electric energy available from the transducer. (4) 



HYDROPHOTOMETER . A sensitive instrument used in 

 water transparency and light absorption measure- 

 ments at sea. The instrument, which contains its 

 own light source, can measure fine graduations of 

 transparency of an individual water mass. (35) 



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. (12) 



HYGROMETER . An instrument which measures the water 

 vapor content of the atmosphere. There are six 

 basically different means of transduction used in 

 measuring this quantity and hence an equal number 

 of types of hygrometers. These are: (a) the 

 psychrometer , which utilizes the thermodynamic 

 method; (b) the class of instruments which depends 

 upon a change of physical dimensions due to the 

 absorption of moisture (hair hygrometer, torsion 

 hygrometer, goldbeater 's -skin hygrometer, carbon- 

 film hygrometer element); (c) those which depend 

 upon condensation of moisture (DEW-POINT HYGROMETER); 

 (d) the class of instruments which depend upon the 

 change of chemical or electrical properties due to 

 the absorption of moisture (absorption hygrometer, 

 electrical hygrometer, carbon-film hygrometer 



58 



