HYDRODYNAMIC EQUATIONS OF MOTION 



hooked spit — See recurved spit. 



horizon — One of sen'eral lines or planes used as 

 reference for observation and measurement rela- 

 tive to a given location. (5) 



horizontal haul — (or horizontal tow) . The tow- 

 in<i' of one or more nets for predetermined 

 periods at a selected depth or depths. In this 

 type of haul, attempts are made to minimize 

 sam])linp; from other levels. See oblique haul, 

 vertical haul. 



horizontal intensity — The intensity of the hori- 

 zontal component of the magnetic field in the 

 plane of the magnetic meridian. 



horizontal tow — See horizontal haul. 



horse latitudes — The belts of latitude over the 

 oceans at approximately 30° to 35°N and S 

 ■where winds are predominantly calm or very 

 light and weather is hot and dry. 



These latitudes mark the normal axis of the 

 subtropical highs, and move north and south 

 by about 5 degrees following the sun. The two 

 calm belts are known as the calms of Cancer 

 and calms of Capricorn in the Northern and 

 Southern Hemispheres, respectively; and in 

 the North Atlantic Ocean, these are the latitudes 

 of the Sargasso Sea. ( 5 ) 



horst — See fault block. 



hostile ice — From the point of view of the sub- 

 mariner, an ice canopy containing no large ice 

 skylights or other features which permit a sub- 

 marine to surf ace. (7) 



hove-to — The condition in which a ship is kept 

 headed into the wind with no headway or by 

 working engines as necessary. 



Humboldt Current — See Peru Current. 



hummock — 1. A mound or hill in pressure ice; 

 the corresponding submerged portion is called 

 a bummock. 



2. Occasionally, any form of pressure ice 

 which has been smoothed and weathered is 

 called a hummock. 



3. Ice pieces piled one over another on a 

 rather smooth ice surface. (74) 



hummocked ice — Pressure ice, characterized by 

 haphazardly arranged mounds or hillocks 

 ( "hummocks" ) . This has less definite form, and 

 shows the effects of greater pressure than either 

 rafted ice or tented ice, but in fact may de- 

 velop from either of those. When hummocked 

 ice has been weathered and snow-covered it re- 

 sembles similarly metamorphosed rafted ice, the 

 term "hummocked ice" is then applied to both 

 formations. (5) 



hummocky floe — An ice floe of hummocked ice. 

 (68) 



hummocky ice field — An ice field composed of 

 hummocked ice. (68) 



hurricane — A severe tropical cyclone in the 

 Nortli Atlantic Ocean, Caribbean Sea, Gulf of 

 Mexico, and the eastern North Pacific off the 

 west coast of Mexico. (5) 



hurricane delta — A deposit formed in a lagoon 

 by sand carried by storm waves washing across 

 a reef. 



hurricane surge — See hurricane wave, also 

 storm surge. 



hurricane tide — See hurricane wave. 



hurricane wave — (also called 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 

 associated more and more with only the dan- 

 gerous semicircle. See storm surge, tsunami. 



(5) 



Huygen's principle — A very general principle 

 applying to all forms of wave motion which 

 states that every point on the instantaneous posi- 

 tion of an advancing phase front (wave front) 

 may be regarded as a source of secondary 

 spherical "wavelets". The position of the phase 

 front a moment later is then determined as the 

 envelope of all of the secondary wavelets (ad 

 infinitiimn) . 



This principle, stated by the Dutch physicist 

 Christian Huygens (1629-1695), is extremely 

 useful in understanding effects due to refrac- 

 tion, reflection, diffraction, and scattering, of 

 all types of radiation, including sonic radiation 

 as well as electromagnetic radiation and ap- 

 plying even to ocean wave propagation. (5) 



hydraulic current — Gravity flow through a 

 channel that results from a difference between 

 water levels at the two ends of the channel 

 because of a difference in phase and/or range 

 of the tide. (50) See head. 



hydraulic gradient — 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. 



(5) 



hydraulic jump — A steady-state, finite-ampli- 

 tude disturbance in a channel, in which water 

 passes turbulently from a region of (uniform) 

 low depth and high velocity to a region of 

 (uniform) high depth and low velocity. When 

 applied to hydraulic jumps, the usual hydraulic 

 formulas governing the relations of velocity and 

 depth do not conserve energy. See also bore. 

 (5) 



hydraulic radius — The quotient of the cross- 

 sectional area of a channel (below the water 

 surface) divided by the wetted perimeter. (5) 



hydrodynamic equations of motion — Three 

 equations which express the net acceleration of 

 a unit water particle as the simi of the five 

 partial accelerations due to pressure gradient 

 force, earth's deflecting force, frictional force, 

 geostrophic and hydrostatic equations are 



79 



