plllng-up of water. The second effect Is the con- 

 vergence of wind-driven currents, which elevates 

 the sea surface along the convergence line. In 

 shallow water, bottom friction and the effects of 

 local topography cause this elevation to persist 

 and may even Intensify It. The low atmospheric 

 pressure that accompanies severe storms causes the 

 third effect, which is sometimes referred to as 

 the "inverted barometer". An inch of mercury is 

 equivalent to about 13.6 inches of water and the 

 adjustment of the sea surface to the reduced pres- 

 sure can amount to several feet at equilibrium. 

 All three of these causes act independently, and 

 if they happen to occur simultaneously, their 

 effects are additive. In addition, the wave can 

 be intensified or amplified by the effects of 

 local topography. Storm waves may reach heights of 

 20 feet or more, and it is estimated that they 

 cause three-fourths of the deaths attributed to 

 hurricanes. Like TSUNAMIS, storm waves or storm 

 surges are often called TIDAL WAVES although they 

 have nothing to do with the tides. (12) 



STRAIN . A measure of the change in size or shape 

 of a body, due to force, referred to its original 

 size or shape. Strain is a non-dimensional quantity, 

 but it is sometimes expressed in inches per inch, 

 etc. 



STRAIN GAGE . The strain gage is an electromechani- 

 cal device which transforms small displacements to 

 changes in resistance which are proportional to the 

 d is plac ement . 



Strain gages are used in ocean bottom pressure 

 measuring equipment. (35) 



STRAIT . A relatively narrow waterway between two 

 larger bodies of water. (11) 



STRANDED FLOE ICE FOOT . See STRANDED ICE FOOT. 

 STRANDED ICE. See FAST ICE. 



STRANDED ICE FOOT . An ice foot formed by the 

 stranding of floes or small icebergs along a shore. 

 It may be built up by freezing spray or breaking 

 seas. Also called stranded floe ice foot. (17) 



STRATH . A broad elongated depression with relative- 

 ly steep walls located on a CONTINENTAL SHELF. The 

 longitudinal profile of the floor is gently undulat- 

 ing with the greatest depths often found in the in- 

 shore portion. (26) 



STRATOCUMULUS . Low clouds (mean upper level below 

 6,500 feet) composed of a layer or patches of 

 globular masses or rolls. The smallest of the 

 regularly arranged elements are fairly large, and 

 are soft and gray, with darker parts. (17) 



STRATUS . A low cloud (mean upper level below 

 6,500 feet) in a uniform layer, resembling fog 

 but not resting on the surface. (17) 



STRAY CURRENT CORROSION . The type of corrosion 

 that occurs when electrical current from some ex- 

 ternal source reaches a metal at one point and 

 leaves It at another. This causes accelerated 

 corrosion where the areas at which the current 

 leaves the metal are in contact with a corrosive 

 solution or substance, such as the earth, wet 

 wood , etc . 



STRAY LINE . Ungraduated portion of line connected 

 with the CURRENT POLE used in taking current obser- 

 vations. The stray line is usually about 100 feet 

 long and permits the pole to acquire the velocity 

 of the current at some distance from the disturbed 

 waters in the immediate vicinity of the observing 

 vessel before the current velocity is read from the 

 graduated portion of the CURRENT LINE. (14) 



STREAM CURRENT. See OCEAN CURRENT. 



STREAMING. ACOUSTIC . Acoustic streaming is the 

 name given to unidirectional flow currents in a 



STREAMLINE . A line whose tangent at any point in 

 a fluid is parallel to the instantaneous velocity 

 of the fluid at that point. The differential 

 equations of the streamlines may be written drxv - 

 0, where dr is an element of the streamline and v 

 the velocity vector; or in Cartesial coordinates, 

 dx/u ■ dy/v "■ dz/w, where u, v, w, are the fluid 

 velocities along the orthogonal x, y, z axes, 

 respectively. In steady-state flow the stream- 

 lines coincide with the trajectories of the fluid 

 particles; otherwise the streamline pattern changes 

 with time. (12) 



STREAMLINE FLOW . See LAMINAR FLOW. 



STRENGTH OF A SOUND SOURCE . The strength of a sound 

 source is the maximum instantaneous rate of volume 

 displacement produced by the source when emitting a 

 wave with sinusoidal time variation. (2) 



STRENGTH OF CURRENT . Phase of tidal current in 

 which the velocity is a maximum; also the velocity 

 at this time. Beginning with the slack before 

 flood in the period of a reversing tidal current, 

 the velocity gradually Increases to the flood 

 strength and then diminishes to another slack, 

 after which the current turns in direction, the 

 velocity increasing to the ebb strength and then 

 diminishing to the slack before flood thus complet- 

 ing the cycle. If it is assumed that the velocity 

 throughout the cycle varies as the ordinates of a 

 cosine curve, it can be shown that the average 

 velocity for an entire flood or ebb period is equal 

 to 2/ IT or 0.6366 of the velocity of the corres- 

 ponding strength of current. (14) 



STRESS . The load on a material divided by the 

 original area of the cross -section through which 

 it acts. Units: psi. Stress, as defined above 



Notural Strain 



and commonly used in connection with routine 

 mechanical tests, is not a true stress. A true 

 stress represents intensity of the internal dis- 

 tributed forces which resist a change in the form 

 of a material, and is equal to the load on the 

 material divided by the instantaneous area of the 

 cross-section through which it acts. 



STRESS CORROSION . Corrosion which is accelerated 

 by stress either residual internal stress or ex- 

 ternally applied stress. 



STRESS CORROSION CRACKING . That cracking which 

 takes place in a metallic structure due to the com- 

 bined effect of tensile stresses and corrosive 

 action. 



STRESS-STRAIN DIAGRAM 

 ted stress vs. strain. 



A graph on which is plot- 

 Such a graph may be con- 



115 



