summer hemisphere when they are very deep, extend- 

 ing to at least 8 to 10 km altitude, and generally 

 not topped by upper westerlies. If upper wester- 

 lies are present they are too weak and shallow to 

 influence the weather. 



In the winter hemisphere, these easterlies are 

 restricted to a narrow belt along the equator. (24) 



EQUATORIAL TIDES . Tides occurring semimonthly as 

 the result of the moon being over the equator. At 

 these times the tendency of the moon to produce a 

 diurnal inequality in the tide is at a minimum. 

 See also DECLINATION. (14) 



EQUATORIAL TROUGH . 1. The quasi-continuous belt 

 of low pressure lying between the SUBTROPICAL HIGH- 

 PRESSURE BELTS of the Northern and Southern Hemi- 

 sphere, This entire region is one of very homogene- 

 ous air, probably the most ideally BAROTROPIC 

 region of the atmosphere. Yet, humidity is so high 

 that slight variations in stability cause major 

 variations in weather. 



The position of the equatorial trough is fair- 

 ly constant in the eastern portions of the Atlantic 

 and Pacific; but it varies greatly with season in 

 the western portions of those oceans and in southern 

 Asia and the Indian Ocean. It moves into or toward 

 the summer hemisphere. 



It has been suggested that this name be adopted 

 as the one general term for this region of the at- 

 mosphere. Thus, the equatorial trough would be 

 said to contain regions of DOLDRUMS ; portions of 

 it could be described as INTERTROPICAL CONVERGENCE 

 ZONES; and within it there might be detected INTER- 

 TROPICAL FRONTS. However, one weakness of this 

 nomenclature is that it alludes specifically and 

 only to the existence of a TROUGH of low pressure. 

 Perhaps an even more general term might be prefer- 

 able, for example, atmospheric equator. 



2. Same as meteorological equator. (14) 



EQUATORIAL WESTERLIES . The westerly winds occa- 

 sionally found in the equatorial trough and sepa- 

 rated from the mid-latitude westerlies by the 

 broad belt of easterly trade winds . 



As the air flow in the lower atmosphere is 

 mostly easterly in and about the equatorial trough, 

 the existence of westerlies on mean charts in some 

 areas has been a subject of much interest and spec- 

 ulation. In some regions, this abnormality can be 

 explained as the result of limited areas of west 

 winds on the equatorward side of frequent westward- 

 moving cyclones in the equatorial trough. Else- 

 where (notably over the Indian Ocean during the 

 Northern Hemisphere summer), the equatorial wester- 

 lies may result from the deflection of Southern 

 Hemisphere air as it flows northward across the 

 geographical equator as part of the monsoon. (24) 



EQUILIBRIUM THEORY . A hypothesis under which it is 

 assumed that the waters covering the face of the 

 earth instantly respond to the tide-producing 

 forces of the moon and sun and form a surface of 

 equilibrium under the action of these forces. The 

 theory disregards friction and inertia and the ir- 

 regular distribution of land masses of the earth. 

 The theoretical tide formed under these conditions 

 is known as the equilibirum tide. (14) 



EQUILIBRIUM TIDE . See EQUILIBRIUM THEORY. (14) 



EQUIVALENT NOISE PRESSURE (INHERENT NOISE PRESSURE) . 

 The equivalent noise pressure of an electroacoustic 

 transducer or system used for sound reception is 

 the root-mean-square sound pressure of a sinusoidal 

 plane progressive wave, which, if propagated paral- 

 lel to the principal axis of the transducer, would 

 produce an open-circuit signal voltage equal to the 

 root-mean-square of the inherent open-circuit noise 

 voltage of the transducer in a transmission band 

 having a band width of 1 cycle per second and cen- 

 tered on the frequency of the plane sound wave. (1) 



EQUIVALENT NOISE PRESSURE LEVEL . The equivalent 

 noise pressure level, in decibels, of a transducer 

 used for sound reception, is 20 times the logarithm 

 to the base 10 of the ratio of the equivalent noise 

 pressure to the stated reference pressure. (1) 



EQUIVALENT VISCOUS DAMPING . Equivalent viscous 

 damping is a value of viscous damping assumed for 

 the purpose of analysis of a vibratory motion, such 

 that the dissipation of energy per cycle at reso- 

 nance is the same for either the assumed or actual 

 damping force. (2) 



EROSION . The phenomenon of wearing away the sur- 

 face of a material by the hydraulic action of a 

 moving stream of fluid. 



EROSION, BASE LEVEL OF . The lowest level to which 

 a river can erode its bed is called the base level. 

 The sedimentary transporting power is minimal or 

 has reached equilibrium at this level. (27) 



EROS ION- CORROS ION . This is the combination of 

 erosion phenomenon and corrosion phenomenon which 

 produces a high rate of local attack of the base 

 metal. Since many materials are corrosive resis- 

 tant because of a protective oxide film adhering 

 to their surface, when corrosive fluids strike 

 this film it is eroded away leaving the base metal 

 unprotected from corrosion. (35) 



EROSION SURFACE . An area which has been flattened 

 by subaerial or marine erosion to form an area of 

 relatively low relief at an elevation close to the 

 base-level (sea-level) existing at the time of its 

 formation. Relics of such surfaces may now be 

 found far above sea-level owing to the falling 

 base-level... or below the present ocean surface. 

 (27) 



ERYSIPELOTHRIX . An organism of the sea which can 

 cause skin lesions in humans. 



ESCARPMENT (SCARP) . A more or less continuous line 

 of cliffs or steep slopes facing in one general 

 direction which are caused by erosion or faulting. 

 (27) 



ESHP . Equivalent Shaft Horse Power. 



ESTUARY. (1) That portion of a stream influenced 

 by the tide of the body of water into which it 

 flows; (2) A bay, as the mouth of a river, where 

 the tide meets the river current. (11) 



ETESIANS . The prevailing northerly winds in summer 

 in the eastern Mediterranean and especially the 

 Aegean Sea; basically similar to MONSOON and equiva- 

 lent to the MAESTRO of the Adriatic Sea. 



According to the ancient Greeks, the etesians 

 blow for forty days beginning with the heliacal 

 rising of Sirius. They are associated (along with 

 the seistan and shamal) with the deep low pressure 

 area which forms in summer over northwest India. 

 They bring clear skies and dry, relatively cool 

 weather. 



In Greece the etesian wind is locally called 

 the Sleeper. In Turkey it is the Meltem. The 

 Romans used the word also for the southwest monsoon 

 of the Arabian Sea. (12) 



EULER VELOCITY FIELD . The Euler method assumes 

 that the velocity of all particles of the fluid 

 has been defined. On this assumption the velocity 

 field is completely described if the components 

 of the velocity can be represented as function of 

 the coordinate and time. 



EULERIAN COORDINATES . Any system of coordinates in 

 which properties of a fluid are assigned to points 

 in space at each given time, without attempt to 

 identify individual fluid parcels from one time to 

 the next. Since most observations in meteorology 



1*3 



