Manual of Current Observations 
General Explanation 
1. For the purposes of this publication current may be considered as the horizontal 
movement of the water as distinguished from the vertical rise and fall of the tide. 
Current is measured by its velocity while the tide is measured by its height. Currents 
may be classified as tidal and nontidal but the actual current found in any locality is 
usually a resultant of both tidal and nontidal movements. Tidal currents are periodic 
and usually arise from astronomical causes. Nontidal currents are not periodic and 
are largely the results of meteorological conditions. Some currents may have their 
origin in meteorological conditions which have a rough periodicity, and so far as these 
currents are periodic they will be classified as tidal currents for the purpose of reduction. 
2. Tidal currents are a part of the same general movement of the sea that is mani- 
fested in the tides by a vertical rise and fall. This movement may be partly of a 
progressive-wave type and partly of a stationary-wave type. <A progressive wave is one 
whose crest advances along a waterway, the times of high and low waters becoming 
progressively later and later as the Wave moves forward. A stationary wave is one 
that oscillates about an axis, the high water occurring simultaneously over the entire 
area on one side of the axis while it is low water on the other side. 
3. The tidal currents have the same periods as the tides and are subject to similar 
variations arising from changes in astronomical conditions. Thus the tidal currents 
may be semidiurnal, diurnal, or mixed, according to the type of tide in the same general 
area. ‘They tend to increase in velocity at the times of spring and perigean tides and to 
decrease at the times of neap and apogean tides and they show an increased diurnal 
inequality in velocity at the times of the tropic tides. 
4, Tidal currents may be classified as reversing and rotary. Reversing currents are 
those which flow alternately in approximately opposite directions with a period of 
slack water at each reversal of direction. Currents of this type usually occur in rivers 
and straits where the direction of flow is more or less restricted to certain channels. 
Rotary currents are those which flow continually with the direction of flow changing 
through all points of the compass during the tidal period. Rotary currents are usually 
found offshore where the direction of flow is not restricted by any barriers. 
5. Nontidal currents may be considered under two classes, permanent and tempo- 
rary. Permanent currents are those which flow more or less continuously in one direc- 
tion and include such well-known features of the oceanic circulation as Equatorial 
Current, Gulf Stream, Japan Current, etc., and also the current in a river due to the 
fresh water discharge. Temporary currents are those of a temporary character which 
are caused principally by changes in meteorological conditions. Currents created by 
the action of the wind form the most common example. 
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