race—A very rapid current through a comparatively narrow channel.

radiational tide—Periodic variations in sea level primarily related to meteorological changes such as the semidaily (solar) cycle in barometric pressure, daily (solar) land and sea breezes, and seasonal (annual) changes in temperature. Other changes in sea level due to meteorological changes that are random in phase are not considered radiational tides.

range of tide—The difference in height between consecutive high and low waters. The mean range is the difference in height between mean high water and mean low water. The great diurnal range or diurnal range is the difference in height between mean higher high water and mean lower low water. For other ranges see spring, neap, perigean, apogean, and tropic tides; and tropic ranges.

real-time—Pertains to a data collecting system that monitors an on-going process and disseminates measured values before they are expected to have changed significantly.

rectilinear current—Same as reversing current.

red tide (water)—The term applied to toxic algal blooms caused by several genera of dinoflagellates (Gymnodinium and Gonyaulax) which turn the sea red and are frequently associated with a deterioration in water quality. The color occurs as a result of the reaction of a red pigment, peridinin, to light during photosynthesis. These toxic algal blooms pose a serious threat to marine life and are potentially harmful to humans. The term has no connection with astronomic tides. However, its association with the word "tide" is from popular observations of its movements with tidal currents in estuarine waters.

reduction factor (F)—Reciprocal of node factor (f).

reduction of tides or tidal currents—A processing of observed tide or tidal current data to obtain mean values for tidal or tidal current constants.

reference station—A tide or current station for which independent daily predictions are given in the "Tide Tables" and "Tidal Current Tables," and from which corresponding predictions are obtained for subordinate stations by means of differences and ratios. See subordinate tide station (2) and subordinate current  station (2).

relative mean sea level change—A local change in mean sea level relative to a network of bench marks established in the most stable and permanent material available (bedrock, if possible) on the land adjacent to the tide station location. A change in relative mean sea level may be composed of both an absolute mean sea level change component and a vertical land movement change component.

residual current—The observed current minus the astronomical tidal current.

response analysis—For any linear system, an input function Xi(t) and an output function X0(t) can be related according to the formula:

X0(t) = oI4Xi (t – J)W(J)dJ + noise(t) where W(J) is the impulse response of the system and its

Fourier transform:

Z(f) = oI4W (J)e-2BifJ = R(f)eiN(f) is the system's admittance (coherent output/input) at frequency f. In practice, the integrals are replaced by summations; Xi, W, and Z are generally complex. The discrete set of W values are termed response weights; X0(t) is ordinarily an observed tidal time series and Xi(t) the tide potential or the tide at some nearby place. A future prediction can be prepared by applying the weights to an appropriate Xi(t) series. In general: * Z * = R(f) and Tan(Z) = N(f) measure the relative magnification and phase lead of the station at frequency f.

reversing current—A tidal current which flows alternately in approximately opposite directions with a 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. When the movement is towards the shore or up a stream, the current is said to be flooding, and when in the opposite direction, it is said to be ebbing. The combined flood and ebb movement (including the slack water) covers, on an average, 12.42 hours for a semidiurnal current. If unaffected by a nontidal flow, the flood and ebb movements will each last about 6 hours, but when combined with such a flow, the durations of flood and ebb may be quite different. During the flow in each direction the speed of the current will vary from zero at the time of slack water to a maximum about midway between the slacks.

reversing falls—A name applied to falls which flow alternately in opposite directions in a narrow channel in the St. John River above the city of St. John, New Brunswick, Canada, the phenomenon being due to the large range of tide and a constriction in the river. The direction of flow is upstream or downstream according to whether it is high or low water on the outside, the falls disappearing at the half-tide level.

rho (D1)—Larger lunar evectional diurnal constituent. Speed = T – 3s + 3h – p = 13.471,514,5° per solar  hour.

rip—Agitation of water caused by the meeting of currents or by a rapid current setting over an irregular bottom. Termed tide rip when a tidal current is involved. See overfalls.

rip current—A narrow intense current setting seaward through the surf zone. It removes the excess water brought to the zone by the small net mass transport of waves. It is fed by longshore currents. Rip currents usually occur at points, groins, jetties, etc., of irregular beaches, and at regular intervals along straight, uninterrupted beaches.

river current—The gravity-induced seaward flow of fresh water originating from the drainage basin of a river. In the fresh water portion of the river below head of tide, the river current is alternately increased and decreased by the effect of the tidal current. After entering a tidal estuary, river current is the depth-averaged mean flow through any cross- ection. See head of tide and estuary.

river estuary—See estuary.

rotary current—A tidal current that flows 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. The tendency for the rotation in direction has its origin in the Coriolis force and, unless modified by local conditions, the change is clockwise in the Northern Hemisphere and counterclockwise in the Southern. The speed of the current usually varies throughout the tidal cycle, passing through the two maxima in approximately opposite directions and the two minima with the direction of the current at approximately 90° from the directions of the maxima.