ROLL 



roll — See ship motion. 



roll angle — See angle of roll. 



roller — 1. An indefinite term, sometimes con- 

 sidered to denote one of a series of long-crested, 

 large waves which roll in upon a coast, as after a 

 storm. 



2. Large breakers on exposed coasts formed 

 by swell coming from a great distance. 



3. See comber. 



ropak — An extreme formation of ridged ice; a 



pinnacle or slab of heavy sea ice standing verti- 

 cally on edge. (59) 



rorqual — Any baleen whale of the family Bal- 

 aenopteridae, a group characterized by having a 

 triangular dorsal fin and a series of parallel 

 grooves running longitudinally on the under sur- 

 face of the throat and chest region ; for example, 

 the blue whale, fin whale, sei whale, minke 

 whale, and humpback whale. 



Rossel Current — A seasonal current flowing west- 

 ward and northwestward along both the south- 

 ern and northeastei'n coasts of New Guinea 

 during May to September; the southern part 

 flows through Torres Strait and loses its identity 

 in the Araf ura Sea, and the northern part curves 

 northeastward to join the Pacific Equatorial 

 Countercurrent. The Rossel Current is a weak 

 branch of the South Equatorial Current. 

 During the Northern Hemisphere winter it is 

 replaced by an east-flowing current from the 

 Indian Ocean. 



rot — See disintegration. 



rotary current — A tidal current that flows con- 

 tinually, with the direction of flow changing 

 through all points of the comjDass during a tide 

 cycle. Rotary currents usually occur offshore 

 where the direction of flow is not restricted; 

 unless modified by local conditions, the change 

 in direction is generally clockwise in the North- 

 ern Hemisphere and counterclockwise in the 

 Southern Hemisphere. The speed of the cur- 

 rent usually varies throughout the tide cycle, 

 passing through two maximums in approxi- 

 mately opposite directions and two minimums 

 where the direction of the current is approxi- 



mately 90 degrees from the direction at time of 

 maximum speed. {See figure for current 

 ellipse.) 



rotten ice — (also called spring sludge). Ice that 

 has become honeycombed in the course of melting 

 and is in an advanced state of disintegration. 

 (74) 



Rotten ice may appear transparent (and thus 

 dark) when saturated with sea water, and thus 

 may easily be confused with newly forming 

 black ice. /See candle ice. (5) 



rough ice — An expanse of ice having an uneven 

 surface caused by pressure ice formations or 

 growlers frozen in place. (59) 



rubber ice — Elastic young sea ice, not strong 

 enough to bear the weight of a man standing 

 still. (59) Rare. 



rubble — 1. Fragments of hard sea ice, roughly 

 spherical and up to 5 feet in diameter, resulting 

 from the disintegration of larger ice forma- 

 tions. When afloat, commonly called brash ice. 

 (59) 

 2. Loose angular rock fragments. 



rudite — (or rudyte). Rock or sediment deposits 

 composed of grains larger than 2 millimeters. 

 See psephite. 



rudyte — See rudite. 



runnel — A trough or corrugation formed in the 

 foreshore or in the bottom, immediately off- 

 shore, formed by waves or tidal currents. (2) 



running ice — Ice in motion or capable of rapid 

 drifting. (59) Rare. 



running mean — See consecutive mean. 



runoff — The water derived from precipitation that 

 ultimately reaches stream channels ; has a direct 

 influence upon the volume of river discharge. 



runoff cycle — The part of the hydrologic cycle 

 undergone by water between the time it reaches 

 the land as precipitation and its subsequent 

 evapotranspiration or discharge through stream 

 channels. (5) 



run-up — (or wprush). The rush of water up a 

 structure on the breaking of a Avave. The 

 amount of run-up is the vertical height above 

 still water level that the rush of water reaches. 

 (61) 



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