330 PHYSIOGRAPHY 



(fresher) than the water about it, the lighter water spreads out on 

 the surface, for the same reason that oil spreads on water. Since 

 variations in the saltness are being produced all the time, motion 

 due to unequal density is constant. Movements brought about 

 in this way are usually very slow, and may be called creep. 



Salinity and color. The sea is generally blue or green, but its 

 color varies from place to place and from time to time. The blue 

 is deeper where the amount of salt is great. Thus inland seas, such 

 as the Mediterranean, which are more salty than the open ocean, 

 are of deeper blue. The cold and less salty waters of high latitudes 

 are often distinctly green. Many of the variations of color are due 

 to the tiny particles of solid matter in suspension in the water. 

 Microscopic animals and plants, and the sediment washed or blown 

 out from the land, or furnished by volcanoes beneath the sea, all 

 help to give the sea-water of different places its different colors. 



THE TEMPERATURE OF THE SEA 



Temperature of the surface. The surface of the ocean, like 

 that of the land, is warmer near the equator and cooler toward 

 the poles (Fig. 216, p. 242). Near the equator its temperature is about 

 80 F.; near the poles, where not frozen, it is about 28 F. When 

 the temperature sinks below the latter figure, the sea-water freezes, 

 and the surface of the ice may become as cold as the air above it; 

 but the temperature of the water just beneath the ice is never 

 much below 28 F. The decrease of temperature toward the poles 

 is by no means regular, as shown by the isothermal charts. In 

 Fig. 216-218, for example, the isothermal lines over the ocean 

 are not parallel with the parallels of latitude. 



In the open ocean, ocean currents make the isotherms depart 

 from the parallels. Some of these currents are of cold water flow- 

 ing into warmer water. These are cold currents. Some of them 

 are of warm water flowing into cooler water. These are warm 

 currents. A cold current turns an isotherm toward the equator, 

 and a warm current turns it toward the pole. Ffg. 217 shows the 

 effect of a warm current in the North Atlantic on the position of the 

 isotherms. 



