Page 579 radio acoustic ranging 6321 



convection water currents, thus mixing it with the layer below. In the colder months 

 there will often be a layer of cold water overlying a layer of warmer water, below which 

 the temperature will gradually decrease with depth, also forming a temperature gradient 

 reversal. 



d. Seasonal temperature distribution. — -An annual cycle of the average monthly 

 vertical distribution of temperature in Monterey Bay, California, to a depth of 55 

 fathoms is shown in figure 129. The observations were made in 1933 at a station 

 inshore, where the temperatures are much affected by wind currents. The cycle is 

 not necessarily representative of offshore temperature curves. The lag in the tem- 

 perature of the water with reference to the seasons is attributed partly to the fact 

 that when the temperature of the water rises, evaporation tends to check the rapidity 

 of the rise; and when it falls, condensation of water vapor in the layer of air overlying 

 the surface checks the rapidity of the fall. 



e. Diurnal variation. — In addition to the seasonal change there is a diurnal 

 variation in the temperature of the surface water. During the day the temperature 

 of a very thin surface layer of water is increased a few degrees, depending on the latitude 

 and the time of year. This may be called a diurnal thermocline. Unless the surface 

 of the water is disturbed so that the heat is transferred downward, most of this heat 

 is lost during the night. This diurnal variation in temperature has little appreciable 

 effect Oil the velocity of sound and is generally disregarded. 



/. Temperature ranges in onshore areas. — The approximate extreme ranges of 

 temperature in the offshore waters adjacent to continental United States and Alaska 

 are shown in figure 130. The Alaska curves are from serial temperatures (see 6321) 

 taken in the northern part of the Gulf of Alaska. The Pacific Ocean curves are for 

 locations offshore in the approximate latitude of San Francisco and they probably 

 represent average conditions for that locality. The curves for the Gulf of Mexico are 

 from observations in the western part. They are very similar to the curves for the 

 waters adjacent to the Hawaiian Islands. The curves for the Atlantic Ocean represent 

 conditions off the Continental Shelf in the approximate latitude of Washington, D. C. 

 The Atlantic Ocean curves are interesting; two of them represent the approximate 

 extreme ranges and the third an extreme temperature gradient reversal. 



6321. Temperature Observations 



The observations required to determine the vertical distribution of temperature 

 and salinity with depth at any one place in the ocean are known as a serial temperature. 

 The temperatures must be measured in situ, and water samples, from which salinities 

 may be determined, must be trapped at various depths and brought to the surface 

 uncontaminated by water from other depths. The temperature is measured and the 

 water samples are obtained with special instruments designed for the purpose which 

 are described in section 47. 



The maneuvers of the ship and the precautions to be observed while measuring a 

 serial temperature are generally the same as those required while taking a wire sound- 

 ing, as described in 3422. The oceanographic instruments used by the Coast and 

 Geodetic Survey are designed to be clamped on a line larger in diameter than stranded 

 sounding wire or piano wire, so a few fathoms of leadline must be placed between 

 the end of the sounding wire and the lead, to which they may be clamped. The water 

 specimen cup is clamped about 1 fathom above the top of the lead and the reversing 



