PHYSICAL OCEANOGKAPHY OF THE GULF OF MAINE 



663 



In this region (fig. 86) the most obvious seasonal change is the very rapid 

 warming of the surface, which takes place from the end of the winter until about 

 the end of July, resulting (on the average) in a rise of nearly 17°. After the first 

 month or so of vernal warming (March to April) , during which the whole column 

 warms nearly uniformly, the rate at which the temperature rises becomes inversely 

 proportional to the depth; and it so continues throughout the spring and summer, 



/^av Dec. Jan fei^. /^crr /^pr /Vay June Ju/y Aug. Sep/: Oct 



Fig. 86.— Composite diagram of the normal seasonal variation of temperature at the mouth of Massachusetts Bay, ofl 

 Gloucester, at the surface, 20 meters, 40 meters, 70 meters, and 100 meters. The curves are smoothed. The station 

 for August 9, 1923, is omitted hecause the water between the 20 and 150 meter levels was much colder that summer 

 than usual, after an unusually cold winter 



primarily because the source of heat is from above and secondarily because the ver- 

 tical circulation is not sufficiently active to prevent a constant increase in vertical 

 stability as the upper strata becomes warmer and warmer. The steadily widening 

 spread between the curves for the surface and for the 20-meter level thus mirrors 

 iucreasing stability. The result of this partial insulation of the deeper strata from 

 the penetration of heat from above is that the maximum temperature for the year is 



