PHYSICAL OCEANOGRAPHY 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, 
Fig. 86. — Composite diagram of the normal seasonal variation of temperature at the mouth of Massachusetts Bay, off 
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 because 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 
increasing 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 
