680 BULLETIN OF THE BUKEATJ OF FISHERIES 



In winter, when the coastal belt is the coldest part of the gulf, the dominant cir- 

 culation tends to carry low temperatures from the western shores out over the central 

 part of the basin, an effect illustrated by the distribution of temperature in Massa- 

 chusetts Bay in February, 1925 (p. 658). 



THERMAL EFFECTS OF EVAPORATION 



The warming of the surface stratum of the gulf by solar radiation is constantly 

 opposed by the draft of heat from the water as the latter evaporates. Quantitative 

 statement of the cooling of the water which this process actually effects over the gulf 

 is not yet possible, but such observations as have been made on the comparitive rapid- 

 ity of evaporation of salt and fresh waters, and the actual measurements of the latter at 

 land stations around the coast of the gulf, afford a rough picture of the order of mag- 

 nitudes involved. 



The latent heat of vaporization of fresh water depends to some small extent on 

 the temperature at which evaporation takes place; the average for the range pre- 

 vailing in the surface waters of the gulf of Maine (0° to 20°) is about 585 to 595 

 calories.^^ 



I know of no determinations of the latent heat of evaporation for salt water, but 

 probably it does not differ greatly from the above. The annual evaporation of a 

 blanket of water about 0.7 meters thick from the surface of the Gulf of Maine, which 

 is probably close to the truth (p. 842), would thus take enough heat from the upper 50 

 meters to cool the latter by about 8° if all the necessary energy were drawn from 

 the water. Actually, however, a large part is supplied by direct solar radiation as it 

 strikes the surface (p. 677), proportionately reducing the draft of heat made from the 

 underlying water by the process of evaporation. No measurements of what percentage 

 of the heat requisite for evaporation is thus supplied direct by the sun seem to have 

 been made at sea, but it is certain that this can happen only while the sun is shraing ; 

 and evaporation is much more rapid in sunUght than at night or under a cloudy 

 sky — on the average about two and one-half times more rapid, according to Kriimmel's 

 (1907, p. 248) summation of the available evidence. The actual hours of sunshiue 

 average only about 50 per cent of the possible number at land stations around the 

 gulf, with the sun above the horizon only about haK of the time for the year as a 

 whole at our latitude. Thus, a rough approximation of the yearly evaporation from 

 the gulf would be about 0.3 meter (out of the total of 0.7 meter, as stated on p. 842) 

 for the one-fourth of the time when the sun shines on the water, 0.4 meter during the 

 remainder of the year. Without going deeper iato this question this implies that 

 the chilhng effect of evaporation is certainly sufficient to reduce the mean tempera- 

 ture of the upper 50 meters in the gulf by at least 5° during the course of the year, 

 and probably by at least 6°. 



THERMAL EFFECT OF THE NOVA SCOTIAN CURRENT 



The distribution of temperature around and in the offing of Cape Sable makes 

 it certain that the cold Nova Scotian drift exerts its chief thermal effect to the east- 

 ward of the cape. Nevertheless, it is now fully established that this cold current 



"Determinations of the latent heat of evaporation of water vary somewhat. The value stated above is calculated from 

 Herring's formula, L=94.21 (30.5-T) 0.31249. (Quoted from Smithsonian tables, Fowle, 1920.) 



