442 



BULLETIN OF THE BUREAU OF FISHERIES 



In July, 1925, there was sunshine for 276 hours and in August for 308 hours, 

 while in 1926 there was sunshine for 320 hours in July and for 256 hours in August. 

 Comparing both years, we find only 8 hours' difference between the two during 

 this 2-Tnonth period and, similarly, less than 1° difference in the mean air and water 

 temperatures. When the number of hours of sunshine and its intensity are above 

 or below the normal mean for the month, there is invariably a corresponding departure 

 in air and water temperatures. 



The number of hours during which solar heating of the water takes place will be 

 found to vary from to approximately 15 per day, and these variations are the 

 fundamental causes of the daily and hourly fluctuations in water temperature that 

 occur during the summer. A good example of the effect of solar radiation on air 

 and water temperatures and the changes it produces are shown in the following 

 table. 



Table 4. — Effect of solar radiation on air and water temperatures, August 1 to 3, 19S6 



Date 



Hours of 

 sunshine 



DaUy 

 mean air 

 temper- 

 ature, ° C. 



Water temperature, ° C. 



Daily 

 mean 



At flp. m. 



Daily 

 increase 



Aug. 1 

 Aug. 2 

 Aug. 3 



2.3 

 7.5 

 14.2 



18.9 

 23.3 

 28.3 



21.5 

 22.4 

 26.4 



20.5 

 21.0 

 23.0 



21.0 

 23.0 

 28.5 



0.5 

 2.0 

 5.5 



On these three days the factors of wind and range of tide were virtually constant, 

 while the chief variable was the number of hours of sunshine per day. On August 1, 

 when there was the least amount of sunshine, the increase in water temperature was 

 exceedingly slight, while on August 3 there was sunshine for 99 per cent of the possible 

 number of hours, which resulted in an increase in water temperature of 5.5° C. 

 during the day. 



AIR TEMPERATURE 



A close relationship exists between the air and water temperatures in inshore 

 coastal areas and the river waters emptying into them. Although the range of 

 water temperature is considerable, it is much less than the range in air temperature. 

 In the following table, a comparison of the characteristics of the two for the summers 

 of 1925 and 1926 is given. 



Table 5. — Comparison of air and water temperatures, monthly data, ° C. 



