464 



KEPORT — 1891. 



la July 1886 the North Sea was 2°-3 cooler than the Wallet, and 

 5°'6 cooler than the estuaries. The water of the shallow estuaries evi- 

 dently heats up more rapidly in spring, attains a higher temperature ia 

 summer, and cools down more rapidly in autumn, than does the open 

 sea. The observed maximum temperatures were as follows : — 



Hence the average maximum for the water was 7"! less than that for 

 the air, and occurred about twenty-six days later. 



In extreme cases, when the tide has risen over a wide expanse of 

 heated mud, the surface temperature has been seen as much as '2° above 

 that of the bottom ; when the sun is shining strongly a difference of 1** 

 may be observed ; but, as a rule, in the estuaries the rush of tide prevents 

 any difference of more than half a degree. When the air is very cold 

 the snrface water lias been observed half a degree or so colder than that 

 at the bottom. During the period of observation, the surface tempera- 

 ture was always in excess of that of the bottom, the greater difference at 

 high water being accounted fqr by the cold current setting in from the 

 open sea : — 



Period 



Excess of Surface over 

 Bottom Temperature 



Hieh 



Tide 



Low 

 Tide 



Mean 



May and June 



July and early August 



Late August and September 



0-35 

 0-17 

 0-03 



015 

 0-20 

 0-07 



0-25 

 0-18 

 0-05 



Low water is usually warmer than high water in the summer season, 

 but, as shown below, variations are experienced in particular years. 



Escess ( + ) or Deficiency ( — ) of Low Water Temperature. 



The effect of solar radiation in heating the water of estuaries during 

 the day, and of radiation from the water reducing their temperature at 

 night, has been arrived at by comparison of the same tidal phase occur- 

 ring consecutively by day and night. The results are as follows : — 



