DISTRIBUTION OF SALINITY, TEMPERATURE, DENSITY 



81 



unsymmetrical, the temperature increasing rapidly after sunrise and 

 decreasing slowly after sunset, but at greater distances from the Equator 

 the curve becomes somewhat more symmetrical. 



In some coastal areas the changes through the year of the range of 

 diurnal variation of surface temperature have been examined. At 44 

 stations around the British Isles, Dickson found that on an average the 

 diurnal range varied between 0.20° in December and 0.69° in May. At 

 individual stations, both the mean annual range and the variation of the 

 range from month to month were dependent upon the exposure of the 

 locality and the depth of the water at which the measurements were made. 

 Similar results were derived by van der Stock from observations at the 

 Netherlands light ship on Schouwenbank, where during the year the 

 range varied from about 0.15° in January to 0.44° in June. This annual 

 variation in range is closely related to the annual variation in the diurnal 

 amount of net heat received by processes of radiation. 



The range of the diurnal variation of temperature depends upon the 

 cloudiness and the wind velocity. From observations in the Tropics, 

 Schott found the mean and extreme values that are shown in table 12. 



Table 12 



RANGE OF DIURNAL VARIATION OF SURFACE TEMPERATURE IN THE 



TROPICS 



Wind and cloudiness 



1. Moderate to fresh breeze 



a. Sky overcast 



b. Sky clear 



2. Calm or very light breeze 



a. Sky overcast 



b. Sky clear 



Temperature range, °C 



Average 



0.39 

 0.71 



0.93 

 1.59 



Maximum 



0.6 

 1.1 



1.4 

 1.9 



Minimum 



0.0 

 0.3 



0.6 

 1.2 



Similar results but higher numerical values were found by Wegemann 

 from the Challenger data. In both cases the numerical values may be 

 somewhat in error, but the character of the influence of cloudiness and 

 wind is quite evident. With a clear sky the range of the diurnal varia- 

 tion is great, but with great cloudiness it is small ; at calm or light breeze 

 it is great, and at moderate or high wind it is small. The effect of cloudi- 

 ness is explained by the decrease of the diurnal amplitude of the incoming 

 radiation with increasing cloudiness. The effect of the wind is somewhat 

 more complicated, but the main feature is that at high wind velocities the 

 wave motion produces a thorough mixing in the surface layers and the 

 heat that is absorbed in the upper meters is distributed over a thick layer, 

 leading to a small range of the temperature, whereas in calm weather a 



