280 RESULTS OF THE METEOROLOGICAL OBSERVATIONS TAKEN DURING 



The following are the mean daily values for the period under review : — 



Mean Visibility from Hourlt Observatioms, Laurie Island, South Orkneys, 

 May 1903 to February 1904. Scale 0-12. 



The following table shows mean hourly values of visibility for the period May 

 1903 to February 1904. It will be seen (see Plate III.) that on the mean of the period 

 under review there is a steady increase in the visibility until 9 a.m., followed by a slight 

 fall during the three hours ending noon. A rapid rise then occurs, the maximum for the 

 day being recorded in the three hours ending 3 p.m., and then the values fall away to 

 the minimum at midnight. The late forenoon minimum is probably associated with 

 the formation of cloud, while the increased clearness which follows is related to the 

 diurnal rise in temperature which occurs at this time. The late forenoon minimum is 

 well marked in Winter and Spring, as well as for the two Autumn months, but in 

 Summer it shows itself more in the form of a retardation in the (general rise, there beino- at 

 this season a pronounced minimum from 5 to 7 a.m. and a secondary minimum at 3 and 



