268 Scientific Proceedings^ Royal Dublin Society. 



to the iufluenee of a passing cloud. In my experience there is only one 

 meteorological factor comisetent to produce a great and sudden rise of 

 temperature at night, and that is a gust of wind, disturbing an evening 

 calm. The gust almost invariably checks the formation of dew, and, if 

 it continue long enough, evaporates what is already there. I have never 

 seen a cloud alone do this. 



Table I gives a summary of the number of times dew was observed 

 in the four years 1905-8, at 8 a.m., 8 p.m., and 11 p.m. The designations 

 " 0," " 1," " 2," at the head of eacli column, indicate roughly the quantity 

 of dew observed at the respective hours ; " " indicating a sliglit dew ; " 2," 

 that everything is streaming wet; " 1," an intermediate state of affairs. We 

 see from this table that dew is of infrequent occurrence in the last quarter 

 of the year, the minimum of frequency falling in November, while the 

 maximum is in April. It must be borne in mind, however, that there are 

 occasional nights of dew in the summer, which do not appear in the record 

 because it has been evaporated by the sun's heat before 8 a.m. For that 

 reason it is not possible to compare the state of the sky with the presence 

 or absence of dew at 8 a.m. in the summer months. 



When frost is taken into account with the dew, the months of maximum 

 and minimum are not changed. June is the month of most frost, almost all 

 of which is observed at 8 a.m., i.e. the temperature rarely falls to the point 

 at which frost can be deposited until after 11 p.m. And here it may be 

 noted that just as there is no definite freezing-point of water (the thawing- 

 point is of course quite definite), so there is no definite temperature which 

 determines whether the air's excess of moisture shall be deposited in the 

 shape of dew or frost. On a still night a water-surface may fall considerably 

 below 32° F. without congelation, while solid bodies at the same low 

 temperatures may remain covered with dew. 



'For an interesting case in point, see J. E. Sutton, " A Lon' Freezing-Point," Symons's Met. 

 Mag., July, 1905. 



