Observations made luith a Black Bulb Thermometer. 81 



Using the same formula, and assuming that the turning-points 

 come at the same time in sun and shade, we get, as expressing the 

 solar maxima, 



A = 86°-2; B=71°-3. 



The monthly means obtained by means of these constants are given 

 in Table L, and also the differences C — between those computed 

 and those observed. 



It is evident from the signs of the values in the C — column 

 that there is some lagging of the temperature, the maxima not falling 

 so fast in the autumn, nor rising so quickly in the spring, as they 

 would if the correspondence between the sun's altitude and the 

 temperature were exact. From a climatological point of view this 

 is important. The greatest differences, minus and plus, are in 

 February and October respectively, these months as it happens 

 embracing the annual maximum of cloud at noon, the October cloud 

 being largely stratiform and the February cloud largely cumulus. 

 Table I. gives further, for purposes of comparison, the differences 

 between the computed and observed monthly mean maxima in the 

 shade. In character they agree very well with — albeit they are 

 smaller than — the corresponding solar differences. 



Table II. gives comparative meteorological elements for the four 

 years 1900-1903. It gives monthly mean values of — 



(1) The mean maximum in the sun ; 



(2) The mean difference between the maxima in sun and shade ; 



(3) The mean percentage of cloud for the hours XI. and XIV. ; 



(4) The mean dew-point, and 



(5) Humidity at Noon. 



A cursory glance is sufficient to show that there is not any very 

 obvious relation between the solar temperature and either the state 

 of the sky or the hygrometric state of the air, beyond the fact that 

 in a rough way the amount of cloud is least, and the temperature of 

 the dew-point lowest, in the winter when the black bulb tempera- 

 tures are lowest and the difference of maxima least. There is 

 nothing at any rate to suggest or to confirm certain previous results. 

 For example, Stow claimed that " solar radiation " (i.e., the difference 

 between the temperatures of sun and shade) is greatest when the 

 vapour tension is less than the average. * His argument and con- 

 clusion seem to have been that the greater the quantity of moisture 



* Eev. F. W. Stow, " On the Absorption of the Sun's Heat Bays by the Vapours 

 of the Atmosphere," Q. J. Met. S., January, 1875, p. 241. The excess reading of a 

 thermometer in the sun above one in the shade as a measure of " solar radiation " 

 is at least as old as the time of Lambert, say the middle of the eighteenth century. 



