ANNUAL VARIATION. 



87 



shown this very clearly by analysing into harmonic terms the mean annual temperature varia- 

 tions of a number of stations in the same latitude having land and sea climates. The results 



for 60° N. are shown in the following * :- 



Table 48. 

 Yearly Variation of Temperature n.nd Insolation in 60° N. Latitude. 



It will be noticed that in each case the amplitude of the fii'st term is much larger than 

 the second term, therefore its phase gives very approximately the phase of the whole wave. 

 Along 60° N. latitude the temperature variation lags behind the insolation by 15° at land 

 stations and 43° at sea stations. As very approximately 1° lag represents 1 dav, the lag 

 is half a montji at land stations and nearly one and a half months at sea stations. 



Analysing the curves on figure 29 in the same way we obtain the following ;— 



Table 49. 

 Yearly Variation of Temperature and Insolation. 



Barrier temperature lags only 8° (i.e., 8 days) behind the insolation, while the temperature 

 in 78° N. lags 35°. Therefore as regards lag, the Barrier behaves like a land station and 

 78° N. like a sea station. The amplitude, however, is a difficulty. The lag at a sea station 

 compared with a land station is due to the absorption of heat bv the water during the 

 season of rising temperature and the giving up of heat during the season of falling tempera- 

 ture. This process of necessity reduces the amplitude of the temperature variation, and the 

 greater the lag, the greater the reduction in amplitude. But we find that the average station 

 in 78° N. has a larger temperature amplitude than the Barrier, in spite of a slightly less 

 insolation amplitude. It is, therefore, impossible to explain the lag of temperature in the north 

 as being due to its sea climate, for its greater amplitude would then be unexplained. 



In order to study more closely the corresponding changes in temperature in the north 

 and over the Barrier, the following table has been prepared giving the change in temperature 

 from each month to the next. 



* Hann Lehrb. d. Meteorologie, 3rd edition, page 98. In the following discussion each month cones!ponds 

 to 30° and the mean value fur January has the zero angle, Februaiy 30'. Marc-h 00'. and so on. 



t For convenience of comparison 180° has been added to the phase of the harmonic terms in the north. 



