16 



H. H. Clayton — Seven Day Weather Period. 



day. Hence the average lowest pressure, even for short inter- 

 vals during 1890, occurred on exactly the same days of the 

 period as the greatest frequency of barometric minima during 

 the ten years' observation. The period was less marked during 

 subsequent years and longer intervals are required to show the 

 same persistency. 



To determine the relation of the period to temperature 

 changes at Blue Hill, the departures from the normal tempera- 

 tures were determined for each day during 1887 and 1888 at 

 8 A. M. and 8 P. M. These were then arranged in seven day 

 periods and averaged. The results in the following table show 

 the departures from the mean or normal of the two years : 



Table VI. 



Day 



1 



2 



3 



4 



5 



6 



7 



Dep. 



-1-0— 1-2 



-0-7 + 0-0 



+ 0-5 + 0-2 



-0-1-0-4 



-0-1 + 0-7 



+ 1-3 + 0-7 



-01-0-9 



The results show the greatest plus departures on the third 

 and the sixth day, or about one day preceding the lowest mean 

 barometric pressures, with the extremes of the temperature 

 and pressure departures on the sixth and the seventh day respec- 

 tively. The greatest minus temperature departures are found 

 on the first and the fourth day, with the lowest mean tempera- 

 ture on the first. 



It must be evident to every one who accepts these inves- 

 tigations that a promising and important field is here 

 presented for weather forecasting. As yet we are confined to 

 averages in forecasting. An effort to follow and separate the 

 individual periods has been attended with only a slight suc- 

 cess. 



It is possible to say that in all parts of the world barometric 

 minima will be from 10 to 20 per cent more frequent on cer- 

 tain days than on certain other days, provided the interval 

 taken is sufficiently long. It is also possible to say that certain 

 days will average colder than other days. I think even this 

 information would prove of value to certain industries, and I 

 have an abiding faith that it is only the beginning of the science 

 of weather forecasting which is yet to come. 



