lO SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 'JJ 



In summer, as shown by tables 4 and 5 and by the charts in figures 

 7 and 8, the high pressure forms farther north and drifts more 

 slowly to the Atlantic coast, not reaching it until some six days or 

 seven days later. In the case of the low pressure there does not 

 appear any evident drift, but only an intensification in the permanent 

 low pressure in the southwestern part of the United States accom- 

 panying an increase of solar radiation from low values to high values. 



The next step in the investigation was to ascertain, if possible, 

 whether the position of formation of the pressure maxima and 

 minima were related to the absolute intensity of the solar radiation. 

 For this purpose the observations of solar radiation were divided 

 into grades of .010 calorie, running from 1.910 and below, to 1.981 

 and above. Taking the dates of the observations in each grade, 

 the 8 a. m. pressures were averaged for 18 stations in the United 

 States and Canada. The results are given in tables 6 and 7. The 

 normal pressures for each station are also given in these tables. These 

 are from 51 -year means in the United States, but at the Canadian 

 stations are from the means of the data used in our research. The 

 departures from normal were plotted on maps of the United States 

 for each day from zero day to three days later. Figures 9 and 10 give 

 the departures of pressure for the day on which the solar observa- 

 tions were made (zero day). The upper chart in figure 9 shows the 

 mean departures of pressure during the winter half-year accompany- 

 ing very high values of solar radiation averaging about 2 per cent 

 above normal, the middle chart shows pressure departures for solar 

 radiation averaging about i per cent above normal, and the lower 

 chart the pressure departures for solar radiation values averaging 

 about 0.5 per cent above normal. It is seen that in each case there 

 was a maximum of pressure in the Rocky Mountain region of the 

 United States. In the case of the very high values of solar radiation, 

 the maximum of pressure is in the extreme northwestern part of 

 the United States and in Western Canada ; in the case of medium 

 intensity values of radiation, it is in the central Rocky Mountain 

 region of the United States ; and in the case of values slightly above 

 normal, it is in the far southwest. In other words, there is a marked 

 displacement of the center of high pressure southward with decreas- 

 ing values of solar radiation approaching normal. With values of 

 solar radiation below normal, a similar march of the area of low 

 pressure was found. With very low values of solar radiation, low 

 pressure is found over the western United States and central Canada, 

 and as the intensity approaches normal the center of the low pressure 

 area is displaced southward to the southern Rocky Mountain region. 



