WHOLE VOL. NOTES. NORTH AMERICA 43 



are for the civil day, midnight to midnight, normal standard 

 time. The monthly means have been obtained by dividing 

 the sum of the mean maximum and mean minimum tempera- 

 tures by 2. 

 Precipitation. 



The rain gages used at the regular Weather Bureau stations have 

 a circular catchment area of about 8 inches diameter, and the 

 snow, hail, or sleet caught within them is melted and mea- 

 sured as water. The rain gage proper is set within an en- 

 closing cylinder, which serves as an overflow attachment in 

 the case of heavy rains and as a snow gage in the winter 

 season. 



The sum total of the depth of rain and melted snow is measured 

 to within o.oi inch at 7:30 a. m. and 7:30 p. m., 75th 

 meridian time, daily. The total precipitation is determined 

 from the amounts recorded daily, midnight to midnight, 

 standard of time in local use. 



The snow caught and retained in the gage is melted and mea- 

 sured as water. No correction is applied for snow that is 

 lost out of the gage by the eddying action of the wind ; con- 

 sequently in some cases the record is less than would be 

 given if the observer had measured cylinders of snow cut 

 from the spots representing the average snowfall on the 

 ground. When it is known that the catch of the snow gage 

 is markedly at fault, an independent ground measurement is 

 made and used as the official record. The loss of both rain 

 and snow caused by high winds, from gages exposed on the 

 roofs of tall buildings in which some of the regular stations 

 of the Weather Bureau are located, is undoubtedly larger 

 than is the case at the cooperative stations where the gages 

 are located in the open country and near the ground, but 

 this loss does not appear to be sufficient to make the monthly 

 sums derived from these two classes of stations wholly incon- 

 sistent with each other. 



CHICAGO 

 Site. 



The Chicago office was moved to the University of Chicago on 

 July I, 1926. 

 Pressure. 



Beginning with July i, 1926, the readings of the barometer are 

 here corrected to the old level of 82^ ft. 



