SEA-LEVEL PRESSURES AT 10° INTERSECTIONS 



OF LINES OF LATITUDE AND LONGITUDE 



OVER NORTHERN HEMISPHERE 



EXPLANATION OF DATA 



(Authority, F. W. Reichelderfer, Chief, United States Weather Bureau.) 



1. The mean pressures are in millibars of 950 plus the amounts given 

 in the tables. 



2. The hour of observation w^as 13^, Greenwich mean time. 



3. The values of pressure were obtained by interpolation between isobars 

 on analyzed daily weather maps. In very high and very low latitudes the 

 isobars were extrapolated into regions of infrequent data by the map 

 analyst. These values (north of 70° and south of 20°) are therefore not 

 so reliable as the values in other regions, although the maps were analyzed 

 as carefully as possible and much attention was given to continuity of the 

 weather. 



4. The pressures were obtained from the daily maps published in 1944 

 as "Historical Weather Maps, Daily Synoptic Series, Northern Hemi- 

 sphere Sea Level," prepared through a cooperative project of the Army 

 Air Forces and the Weather Bureau. 



5. On each daily map the pressures were read twice for accuracy, 

 punched on cards, and monthly totals and averages obtained by the use of 

 punched-card machines. Although errors are not entirely eliminated by 

 this procedure, discrepancies greater than i millibar are extremely rare 

 in the daily values, and it is believed the monthly mean pressures are de- 

 pendable to an accuracy of 0.5 millibar, except in the regions where avail- 

 able data were consistently scarce. 



6. Although the daily maps were for the standard hour of 13'', G.M.T., 

 not all the basic data were synoptic with that hour. In such cases the analyst 

 took account of the difference adjustment. 



In cases where no more than 2 months were missing, the yearly means 

 were obtained by using normal values for the missing months. — Editor. 



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