INTRODUCTION 6 



results in the following definition of the U. S. nautical mile, which is used 

 throughout this volume: 



1 U. S. nautical mile = 1853.248 meters = 6080.21 feet 



Other definitions of the nautical mile are in use. The British Admiralty 

 defines the nautical mile as being 6080 feet (exactly) ; the international 

 nautical mile is defined as being 1852 meters (exactly). 1111 



STANDARD GRAVITY FOR REDUCING BAROMETRIC OBSERVATIONS 



Prior to the introduction of the millibar as the common unit of pressure in 

 meteorology, it was customary to express barometric pressures in terms of 

 the height of a column of mercury reduced to standard conditions of tempera- 

 ture and gravity. Conventionally, the standard value for the acceleration of 

 gravity adopted was that at latitude 45° and sea level. 9 



To reduce units of pressure expressed in terms of the height of a mercury 

 column to standard gravity, it is only necessary to know the ratio of the local 

 acceleration of gravity to the standard value. In general, this ratio is de- 

 terminable with more precision than the absolute value of the acceleration of 

 gravity at a given place. However, if pressures are to be converted to 

 absolute units (e.g., millibars), it is necessary to know the standard acceleration 

 of gravity. 



At the 1891 meetings of the International Committee on Weights and 

 Measures, Defforges and Lubanski 10 announced the results of an investigation 

 made in 1888 which yielded an acceleration of gravity at latitude 45° and 

 sea level of 980.665 cm. sec." 2 This value has been used extensively since that 

 time by physicists and others as an arbitrary standard value of gravity, al- 

 though it has long been known that it does not represent the absolute value of 

 gravity at latitude 45° and sea level. 11 The most recent determinations indicate 

 that the best value is near 980.616 cm. sec." 2 (see Table 167). 



Most meteorological services, including the U. S. Weather Bureau, first 

 reduce barometer readings in terms of inches or millimeters of mercury to 

 gravity at latitude 45° and sea level (g 45 , o) by means of a correction depending 

 on the ratio of local gravity to g i5 , (see Table 47) and then convert to abso- 

 lute units. Strictly speaking, it is therefore necessary to use the best estimate 

 of the value of gravity at latitude 45° and sea level in converting the inch or 

 millimeter of mercury to millibars. This procedure was adopted by the Inter- 

 national Meteorological Organization in 1939 (Resolution 25, Berlin). How- 

 ever, for many physical applications (e.g., the definition of the International 

 Temperature Scale, 1948, see footnote, page 17) one atmosphere is defined as 

 1013.250 mb. This pressure corresponds to the pressure exerted by a column 



International Meteorological Tables, Paris, 1890. 



10 Defforges and Lubanski, Com. Int. des Poids et Mes., Ann. I, p. 135, Paris, 1892. 



11 Dryden, H. L., Nat. Bur. Stand. Journ. Res., vol. 29, p. 303, 1942. 



lla Effective July 1, 1954, the international nautical mile was adopted bv the L. S. 

 Department of Defense and the Department of Commerce. 



