XXX INTRODUCTION. 



REDUCTION OF TEMPERATURE TO SEA LEVEL. 



Table 44. English Measures. 

 Table 45. Metric Measures. 



These tables give for different altitudes and for different uniform rates 

 of decrease of temperature with altitude, the amount in hundredths of a 

 degree Fahrenheit and Centigrade, which must be added to observed tem- 

 peratures in order to reduce them to sea level. 



The rate of decrease of temperature with altitude varies from one 

 region to another, and in the same region varies according to the season and 

 the meteorological conditions; being in general greater in warm latitudes 

 than in cold ones, greater in summer than in winter, and greater in areas 

 of falling pressure than in areas of rising pressure. For continental plateau 

 regions, the reduction often becomes fictitious or illusory. The use of the 

 tables therefore requires experience and judgment in selecting the rate of 

 decrease of temperature to be used. Much experimental work is' now in 

 progress with kites and balloons to determine average vertical gradients. 

 It must be remembered that the tables here given are not tables giving the 

 data as recently determined for various elevations. 



The tables are given in order to facilitate the reduction of temperature 

 either upward or downward in special investigations, but the reduction 

 is not ordinarily applied to meteorological observations. 



The tables, 44 and 45, are computed for rates of temperature change 

 ranging from i° Fahrenheit in 200 feet to i° Fahrenheit in 900 feet, and 

 from i° Centigrade in iot> meters to i° Centigrade in 500 meters; and for 

 altitudes up to 5000 feet and 3000 meters respectively. 

 Example, Table 44. 



Observed temperature at an elevation of 2,500 feet, 52^5 F. 



Reduction to sea level for an assumed decrease in tem- 

 perature of i° F. for every 300 feet, + 83 



Temperature reduced to sea level, 60 8 F. 

 Example, Table 45. 



Observed temperature at an elevation of 500 meters, 12^5 C. 

 Reduction to sea level for an assumed decrease in tempera- 

 ture of i° C. for every 200 meters, + 2T5 

 Temperature reduced to sea level, 15^0 C. 



BAROMETRICAL TABLES. 



REDUCTION TO A STANDARD TEMPERATURE OF OBSERVATIONS MADE WITH 

 MERCURIAL BAROMETERS HAVING BRASS SCALES. 



The indicated height of the mercurial column in a barometer varies not 

 only with changes of atmospheric pressure, but also with variations of the 

 temperature of the mercury and of the scale. It is evident therefore that if 



