The use of dew-point temperature in humidity calculations

Abstract

Th e de w-po int te mperature has a number of desi rable features as a means of e xpressin g humidit y.Th e Antoin e Equ ation, log ew= A -B ( T + C) -I , wh ere ew is the pa rtial press ure a nd T is th e te mperature of satura te d aqu eous vapo r , re prese nts th e Go ff-Gra te h formul ati on quit e well over th e ra nge of te m perature from 0 to 140 of.Th e press u re ew , in in ches of me rc ury, is obta in ed by ta kin g th e co ns tan ts A=6.70282 , B = 3150 .5 15 (OF ) -l a nd C = 391.0of, ca lc ul ate d fro m va lu es given b y Dre is bach.It is s hown th at the d e w-point DP is rela te d to th e rela ti ve humidit y RH b y th e relation : (D P + C) -l = (T + C) -I+ B-l log (RH )-l " Lin es of nearl y co ns ta nt pos itive slope rep rese nt co ns ta nt re lat ive humidit.yvalu es on gra phs of dewpoint aga in s t te mpe ra ture.The valu e of th e s lope d ecreases fro m unity fo r RH = 100 pe rce nt to abo ut 0.76 for RH = 10 perce nt, corres pondin g to th e lin ea r equ a ti on DP = [ I + 0.147 1 log (RH )-I]-'( T -70 ) + DP,o whe re DP70 = [2169 + 319 log (RH )-l] -l x 10li -391.