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 =

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... 391.0 of, 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 : Lin es of nearl y co ns ta nt pos itive slope rep rese nt co ns ta nt re lat ive humidit.y valu 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. P sychrome tri c c harts s ho win g de w-point a nd dry-bulb te mperature as coo rdin a tes with lin es re presenting co nsta nt relati ve humidity a nd cons ta nt we t-bul b te m pera ture (o bta in ed from t he Fe rre l Eq uati on) a re extre mely useful , since give n valu es for any two of th ese four va ri ab les se rv e to loca te a po int, from whi c h the valu es of th e oth er t wo varia bl es ca n be read d irect.ly. Key words: Antoin e Eq ua ti on; de w po int; humi d ity; hyg rome try; psyc hromet ric c hart; re lat ive humidity; vapor press ure of wa ter; we t-bulb te mpe rature.