The differ ence in t he mon t hl y mcan values of t he r efractiv c index at ""("ound le ve l a nd at one kilometer above the ground level is often u sed for t he purpose of predict in g th e a nnu a l q ' cle of radio trans mi ssion loss. The. present study investigates t he possibili t. v of u t ili zin g differe nces to h eights o t her t h an one kilometer. A co mparison of 100 megacycles per seco nd transmISSIOn lOSS r ecorded over t we nty-one paths with various refractivity difl'e rences from t h e surface to t hree kllom eters re veals that the surface value of t he r efr active index y ields as good a co rrela t io n as any of t he refractive index differences du e to the high co rre la t io n bet ween t he surface values and t hese differences; t h erefor e the mo re access ib le s urfacc valu c can be effectively substit uted for the dir-ferences. Specifyin g t hc r efra ctivc index profilc at t wo or t hree additive points and u sing mul t iple co rrelat ion techn iqu es does no t sign ifi cant lv increa se the cO rl•elation. • The u se of r adio d ata over t hc same p a th docs no t signifi cant ly improve t he co rrelat ion ovcr t hat obtained from o nly mcteorological d ata, indicat in g t he very pract ical res ult t hat in ex pe ns ive meteo ro logical data may bc used to pred ict th c seaso na l t rend of V II F radio fi e ld stre ngths with as much accuracy as ex pe nsive radio path meas urem ents.
Threc Illcthod~ for prcdi cting t he bcnd ing of r adio mys ",hcn the rcfracti,'c index I~,;ofi l c a bove t hc s Ul'facc laye r is unknown havc bec n d C\'cl~pe d !'('cently by, the auLl:o r:,:, ~ hl'o;l' m ethods a re : a s t atistical tec hniqu e for rcfraetlOn at hi g h llll tIal l' le \'atlOn a ng les, l st."l1ctt!on of bend in g from an cxp o ncntial mod el of at mosphe ric l'ef rad lvc Index, a nd a 1Il0dlfi ?atlon of the expont'nt ial Illodel t o accoun t for the h.eavI ly \\,l' lghtl'cl dlccts of ll ilomalolis In It Ial re[racti,'c in dl'x grad ie nts at s mall initial eiC'\'at ion angll's , 1';ac l1 model IS dl' l?eudent UP O!I t he ,'alue of the rcfractiH' index at g rou udle,'el 01', III L1w CH'iC of S Ul)(' rrc fracL l ~n , t he add l-Lional kn o\\'ledge of L1,e refract i,'e index ,grad il' n L next to the ea rlil 's s urface" I-,nch method works best in a parti cu lar ra ngl' of JII lk ll r1entilOn 1tnglr's 01' Ille teorolog lcal cond lL10!lS, The hcig llt a nd a ngu lar mngl's of a ppli catio n of t':w h lllct hod an' che cked by co mpa l'l so ll Wl t,h valllcs obtain ed frolll 77 di verse rl'fractl vc Inde x profill' s rl' prl'sc nt a t'." c of \\ Ide clilnaLlC "l1ria lio n, H is found Lhat 1I11' ll ~C of Lite b('st of Lhe t hree Ill ClilOcl s \\:111 always l:csul L In a prl'cl iction of the tolal aL mos phl'ric bl'nding wilhin 10 p Cl'cc nt for ' ,l lI tm l l'll'ntt lo n angles from zer o to 10 lIli ll iraci ia ns an d to \\'i th in •1 p e rc e ul fol' Itl llIal l'lcnLt lo n a ll g l e~ g reater t han J 7 m ill iradian s ( ...... I dl'g) ,
The tracing of radio rays is normally carried out under the ass umpt ion that t h e r efr active index varies only in t he vertical direction. Although this assumption appears to be quite reasonable in the average or climatic sense, it is seldom satisfi ed under actu al conditions a nd is strongl y violated by horizontal airm ass changes occ urrin g near frontal and land-sea interfaces. This lattcr casc is investigated by tracin g rays t hrough two i nstances of obser ved marked h orizontal variation of t h c refractive ind cx. The bending for t hese ray paths was t hcn co mpa red wIth valucs obtflin cd und er t h e no rmal ass umption of hori zontal homogeneity .Although at 1 kilom ctcr and abo\'c t hcse horizontal changcs appear to have littl e cffect !"a.VS emitted at low clcvation a nglcs a rc se nsitive t o cxtrcme horizontal variation s of th~ atmosphere nea r t he s urface, such as those assoc iaLed with ducLing con di tion s. H O" 'cver sin ce it ap pea,rs that s uch conditions occur lcs t ha n]5 p erce nt of t he t im e at most locations' t hc m a jority of r ay-pat h calcu la.lions may b e ca.rried Oll t under thc norma l assl.l mp Lion o·r hori zo ntal straLifi cation of the r cfrflct ivc ind ex.
Maps of the variation of absolute humidity over the United States are presented for the values exceeded 1, 50 and 99 percent of the time during the months of February and August. Cumulative distributions of absolute humidity are presented for nine climatically diverse locations for the four seasons of the year. In addition, regression equations have been derived to enable one to estimate the values of absolute humidity exceeded 1 and 99 percent of the time at any location for which average values are available.
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