function tau=tauPW(lambda,PW,m);
load MODTRANtrans
lambda=lambda/1000;
dlam=0.01;
ind=find(RES.lambda>lambda-dlam/2&RES.lambda<lambda+dlam/2);
Tr=squeeze(mean(RES.Tr(:,:,ind),3));
Trtot=interp2(RES.PW,airmass(RES.el),Tr,PW,m);
TrPW=Trtot/exp(-m*rayleigh(lambda*1000,1013.25));
tau=-log(TrPW)/m