mod5<-glmer(Fat_Category ~ Site+(1|Year) + (1|Julian.Date) +
            barometric.pressure, data= fat.dat, family=binomial, na.action=na.omit)

Warning messages:
1: In checkConv(attr(opt, "derivs"), opt$par, ctrl = control$checkConv,
:unable to evaluate scaled gradient
2: In checkConv(attr(opt, "derivs"), opt$par, ctrl = control$checkConv,
:Model failed to converge: degenerate  Hessian with 1 negative eigenvalues