))
               (defined in gcm_vert_diff_down -- not used elsewhere)

     delta_t = the increment in temperature in the lowest atmospheric
               layer (((i+1)-(i-1) if atmos model is leapfrog) (K)
               (defined in  gcm_vert_diff_down as the increment computed up
               to this point in model, including effect of vertical 
               diffusive flux at top of lowest model level, presumed
               to be modified to include effects of surface fluxes 
               outsi