Dave,
Then look up what latent heat of vaporisation has to do with it - and why it increases at altitude, when I would expect it to decrease with falling vapour pressure.
Again, not terribly relevant but, at a saturated steam pressure of 3208psi (705F) the latent heat of vapourisation is zero. Not many people know that
Re your boiling points, my Callendar Steam Tables seem to disagree. They're psi/Btu/lb/deg F so perhaps I've got a conversion wrong.
How's that for thread creep? - from an advanced FBW aircraft to a 1960 steam turbine engine room.