Water isotope ratios D/H, 18O/16O, 17O/16O in and out of clouds map dehydration pathways.

Water isotope ratios have been measured by laser absorption spectroscopy in and out of cirrus clouds formed in situ and convectively generated in anvils over subtropical regions. Water vapor in the tropical and subtropical upper troposphere shows a wide range of isotopic depletion not observed previously. The range suggests that dehydration of upper tropospheric air occurs both by convective dehydration and by gradual dehydration mechanisms. Twenty-five percent of upper tropospheric water sampled is in ice particles whose isotopic signatures are used to identify those grown in situ from those lofted from below.