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Hypothesis. Ortho and para water are the two nuclear isomers where the hydrogen protons align to give a total nuclear spin of 0 or 1.The equilibrium ratio of 3:1 is established slowly in freshly evaporated water vapour and the isomers then behave distinct gasses, with their own partial pressures. Magnetic-field-induced ortho to para transformations are expected to alter the evaporation rate. Experiment. Evaporation from beakers of deionized water and a 6 M solution of urea is monitored simultaneously for periods from 1 to 60 hours with and without a 500 mT magnetic field, while logging the ambient temperature and humidity. The balances with the two beakers are shielded in the same Perspex container. Many runs have been conducted over a two-year period. Findings. The evaporation rate of water is found to increase by 12 % of in the field but that of water in urea solution decreases by 28 6 %. Two effects are at play. One is dephasing of the Larmor precession of adjacent protons on a water molecule in a field gradient, which tends to equalize the isomer populations. The other is Lorentz stress on the moving charge dipole, which can increase the proportion of the ortho isomer. From analysis of the time and field dependence of the evaporation, we infer an ortho fraction of 39 % in fresh vapour from water and 60 % in fresh vapour from urea.