A curiosity regarding the relation between surface level humidity and precipitable water

We have examined the global relation betwen the mean monthly surface level humidity and the precipitable water (Q-W relation) developed by Liu (1986) for its applicability to instantaneous data sets as proposed by Hsu and Blanchard (1989) over the Indian oceanic region. Our analysis using regional instantaneous values of W from Monex-79 in-situ data sets gave an rms error of 5.66gm/kg. This is considerably larger than the rms error of 0.4 gm/kg as obtained by Liu using the global mean monthly relation, thus failing to support Hsu and Blanchard's conjecture. However, a fifth-order polynomial in W derived by us based on instantaneous data indicates an rms error of 0.92 gm/kg.