The Case for H₂O Clouds on Venus

The long-recognized clouds on the planet Venus are now almost universally attributed to some substance other than water or ice droplets.The major arguments for the existence of dust rather than water clouds are :(A) No spectroscopic evidence exists for the presence of water vapor, which should accompany such clouds.T. Dunham's 1 analysis of Mount Wilson data places the upper limit of water vapor in the atmosphere of Venus at from 2 to 5 percent of that above the Mount Wilson Observatory, with a partial pressure of the order of about 7 mm of Hg (T = 279° K). (B) According to G. P. Kuiper, 2 the color of the planet is slightly yellowish with a color index of 0.8 mag., about 0.2 greater than the color index of the sun.Water clouds on the earth, according to H. G. Houghton, 3 consist of droplet sizes from 3 to 56 microns and are almost perfectly white reflectors.Furthermore, the albedo of the earth as measured by A. Danjon 4 is greater in the blue than in the red, with a substantial percentage of this reflection coming from water clouds.(C) The radiometric measures by E. Pettit and S. B. Nicholson 5 indicate temperatures near 250° K on both the day and night hemispheres of Venus.The vapor pressures of water clouds