ATMOSPHERIC WATER VAPOR: A NEMESIS FOR MILLIMETER WAVE PROPAGATION

Abstract : Millimeter waves offer an attractive way of solving unique system problems because of their ability to penetrate clouds, smog, dust, or fog. This makes them a logical choice over electrooptical devices for adverse weather applications. Spectral lines of oxygen and water vapor ultimately limit the atmospheric transparency; hence, most applications operate between the absorption lines in four window regions (that is, 24 to 48, 72 to 110, 128 to 160, and 200 to 260 GHz). Observations have established the existence of excess water vapor absorption (EWA), which is most evident in these windows. Excess implies that the effect is not related to the known spectral properties of the water molecule. EWA is found to increase in nontrivial manner with humidity and the discrepancies can be as large as a factor of 10. Several groups (most notably at the Appleton Laboratory, UK), have gathered evidence of EWA from laboratory and field observations and brought forward hypotheses to account for the data. Qualitative explanations are based on the assumption that water molecules in moist air form larger molecules with a dimer being the first step in a series of stable species. Hydrogen bonding, ion attraction and attachment of the polar H2O to foreign particles (aerosol growth) are the ordering forces considered in the clustering process. An assessment of the current EWA picture will be given and avenues of research attacks are discussed to solve the enigma in the quantitative description of the interaction between millimeter waves and moist air.

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