Precipitable water (PW)—is it a greenhouse “gas”? This complex material is in large part a true gas, but the remainder, condensed from pure vapor, is not. Still, the remainder has certain features that have much in common with water vapor. For one thing, almost every bit of it is composed from the exact same H2O molecules. The only exception would be the tiny foreign particles around which water droplets are formed via vapor condensation. Their weight is not a significant part of the whole but they are a real part of PW and should at least be kept in mind, were it somehow to matter.
PW in its entirety has two important features in particular that cannot be distinguished from those of water vapor and every other greenhouse gas. One is simply the fact that, by definition, it all exists as part of the atmosphere, floating around above Earth’s surface. That’s what makes every bit of it precipitable at some point in its lifetime, and in fact it all soon drops back down to the surface. By virtue of being part of the atmosphere all PW material is in a perfect position to capture longwave photons of energy that have radiated away from Earth’s surface and are headed toward outer space. By common understanding, any such capture will be reversed via re-radiation, ultimately causing what we call the greenhouse effect.
All greenhouse gases, by definition, have this power of re-radiation, but only to a limited extent, specific to each gas. Water vapor happens to exceed all of the others in this respect, giving it extra “muscle power.” Ultimately, the effective greenhouse power of each gas also depends on its volume of concentration in the atmosphere as well as its wavelength band coverage. Here again water vapor is unusual, due to the fact that variations in its volume of concentration in different locations range from one extreme to the other. Concentrations are further subject to a continual amount of shifting up and down in each location. All other greenhouse gases are functionally just the opposite, with concentrations that are nearly the same in all locations and slow to change over time.
So much for water vapor. What about the remainder of materials that constitute PW? Do they or do they not add to the considerable strength of vapor alone? There are two principal considerations. One concerns the nature of any such possible greenhouse effect they might have, which should differ from the characteristics of gaseous molecules due to their large-body type of composition. These materials also have a different kind of distribution from that of the water vapor, and in addition the overall nature of the mixture is quite variable in a number of ways. For illustration, just think of the difference between a sky that is clear and one full of heavy clouds and maybe even rain. The potential for effective variability of greenhouse power between vapor and the other PW materials would appear to be endless.
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Sorry to be rambling on so long, but I need to finish this train of thought by taking a different view of the non-vapor greenhouse power. Let’s just assume that all non-vapor material, like clouds, does have greenhouse power that cannot be directly and separately measured by any acceptable means. Does that matter? Maybe not. What I believe we can measure, in an indirect way, is the greenhouse effect of PW in its entirety. We know almost exactly what that entirety means in terms of overall weight—at practically any location, on every day of the year. And we know almost exactly how temperatures differ from average at each of those locations on each day of the year. We also have a pretty good understanding of those many other different things, aside from the total greenhouse effect of PW alone, that might have a bearing on the temperature differences. If we simply subtract the net effect of all these things whatever is left of the anomaly could then presumably be attributed to the overall greenhouse effect of PW, vapor and all, in combination..
This is basically what I have been doing, in an admittedly primitive way, mainly using the Weather Maps. As often reported, I keep getting results that have extraordinary regularity. This outcome suggests a conclusion that it does not much matter what the mix may be for the entire PW component, just the weight. In other words, all the other-than-vapor H2O material, by weight, may realistically have effective greenhouse power similar to that of water vapor alone. This unexpected finding will need to be tested and validated by more rigorous methods. If confirmed, the knowledge gained would almost certainly have many useful applications.
Carl