Climate Letter #1667

Precipitable water is a textbook term intended to include every kind of water, from individual molecules to a number of different kinds of compact assemblies of molecules, all of which exist in suspension in some part of Earth’s atmosphere.  I would prefer using the term airborne water as a substitute, having the very same definition, for several reasons.  One is that the latter is more perfectly accurate, and more perfectly inclusive.  The word precipitable implies some degree of readiness to precipitate.  It is not clear that single molecules, which by weight are of greater measure than all of the compact assemblies combined, have a sufficient degree of such readiness to be properly included.  Indeed, they may have no ability to precipitate at all until after having taken the initial step of condensation.  Even that first step may not be enough to enable precipitation unless the molecule is quickly included in a compact assembly of sufficient weight to assure its eventual descent. Apart from that detail, all of the molecules, assembled or not, are airborne, and that is an important distinction for purposes of climate study, as opposed to all the rest of Earthly water.

With such a change in place we could go on to apply the term precitable water only to molecules that have condensed and become part of one of the bits or pieces that subsequently exist in either a liquid or icy state, as opposed to others that remain in the form of vapor, or gas.  Having these two clearly distinct categories can be useful for study purposes, especially when we start discussing greenhouse effects.  Water vapor is a true greenhouse gas, even though conventional climate science, whenever possible, prefers to keep it from being identified or listed that way.  Almost everything about water vapor is indeed substantially different from the common properties of all other greenhouse gases, including the relative strength of its power to affect air temperatures. That’s precisely what makes it interesting, at least for some people.  

At this point I want to introduce some thoughts about the precipitable kind of airborne water—under its new definition. Except for having the same basic molecules, its properties must not to be confused with those of water vapor or any other greenhouse gas. Nevertheless, since these bits do exist in the air, mixed in with everything else in the air, it seems possible that they could have an independent greenhouse effect of their own which could then be compared with that of the gases. I would say yes to such an effort, and I am even going to stick my neck way out with a prediction: Whenever a full study to that effect has been completed, it will become apparent that the greenhouse power of any weight of precipitable water is fairly comparable to that of a similar weight of water vapor in any location where they coexist. In other words, condensation of the vapor, wherever it may be located in the atmosphere, shouldn’t change anything very much with respect to total greenhouse calculations at that location.

I base this judgement on the more general idea that everything in the air that is capable of absorbing longwave radiation, no matter where it comes from, will re-radiate the same amount of energy.  (The only exceptions I can think of are nitrogen, oxygen and a few other trace gases.)  Greenhouse gases are special because each one only absorbs a limited number of wavelengths from the longwave part of the spectrum, with water vapor happening to absorb somewhat more than its fair share.  Once the vapor has condensed into either a liquid or solid form I believe it will go right on absorbing, but in a quite different way. Presumably that could mean taking in a much larger part of the longwave spectrum, making it yet more powerful, but in this case the situation is complicated by having a surface and interior of each bit to deal with, and ice having its own unique properties, and so on.  I can only speculate that, when all is said and done, the net result for re-radiation that returns to the surface will be similar to that of a comparable amount of vapor.  The albedo effect of cloudiness—due mostly to precipitable water, and which has an opposite effect from that of the greenhouse one—is an entirely different subject, being studied more thoroughly, but with widely inconclusive results to date.

What this all means is that when I see a reading of so many kilograms of “precipitable water” on a Weather Map I treat it as being equal to the same amount plain water vapor with respect to impact of greenhouse warming.  That cannot be perfectly true, but it does seem to work out in a consistent way when coming to an understanding based on readings of all the different observations that show up as current realities on the Weather Maps. 

Airborne water can be classified in another way that has significance from the standpoint of greenhouse effects, and that is by altitude.  Low-level airborne water has one set of effects based on its own content and pattern of behavior while at a higher level considerable differences exist.  The “high-flying” pattern I have written about recently is one type of upper-level behavior that is coming more and more sharply into focus for me each day as I study it and think about it, with more of such thoughts forthcoming.

Carl