Climate Letter #1998

Why is it important for scientists to fully understand the greenhouse energy effect of precipitable water (PW)?  It all boils down to one major consideration:  because we have the ability to measure the effects of this energy on surface temperatures with great accuracy.  We actually obtain readings of the total amount of PW (meaning molecular H2O) in the atmosphere, expressed as total weight, over every location on the surface of the planet, in amazingly fine detail.  This has been done a number of times each day for several decades and records are saved. Daily averages are reproduced at the University of Maine by use of color coding laid out on global maps, which are published online (see https://climatereanalyzer.org/), mostly for meteorological purposes and also for general public information.  

This same institution, using the same set of maps, provides a large amount of other weather-related information in similar scope and detail.  Of specific interest, we get daily maps of both the average temperature for the day and the anomaly for that day respective to an average for each of these days as created from a baseline period of 1979-2000.  The magnitude of this anomaly is ready for comparison with the magnitude of any similar type of anomaly, if known, for any other weather-related factor that may have helped to produce the temperature anomaly observed at any particular location for the day.  No such information about potential factors is currently available, but there are sources ready to be tapped if a need is recognized.  Historical records of PW readings are in place that could be applied to meet these requirements.

The idea of matching PW anomalies with temperature anomalies on a single-day basis is what inspired the development of Carl’s theory.  PW anomalies could be estimated in a number of ways that seemed close enough to have reason for viability. Whenever the magnitude of change was obviously large, which was quite often the case, the need for precision in the baseline average could be reduced..The whole point in matching different anomalies is simply to see how often a large outcome for one is matched by a large outcome in the other—in the same direction of course.  Statistically speaking, there should be no close correlation unless there is a close underlying physical relationship.  In this case, after testing hundreds of examples, I was getting results that had startling predictability.  They ended up with formation of the claim that any doubling of total PW content in the column of air over any location—outside of the tropical belt—produced enough greenhouse energy to raise surface temperatures at that location by about 10C.  It could even be seen that way from one day to the next as well as over decades of time, and all outcomes were fully reversible in cooling situations.  This was all done by only using estimates for PW’s history.  The hard numbers held in storage would provide much more accuracy, if extracted.  Similar relationships might also be discovered by matching temperature anomalies from past averages regarding amounts of cloud cover and rainfall, which have visible effects on the cool side that in some situations look quite competitive in scale with those of PW.

When doing this work I also learned that the energy effect of PW, as reflected in surface temperatures, is not greatly affected by different states of H2O in the physical structure of PW.  What this implies, if correct, is that the actual greenhouse energy effect of water vapor alone is about equal to that of PW in any configuration. The latter just happens to be easier to measure. Once measured the data can be usefully applied to water vapor for various purposes. Now let’s suppose this calculation, 10C per double, is in fact an accurate way to describe the power of water vapor’s greenhouse effect, outside of the tropical belt.  Relative to all other greenhouse gases, including CO2, that seems to be a pretty high number.  Thankfully, doubles are always localized and relatively infrequent, and are regularly offset by halvings and other reductions from average.  Also, what about vapor’s strength inside the tropical belt?  Is it still the same?  I have no idea, but there is one observation I can make by looking at the weather maps and also the 5-day animation.  That is, I would estimate that the total amount of PW (which, remember, has about the same greenhouse effect as vapor alone) within the tropical belt is around ten times higher than the combined total within the two hemispheres on the outside.  Local variations are small, and doubles are practically unheard of, thanks to an abundance of quick precipitation in saturated atmospheres. 

Carl