I want to write some more about the greenhouse effect of precipitable water (PW), comparing it to the greenhouse effect produced by CO2 and a number of other gases. The effect is always produced in the same basic way—by any kind of material contained within the atmosphere that captures and re-emits photons of longwave energy. Such activity impedes the rate of flow of energy of the same kind that Earth is constantly emitting from its surface toward outer space in order to keep itself cool. As a result the Earth’s surface tends to lose heat more slowly, leaving it a little warmer than it otherwise would be. That’s all there is to the greenhouse effect When we compare the effect of PW to that of CO2 and the other gases, while the method of production is similar, there is an absolute world of difference in the way things heat up at the surface.
You probably know a lot about how the gases heat things up. Essentially, the effect is evenly distributed over all parts of the surface, it builds up in tiny increments over long periods of time, and once in place it tends to stay that way for long periods of time, ranging from decades to millennia. The tiny increments may not even be noticed over the course of a handful of years. This is exactly the way the gases themselves build up in quantity and remain in the atmosphere, with ample time available to achieve wide and even distribution. Their levels of concentration can essentially remain stabilized and steady over long periods of time, affected only by small-percentage increments of change while continually seeking a balance in the number of molecules being added and subtracted.
So let’s see how PW compares. PW’s greenhouse effect is mostly produced by just two kinds of material, water vapor and the bodies of clouds, working in concert. Is there even distribution? Yes, at least fairly much, but only over about one-third of Earth’s surface, the tropical belt. This is the zone of heaviest concentration, and there is little room for growth before the limiting effect of precipitation takes over. This zone can expand in size over time, or shrink, and has seasonal movements, but is otherwise not too interesting from the standpoint of changes in greenhouse effects. The PW impact on the other two-thirds of the planet is of total contrast, full of constant distributional variation as well as atmospheric content. Content, measured by gross weight within a vertical column of air (1sqm), ranges from highs of as much as 50 kilograms in the lower of latitudes to lows of less than 100 grams close to the poles. Locally, these numbers are subject to daily and seasonal changes over a rather wide range, with a significant widening of percentage changes trending between the lower to higher latitudes. All of these factors have a corresponding type of influence on the realization of greenhouse warming impacts, where we see varying degrees of irregularity everywhere, every day, often wildly exaggerated, precisely the opposite of how the gases operate.
There is a special mechanism that causes PW irregularities in the mid to upper latitudes, covering two-thirds of the planet, to be even more exaggerated than one could possibly expect without knowing all about it. This mechanism is created by the plain fact that a certain amount of PW, I think well below 10% of the grand total, rises to an unusually high level of the atmosphere following evaporation from qualified locations on the tropical margins. It does so in the form of concentrated streams that proceed with steady movement on tracks that are both easterly and in the general direction of the nearer pole. While the streams disintegrate within days, the fact that they begin as bulked-up concentrations means that as long as they hold together the greenhouse effect they express on surfaces immediately below will also be concentrated. The potential for leveraging can lead to local single-day anomalies as high as 20C or more on those surfaces. Exposure to really extreme anomalies is not limited to places nearest the poles. It can happen almost anywhere when concentrations stay intact. Here is an example I see today:
In northern Manitoba you can see a temperature anomaly recorded by color coding between 24 and 28C. Use magnification for a clear view. (The Weather Maps have recently made improvements in the way the coding is presented.) Now we’ll switch to the PW map, where high-altitude PW concentrations, as usual, are highly irregular:
Here, in the anomaly location, with magnification, you can see a small spot (light brown) where the concentration is 11kg, surrounded by a larger area of 10kg. The average PW value for this area on this day, which would result in almost no anomaly at all, could easily be a little less than 2kg. As a rule, remember that every double in PW above its normal value should add about 10C to the surface temperature of the location, but only for just that one day. Gases of the CO2 type, combined, probably added about one-half of one degree to this location’s total anomaly of 24 or more, and will add the same amount again tomorrow, and every day beyond for quite some time, which gives you a good idea of how to compare the greenhouse effects of the two main kinds of providers.
Carl