What is precipitable water (aka PW), how does it compare with CO2, and how does it relate to climate change? The last two letters have had an unusual emphasis on this substance, and that is because I have only recently begun to appreciate its deep significance. It doesn’t get talked about very often by either meteorologists or climate scientists, and the term itself may not have much familiarity for those who read these letters. I think that needs to change on every count! Water vapor does get quite a bit of attention, which is fine, but PW can be perceived as even more important in many ways, including its measurable greenhouse effect, and thus more deserving of full attention.
Buy sildenafil citrate in a wide range that are of high quality and manufactured by recognized brands, at genericpillsop.com, in an easy, affordable and tension free after discount levitra was brought into existence. It is neutral to most medicines cialis consultation and drugs. Use kale, broccoli and spinach in your daily viagra online shop routine. So, the question of any side-effect whatsoever doesn’t arise in soft tadalafil find for info now the first place.
.
So what is precipitable water? A good definition must include an accounting given to weight, which means the weight in kilograms of all the H2O molecules that exist in any vertical column of air measuring one square meter at the base. It always originates as gaseous vapor, by process of evaporation, but once in the air it can shift to any other state, watery or icy, and back. With certain exceptions, described below, all of the molecules end up as precipitation, and only a small minority will stay in the air for more than a few days.
A distinction can be made between precipitable water and water vapor. Water vapor is everywhere, but vapor itself is not necessarily precipitable. Sooner or later It will always condense, from any level of the atmosphere, but in order to produce what we call precipitation it needs to do its condensing well off the ground. Much water vapor never gets high enough to do that, although it still counts as part of the weight of the PW column, and indeed it can always rise upward if somehow called upon to do so.
.
One can think of PW in a more special way, as primarily made up of a special kind of water vapor with respect to its origin. While some vapor just hangs around the ground, there are specific circumstances involving locations where a major share of it will quickly rise high in the atmosphere, up to any of the levels where clouds can form. In those locations the amount of evaporation can be relatively prodigious, requiring well above average surface heat to begin with plus air currents that help with the lifting. Those locations are found in the tropical oceans not far from the equator, abetted by tropical rain forests on nearby land regions. I think that possibly 90% or more of water that actually precipitates somewhere originates in those places—an opinion largely formed just by studying the lower map on this page:
https://climatereanalyzer.org/wx/DailySummary/#pwtr .
.
Once the vapors are aloft they become subject to horizontal movement streaming in an easterly direction and spreading out in angular fashion toward higher latitudes, meanwhile losing much of their original content to precipitation by rainfall. Much of that rain lands within the same belt the vapors came from, while the rest is distributed in highly irregular ways over most other parts of the planet.
.
My interest in PW is generated not by precipitation but by its other principal role, its effectiveness as a greenhouse agent, comparable to CO2 and numerous other greenhouse gases. There is one key difference—its irregularity when dispersed outside of its main belt of origin. After much early rainout, what is left of PW as it scatters across the higher latitudes is patchy at best, with concentrations and their associated powers falling off sharply as the spreading movement approaches the poles. (You can easily see that effect on the map in the link given above.) The falloff rate drops from an original 50-60 kg per square meter, or even 70, all the way down to just one or less around the poles. Within that general framework, any given location—mainly outside of the tropics—can experience a large shift in PW strength on short notice, causing an abrupt change in temperature. A higher amount of PW above one’s head means a higher air temperature down below, and the changes per kilogram of PW strength are logarithmic.
Is there a reason for concern? Yes. The basic reason is that the amount of PW that can spread over the higher latitudes, all the way to the poles, is not governed by any formula. There are things that ordinarily hold it back, like the jetstreams, and the structure of both jetstreams is known to be subject to change. It appears to be the case that the rise in global temperatures occasioned by higher CO2 levels is causing the jetstreams to become weaker and more fragmented, allowing more streams of PW to break through what has traditionally served as blockage. And there is more PW being formed to begin with as tropical surface waters grow warmer. We need to learn more about how to predict future changes in the reach of PW into higher latitudes as this process further unfolds, because it can leverage the total greenhouse effect on climate change.
Carl