Why is water vapor distributed so unevenly in the atmosphere, and what does it mean? Yesterday’s letter just touched on the subject. Let’s take a closer look.
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Typical of all well-mixed greenhouses gases, the concentration of CO2 in the air, measured in parts per million from carefully collected air samples, is almost always about the same, everywhere. At all of the official measuring stations, over the course of a typical year, the difference between the highest and lowest of the year’s daily readings, unless corrupted in some manner, will always be less than ten percent. (At Mauna Loa it is around three percent.) Water vapor, alone among these gases, is different in every way. It is measured not by ppm, etc., but by finding the weight of one-square-meter columns of precipitable water content between the surface and the top of the atmosphere, which can be done with accuracy over every bit of the globe using radio waves tied to satellite responders. Results are reported either by weight in kilograms or by volume in milliliters, both having the same number. They range from around 15 grams up to more than 75 kilograms, for a remarkable differential of no less than 5000 times! In a place like Antarctica the whole continent can stay mostly below 1 kg for months in the winter, while on all of those same days there are many places in the tropics aiming for the other extreme. In terms of quantity of airborne substance, precipitable water, largely composed of vapor, is simply prodigious.
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Now we need to look at another statistic, namely, that the average life of a water vapor molecule as a gas is only about ten days. During this brief span it will all, on average, have both condensed and precipitated back to the surface. So here we have a gas that in total weighs out prodigiously day after day in spite of a very short lifespan per molecule, a good indication that its source of production is equally prodigious as well as unfailing in regularity. Where does it all come from? No study that I know of has come up with a rigorously detailed answer, so I will give you my own rough guess. I think that around 80% of all the water vapor in the atmosphere, and posibly more, originates by evaporation from certain ocean water surfaces in the tropical zone. The particular surfaces I have in mind are basically located within the ruddy-colored portions of this weather map from Climate Reanalyzer.org, with extra emphasis on the darkly-shaded areas within those portions (one of the darkest of which, by the way, is just to the north of Australia):
Here is another map from the same source that should help you understand why I see such a strong relationship between surface water temperature and its evaporation rate:
Water vapor cannot actually “originate” over any land area, as the map might seems to suggest, when there is no large, preexisting body of water to draw from. It can, however, be lofted high into the atmosphere over its point of origin and then be carried by upper-level winds that happen to be moving in a direction that heads its way over any nearby land area. Good examples of such action are on display in the lower map, as seen in much of South America, lower parts of Africa and many smaller regions, like southern parts of the US. A close study of the map strongly suggests that the same kind of dispersal activity goes on across selective portions of ocean water as well. One can see oceanic zones in mid to higher latitudes that exhibit high readings even though it is most unlikely that they serve as major originators of the vapor in their own right. That kind of power may well be limited to the waters in the ruddy-colored zone that we see in the top map.
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The role of upper-level wind in the dispersal of water vapor from its tropical sources toward the poles is most likely the key to getting the rest of the job done, and there are only so many days available for moving and spreading before each new batch of vapor is exhausted by precipitation. Those winds are not especially dependable in any one place, thus not always cooperative, which is enough to explain why the final distribution is so uneven. By the time of exhaustion there is usually little vapor left to add much moistening to air in the polar regions. On that note, let me leave you with the thought that those upper-level winds may have their own way of being modified as conditions change. There is no reason why they cannot change their habits in ways that are more friendly to the movement of new vapor into the polar regions, thus bringing opportunities for new and different kinds of weather, including more warm air as well as more clouds and precipitation, and so on.
Carl