Climate Letter #1669

Go back to yesterday’s letter, look at the chart, and take note of the mostly horizontal band of area along the equator, the edges of which are color-coded in light green. Light green carries a PWat measure of 50kg, and everything inside its borders will be higher than that, all the way up to 70 or so where you see magenta. If you go to a live Climate Maps link, open to the Precipitable Water map, and toggle back and forth with the one called Precitation/Clouds, you’ll see that every bit of the area with a green border is experiencing an abundance of rainfall, heaviest in the magenta spots, and of course heavy clouding along with it. As soon as you look outside of the green-banded region, as we were doing yesterday, cloud cover rapidly declines, and you may even see bright blue sky situated literally edge to edge with the green zone. At that particular marker the PWat reading will in every instance drop abruptly to around 40, with a preference for 35 or 30 when the skies are completely clear. The implication is this: on the clear side the lowered PWat reading represents maybe 99% vapor with almost no condensation, while the rainy zone next door has a ratio up to 50% condensed, and possibly more. The surface water on both sides is very warm, and evaporation is plentiful, but any fresh vapor being produced on one side is soon swallowed up by the thick cloud cover and held in place for a time before soon dropping back to the surface as liquid precipitation. On the other side, only a short distance away, there is no holding back, nothing held in place, and the newborn vapors are free to sail off toward space in a continuous stream, with a nice long life ahead of them.

This contrast certainly raises some questions, such as, what is the main cause of the difference between being born as a vapor inside or outside of the rainy zone? Or, why do the clear skies sometimes stay clear for many days on end while the cloudy ones stay cloudy? I cannot spot any kind of a clue from the maps, except to note that there may be a small difference in surface water temperature—although not necessarily. But how can such a small difference cause such a radical difference to be not only formed but maintained for days in the structure up above? Anyway, it does happen, because we can see it happen, and as a result one will soon be led to conclude that the consequences are profound, after studying what happens next to the vapor that has successfully gone aloft. Previous letters tell that story. What it all means, in essence, is that air temperatures in large parts of Earth’s middle and upper latitudes are greatly influenced by the total amount of ocean surface area in the tropics having a specific combination of properties, as described yesterday.. In that case, is there any way that we can look the future and make predictions of the absolute amount of qualified open sky there will be, relative to the average amount today, as the overall warming of the planet continues? I think this is a key question for climate scientists to be pondering.

One thing we can be sure of is that the oceans will continue to absorb more energy because of the greenhouse gases now in place, and ocean surfaces are therefore likely to keep adding warmth, with the temporary exception of those being cooled by glacial meltwater. More surface water having higher temperatures should bring an increase in total area over 25C into play, but other things will also be changing, that we maybe don’t even know about, that could affect today’s close balance between cloudy and open skies one way or the other.

With respect to the complete operation of “high-flying airborne water,” as described in previous letters, the story is a little different.  We are keenly interested in what happens to the water carried by any one “spike” after it has moved up and away from its spawning ground.  How long will it last before precipitating, and how far toward the poles will it go while also spreading out over regions that normally have very low PWat readings?  The answer to both of those questions is greatly affected by expectations for change in the positioning and relative strength of jetstream winds, which in turn are influenced by changes in the patterns of air pressure at high altitudes, both north and south.  These things can be readily observed on the Weather Maps, and as of now the trends seem to favor considerably more airborne water penetration over higher and higher latitudes. So far, those in the south have been observed lagging behind the north in this respect, with no good reason to believe it will stay that way.

Carl