If you have not read yesterday’s letter please take the time to do so. It provides a perfect example of what could be called compelling evidence, first of all, that precipitable water (PW) has a greenhouse effect, and that the intensity of the effect is linked to the level of concentration (by molecular weight) of the quantity of PW in the atmosphere directly above any given surface location. Second, the level of PW concentration can show an extraordinary amount of variation between one location and another that has the same basic features and is relatively close by. Third, we can diagnose the exact reason for why there is so much variation in the level of PW concentration between two such locations on a given day, and why it is always temporary. On other days the relative levels could be completely reversed, or there could be no difference at all. For all locations that exist outside the planet’s tropical belt, the cause behind these variations can be summed up in one phrase: the erratic behavior of atmospheric rivers (ARs).
ARs are composed of one substance, PW, and nothing else that matters. PW is composed entirely of H2O molecules in any of three different states, vapor, liquid or ice, in the form of either independent gas molecules or condensed aerosols. ARs originate as pure vapor, derived from a narrow range of warm ocean waters with surface temperatures of not less than 24-25C. Evaporation is a continuous process, all 24 hours of the day. The vapor very quickly and continuously moves straight upward, the same way a kite moves, to an altitude of several miles if the sky is relatively clear. At some point these rising streams of vapor are likely to encounter wind currents that move horizontally at relatively constant speeds and direction. When the evaporation waters are near the outer borders of the tropical belt these winds will typically be of a sort that moves in a direction leading to higher latitudes of the hemisphere. This is how ARs are established, with an additional feature being a tendency to maintain a good share of the collected vapor within narrowly-banded streams that hold unusually high concentrations of vaporous content.
Now for some illustrations. This first map shows the geographical locations of the particular ocean waters that give rise to most ARs. Any region shaded in red plus the yellow band qualifies, as long as the red part is not too close to the center of the belt. (ARs can also originate from the copious moisture that exists—we hope forever—in the rainforests of South America and central Africa.)
This next image shows new rivers as they are being formed along the warm waters bordering the tropical belt, plus continuous snapshots of the progress that has previously been completed over each of the past one to five or six days by each of the separate rivers. A river will normally keep losing strength as it moves along but then will sometimes regain strength by merging with parts of another one in an accidental way:
Practically all of the precipitation that falls in each hemisphere beyond the inner confines of the tropical belt has at first been carried away by one of the rivers as vapor from source waters, later condensed into aerosols and ultimately dropped at some point along the journey that follows. Today, as usual, notice how quite a bit more of it is dropping into various ocean waters than over continental land masses.
All ARs are composed of nothing but PW. Any PW that has not fallen out of a river constantly exercises its greenhouse energy powers over the surfaces below the river, generally resulting in a warm anomaly. PW concentrations carried by any AR tend to be quite a bit higher, perhaps by multiples, than the mostly vapor concentrations that exist over any given location when no AR is passing through the sky directly above. Here’s a look at today’s anomalies:
Most of the largest and longest ARs are composed PW from vapors that originally were transported all the way up to an altitude having the special kind of wind system that features the activity of strong and fast-moving jetstream winds. The strength and positioning of individual legs formed by these jets has a great deal of influence over the progress of each AR as it continues its flowing momentum. Jetstream legs tend to be shorter, more convoluted and more intermittent when moving over continental land masses, leading to noticeably different results between the two hemispheres:
Some ARs are carried for considerable distances by winds in the upper part of the lower wind system. Their behavior is generally similar to that of the ones that flow within the higher level wind system but there are also a number of differences, most notably when hurricanes are formed. Today’s images include a more common example of this type of AR, showing vapor carried from the Gulf of Mexico by winds that proceed on a northerly path through the heart of the North American continent, leaving tracks seen on this and three more of the above maps:
Carl