I just love the new jetstream map. It brings out details that no longer need to be assumed, plus a few new things. We can now see the complete pathways containing these winds, and exactly what causes the winds to be stronger in some places along a pathway but not others. Both of those features are fully under the control of high-altitude air pressure differences, so let’s put that map up first:
There are three major jetstream pathways, all based on color-coded pressure differences. One tracks the yellowish fringe of the green zone, another the border between light red and dark red inside the red zone, and third, the light blue line on the fringe of the blue zone. The last of these barely registers at this time in the Northern Hemisphere but is prominent in the SH. Besides these three there are several more pathways now visible at times bearing weaker jets of an irregular type. One lies on the fringes of the deepest blue parts of the blue zone, quite visible in the SH. The strength of jetstream winds is mainly dependent on the relative proximity of any two major pathways, which is naturally irregular. A sharp bend in any of the major pathways always tends to reduce windspeed. With these rules in mind let’s bring up the new map and let you study how well they apply:
My reason for paying so much attention to jetstream formation, as you may know, relates to a specific effect they have on air temperature and other weather events and perhaps also on the longer-term fundamentals behind climate change. The key to understanding this effect is tied to observations of the way strong jetstream winds influence the disposition of the many relatively large batches of water vapor, of limited lifetime, that successfully make their way into the upper atmosphere every single day of the year. These batches take on the form of well-defined streams that are constantly moving forward in a generally easterly and poleward direction. Jetstream winds, depending on their location and relative strength, have a powerful influence on the progress of these streams. Today we have a particularly clear illustration of this relationship, as observed on the Precipitable Water map:
Give your attention to the stream that has arisen from tropical waters in the eastern Pacific Ocean and is headed toward Alaska, and see what happens to it. Now do the same with a stream that has emerged from warm waters on either side of Mexico. I see both of their courses being seriously altered along the lower edges of a pair of extra-strong jets that both lie on the same (red-zone) pathway and are getting reinforced by nearby winds that travel along sections of the green-zone pathway. The maps display similar effects in a number of other places, but this is the one having particular interest to many people in North America who have lately been affected by the unexpected approach of an unusually deep cold spell:
Think of what it must be like, for example, to be an outdoor camper on Minnesota’s Canadian border. We should not forget that water vapor is an extraordinarily powerful greenhouse gas. On most late summer days there is normally a fair amount of it passing overhead at high altitude, not greatly impeded by the presence of strong jetstream winds, and thus quite capable of having a substantial warming effect at the surface. This day was different, with the doors being tightly closed by an accidental jetstream formation.
Carl