California is again struggling with too much heat and not enough rainfall, spelled out in the form of major wildfires. The warm anomaly, up to about 10C or 18F in places—as Death Valley reached a potentially world record high of 130F—covers a large-sized portion of western North America that we see on this map:
As customary for this space, I immediately suspected that water vapor was involved in creating this situation and so opened the Precipitable Water map, shown next. The thing to look for is the signature of incoming streams containing high amounts of vapor that probably originated in warm Pacific Ocean surfaces to the west or south, and sure enough, there they are. In particular, an unusually wide stream is moving in from the southwest while a more narrow one is seen creeping up and into the coastal area from points directly to the south. (Both of these are revealed in more detail by going to the animated site at http://tropic.ssec.wisc.edu/real-time/mtpw2/product.php, where images cannot be copied and do not stay for long.)
Now for the big question: With such a large amount of water vapor moving in why are we seeing nothing other than a lot of heat and no rainfall? Shouldn’t all that newly evaporated water vapor be condensing and falling out, probably with a cooling effect, just like it does every day in so many other places? That’s what could have happened, but not this time. This was an entirely different kind of situation, where the evaporation arose from very warm surfaces under basically clear skies. It can keep going on like this for 24 hours a day, and for days on end, with virtually no sign of cloud formation. So where does all the vapor end up after it rises? As I’ve said here before, I think it forms into continuously moving streams and is quickly lofted by updraft winds to levels more than three miles high, where it enters into a new and different horizontal wind system. There it is picked up by passing currents of very cold and dry wind which are able to transport it for long distances toward the east and north without stopping or condensing. This kind of vapor may be said to have found a world of its own, call it “water vapor heaven,” where it has a special way of life and a great view of the world below as it passes over. On this map see how clear the skies are over much of the trail. The bar-shaped cloud formation you see is of a thin, high-altitude type, which I checked out on the Windy site, probably created by compression when the two vapor streams came together:
The rest of this story, which fully explains the cause of the temperature anomaly, you should already be familiar with. For an interesting sidelight, these same maps show a smaller scale anomaly in the Alaska area that seems to have been created in the same way by another stream that looks to have pursued a more convoluted course. You can also spot some other parts of the US being hit with rainfall that apparently originated from evaporation in the Gulf of Mexico by vapors that were unable to move high enough to bypass the lower-level wind system. They don’t have much time to waste in that situation before condensing and raining out.
I also want to reproduce the image of the high-altitude air pressure configuration that governs the flow of ordinary wind currents and jetstreams of the upper-level wind system. You can see how air temperatures at the surface tend to be reflected in the pattern of higher and lower relative pressures up above, which I think is basically the product of a feedback mechanism. Notice the dark red coloration of the area that lies over the very high temperature region of North America, probably existing as a marker of upward forces generated by the extraordinary expansion of warm air near the surface. Also notice how the Alaskan warm air anomaly seen in the top map neatly coincides with the shape of the “slot” between two green lobes in the air pressure image. That feature has the same kind of explanation but with lower relative temperatures for its cause.
Carl