Today I noticed that almost the entire Australian continent was having a heatwave on the order of 5-7C, which might be worth investigating even though it’s not a thing to worry about with summer still a long way off.
This turned out to be interesting because when I opened the Precipitable Water map all I could see for Australia was a large blob or relatively low water vapor reading (upper left) surrounded by places having much higher values. So why the anomaly?
That prompted me to open the animated version of precipitable water on another site to see what the last five days were like, and that was a revelation, one which I can describe but have no way to illustrate. This site showed very small batches of vapor, on the order of 12-18kg, drifting across the continent on some of the days. They were carried in by pieces broken off of two separate vapor streams coming from the Indian Ocean. These streams normally would have been moving in a more southerly direction except for a jetstream wind that had gotten in the way and pushed them off course. The incoming batches were spaced out in a way that left gaps, and within those gaps I could see vapor readings in a range of about 5-7C. Those really dry numbers probably represent something close to normal at this time of year for the continent’s interior, where overhead vapor streams are relatively uncommon. The southern tip of Africa is a similar spot that is seldom visited by these streams and is in fact having its own heatwave right now very much like the one happening in Australia, maybe even a little stronger.
If you look again at the images above you will see that Antarctica is having a really super heatwave, but it’s one that gets no attention because they happen so often and the place is so cold and forlorn that it seems to make no difference. The heat must still have a source, and it does, via influxes of high-altitude water vapor coming in from three sides. We can’t see how much there is, or what it means, because all of the vapor readings in the interior that produce the warming activity and need to be interpreted are less than 1kg, thus too small to be shown. A normal ambient reading might be around 100 grams, with associated air temperatures of 60 below or thereabouts. My understanding is that anytime total water vapor content is doubled, from no matter what existing level, and nothing else changes, the surface air temperature will increase by about 10C. That principle works even with temperatures of 90 below zero and about 15 grams of normal vapor serving as a starting point. In such situations obviously not much incoming vapor is needed to get the temperatures up, which means all incoming streams or remnants thereof have an abundance of leverage when they find a path that takes them directly over those spots that are so relatively dry and not often visited.
Australia is not getting enough vapor for a 10C anomaly, but it easily could on any day if an ordinary vapor stream measuring 20kg or more came across and was largely intact. Siberia, which is also a dry type of region, gets them all the time, and this year has been their worst on record. In all of these situations there is ample evidence of the important effects exclusively created by the randomized movement of discrete streams of high-altitude water vapor.
Carl