While looking at the weather maps yesterday I found an opportunity to measure the temperature effect of heavy rainfall in an extremely hot region during the peak of summer, with the sun at a high angle. Central Australia meets those qualifying solar conditions very well right now’ It is also experiencing a perfect setup of unusual differences in weather conditions in regions that are close to each other and would normally be very similar in temperature and other weather categories. One result of the current differences is clearly indicated on the temperature anomaly map:
With the help of magnification, I am seeing a lowest reading of 11C on the cold side and another 11C on the warm side, for a total difference of 22C, or 40F, for regions that are normally about the same. Let’s see how this looks on the temperature map:
Everything checks out well, with an average reading of +18C in the spot of the coldest anomaly and +42C in the warmest spot, for a difference of 24C. On a normal day at this time of year they should both have an average within a degree or two of 30C if all else were equal in the way of weather-making conditions—which is clearly not the case on this occasion. We’ll first check to see if there are enough differences in overhead precipitable water (PW) content to have that kind of effect on temperature comparisons tied to variations in greenhouse energy production:
Something is not right. When the warmest and coldest temperature locations are lined up just right with the PW values at the same locations I get a PW reading of 47kg on the cold side and 27kg on the warm side—completely the reverse of normal expectations. Normally, a 70% higher PW reading would be a factor that adds around 8 degrees in relative temperatures, but here we are apparently subtracting 24 degrees. Is this a graphic mistake, or is something unusual going on that has an effect even more powerful than PW content? We need to look for clues in the clouds and precipitation map:
There it is, an area of heavy rainfall that almost exactly matches up with the area covered by the cold anomaly in the map at the top. The warm anomaly area, on the other hand, does show cloud cover, presumably light, but not a bit of rainfall. The cloud cover alone does not have a cooling effect large enough to prevent the warm anomaly that goes with the observed relatively high PW reading over a normally dry desert region. And yet, a PW reading that is even higher still is accompanied by temperatures that have turned much, much colder. How does this make sense?
I have no complete description of this day was like in a remote part of Australia, but I do have personal memories of midsummer days in the US when the skies turned pitch black and rain came down in torrents for an unreasonably long time. This combination was capable of quickly producing temperatures of around 60F when normal would have been in the 90s. The major decline in solar energy reaching the surface must itself be great enough to cause a large part of the swing in temperatures. In addition, I can how the continuing stream of thick and heavy raindrops should cool the air at the surface even more, assuming that those drops were created at elevations of normally cold air to begin with and never had much of a chance to warm up. The prime requirement for a phenomenon like this is an atmospheric river that is heavily loaded with liquid or icy states of PW. It still has a greenhouse effect, but on balance the albedo effect plus ice-cold rain win out by a wide margin. We don’t see anything like it in winter that I can remember.
Carl