Today, a bit of map study about the critical role played by high-altitude streams of precipitable water in establishing surface air temperatures. We are going to zero in on a specific portion of North America and use temperature anomaly readings for this date as our guide to the effect on temperatures. This selected region offers a great working example because it contains a long series of alternating cold and warm anomalies, all of them on land and all at nearly the same latitude. See how well the borders are sharply defined in several cases. Here is how they map out:
There are six anomalies that I want to call attention to. The first is a strong warm one over eastern North America, with a well-squeezed view appearing on the far left edge. Second, a cool one covering northern Europe and a bit of Asia. As an interesting detail, notice how the Asian part makes a pronounced dip that takes if well to the south. Third, a warm anomaly covering much of southern Europe. Fourth, a strong warm one that stretches from the Caspian Sea to the Arctic Ocean. This one was featured in a letter earlier this week. Then up close to it we see a cool anomaly that is smaller but quite strong. Finally, a warm one near the right edge of the map that extends almost as far north as the Arctic Ocean much like number four. Now for a map of corresponding vapor streams:
In case of each anomaly you should be able to detect an almost perfect overlay of relative vapor stream strength. The four warmest streams are all decked out in bright blue with a dark brown fringe. Their total water content (to the top of the atmosphere) runs from 25 to 35 kilograms per square meter, which proves sufficient to effectively raise air temperatures by 5-10C over normal throughout each of the covered areas. The two cool anomalies both get that way mainly because of a relatively low amount of overhead vapor, as viewed in totals ranging from at most around 18kg down to lows near ten. Calculations based on averages for all overhead vapor relevant to these locations on this day, which are not available, would presumably be higher than those numbers. The result is observed anomalies ranging from just a degree or two below normal to as many as ten. A mix of clouds and rain can be viewed on other maps in parts of both of these kinds of anomalies. Their effect will always be on the cool side, but not necessarily uniform.
The purpose of making studies like this, which can be done every day with similar results, is simply to show broad evidence of a correlation that is rarely discussed anywhere else. It should properly begin with a reminder that a significant part of the total amount of precipitable water that we see measured within visible streams on the map will surely be located several miles high in the sky, but not all of it. There is a regular portion that will always be found lying close to the ground, attributed to local sources of evaporation, and that will be true whether or not there is anything coming in via overhead streams that originated in far-off places. I do not know of any source of separate data that would tell us how to make a distinction between amounts of “high-water” and “low-water,” or just what is regular for low-water. We can only be confident that the average amount of low-water in any one place or time will always be lower than the average of the two combined. The latter is actually measurable, just like average daily temperatures everywhere are measurable, and all of the necessary data has in fact been gathered, but I don’t think anyone has been doing it. (It’s on my wish list.)
Carl