In yesterday’s letter I left out part of the story, which would have made it even more interesting. Why are we seeing this strong surface wind, carrying such a huge amount of precipitable water (PW), following the particular route it has taken? After traversing over a long stretch of the Caribbean Sea from east to west, why has it suddenly made a sharp turn to the north, into the center of the US? The answer is not hard to find, by opening the map of Sea Level Air Pressure:
There are are a couple of basic rules to keep in mind here. One is that surface winds regularly tend to follow courses set by the isobars that surround all of the many existing pressure centers, both high and low. The other is that in the Northern Hemisphere the wind direction around a high pressure center will always be clockwise, while movement around a low one is always counter-clockwise. In the SH everything is reversed, while in the upper level wind system, where jetstreams are operative and follow courses established by a separate set of isobars, the entire setup has a number of novelties. The principal surface wind we will observe today is similar to the one yesterday, except that now its strength is being delivered even deeper into Canada. You can also see how it must shift again as it progresses over a much greater distance and curves back to the east and out over the Atlantic:
With reference to the pressure map, we have several examples here of what happens when winds from one kind of pressure center press up against those of the other kind. They will be moving in the same direction, and I can see possibilities for interaction. One thing for sure is that the heavy PW concentrations borne by this stream have been extended more deeply into Canada today, still without any sign of help from high-altitude inputs.
My general theory concerning the substantial nature of the greenhouse powers of PW is based on readings of its total value, as derived from all levels of the (non-tropical) atmosphere. These are the readings that are obtained by radiosonde instruments and used in constructing the Weather Maps and also the real-time animation series. There is no separation made by altitude, or layers, or different components or anything of the kind, just a compilation into one final number expressed in terms of the total weight of H2O molecules in a vertical column. What I have found is that this one number is very useful when applied to calculations aimed at determining the causes of the warm and cold air temperature anomalies that appear every day all over the surface of the planet. This is especially true when the anomalies are large. All of the other causes to be considered seem to fade into the background, leaving only this one particular item of causation and the consistent quality of results that it yields.
The attention I keep focusing on the unique concentrations of PW content in the upper level of the troposphere has a separate purpose. This material can do things that concentrations of the same kind of material in the lower level would find virtually impossible, due to opportunities for higher concentrations, broader range of movement and a more versatile range of behavior within that exclusive environment. The event we have been looking at these last two days, while impressive, has nothing comparable in the way of commonly enlarged potential that I can see.
Carl