I am not entirely happy with the explanation given by CL #1690 on June 1 pertaining to the way changes are made in the configuration of the 500hPa map. There has to be a simpler and more direct way to describe things that will end up with the same or an even better result. Let’s try again. Start by thinking of the 500hPa level we see on the map as a thin, flexible diaphragm surrounding the entire globe a few miles above surface. By definition, every square inch above the diaphragm will have exactly the same weight of molecules pressing down on it at all times because of gravitational pressure effected by the great mass of the planet below. The number of molecules over any one spot should never show much change because their total weight never changes, but the diaphragm is always free to move up or down. Why would it do so? Probably because of something either pushing up on it from below or causing it to contract downward, but that would not be gravity. There is only one other choice—the pressure of air itself, expanding or contracting, because air is a gas, and that’s what gases always do when they get warmer or cooler. (In fact, if the air gases were cooled to absolute zero on the Kelvin scale there would be no atmosphere at all. We only have one because there are gaseous elements and they do get warmed above zero, way above.)
This leaves us with two different concepts of “air pressure” to deal with, an inescapable reality because they both exist and are constantly in play. One kind of pressure is based on the weight of a body of air molecules, the other on its temperature. Each kind of pressure involves a different kind of force, and these two forces will always be meeting and competing when an atmosphere like ours is in existence. One of the forces, gravity, does not change, but the other one does, and whenever it changes effects of a complex nature are generated and felt in both weather and climate.
We know from first hand experience that air temperatures near the surface do a great deal of shifting between warm and cool, from causes starting just with day versus night, then on to all the many other things that make things uneven. We also know that when you measure temperatures at higher and higher elevations they tend to get colder and colder, and they also tend to even out the differences between warmer and colder. By the time you have risen to the 500hPa level, or maybe even lower than that, they are indeed very cold, and there is no longer much change between warmer and cooler. What this means is that the up and down type of action due to air temperature pressure should always affect the diaphragm from below, because that’s where the big temperature swings are happening. The diaphragm will be either pushed up or contracted down depending on the strength and duration of these swings. Now let’s take a quick look at a current global map of the 500 hPa diaphragm:
What does this all mean? One thing it tells us is that surface air temperatures have a powerful effect on the shape and configuration of the diaphragm, quite possibly all the time, as well as in more lasting ways. We also know from other studies that the shape and configuration of this same pressure distribution has a critical role in a sequence of events that can cause temperatures to change at the surface below. The pressures exerted by warm air temperature below the diaphragm alter the shape of the diaphragm in a manner that effectively reduces the strength of jetstream winds that are housed within the depressed sections regularly established over each of the high-latitude parts of the globe. Any such weakening inhibits their ability to control the movement of a particular and very powerful greenhouse gas, water vapor, bunches of which are always likely to be hanging around in high altitudes, trying to become more widely diffused throughout the atmosphere before condensing and falling out as precipitation. The movement of any extra amounts of this vapor over either polar region has an immediate warming effect on surface air temperatures below, thereby adding still more to upward pressure on the diaphragm and thus the potential for further weakening of associated jetstreams. I do not think there is any better explanation than this uniquely situated feedback loop for the severely unbalanced rate of change we are presently witnessing in the climate of the Arctic region.
Carl