I’m going to try something today that is not easy. I think I know how to explain the manner by which the upper-level pattern of hPa air pressure configuration is composed, and want to pass this information on as clearly as possible, but there is no easy way to do so without personal communication and use of pointers, etc. I will still make an attempt through this regular format. The shape of the pattern is largely responsive to air temperatures at the surface below, but not directly. It has no way to measure those temperatures the way we do. Units of 500hPa downward pressure, for example, can only respond to the upward pressure physically exerted by the air directly below, for no matter what reason, which in some cases may be irrespective of the temperature of that air.
We know that upward pressure is closely associated with temperature, simply because warm air expands in volume and cold air contracts, but that is not the whole story here. Elevation of the surface also comes into play. Parcels of air over an elevated surface are always cooler than they would be lower down. That’s because the air above an elevated surface is thinner and thus contains fewer molecules of overhead greenhouse gas obstructing the outflow of surface radiation toward space. This fact makes no difference for the hPa level. What appears to make a difference is the fact that elevation reduces the volume of air that is able to expand or contract between the surface and the lowest part of the hPa pressure system, which is lower than 500 and may even intersect with the surface in some places. As a result it seems possible that the hPa system may end up with an upward pressure reading comparable to that exerted by nearby regions that are not elevated.
This idea is readily corroborated by looking for special color-coding effects that either Greenland or the Himalaya mountain range might be having on our regular 500hPa map due to their relatively cold temperatures. There is nothing to see. They both have effects that blend right in with nearby areas that are not elevated. Presumably, the same outcome might be expected or watched for with respect to any of the lesser elevated regions that report cooler temperatures than the average of surrounding areas. It’s something to keep in mind when using air temperature maps as a way to explain hPa patterns, which we will next be doing.
I have set up two maps below for comparison, one showing hPa pressure and one of actual temperatures, with the intention of focusing on the Northern Hemisphere. Today is a good example to work with because there is so much complication to sort out. The main challenge will be to see how the shape, intensity and borders of the green zone on the hPa map are determined—wherever possible—by surface air temperatures. Use the line of dark green 5C temperature shading as a primary guide, and watch out for effects of elevation changes. When you are done with the green zone try using similar methods with the three smaller areas of blue zone that are prominent, and then do the same in the south, which is like a piece of cake. In spite of all the confusion you should end up by gaining confidence that this particular hPa pattern is really and truly determined in large part by temperature-related things that are going on simultaneously at the surface.
A proper understanding of how the hPa pressure pattern is established, as seen on a daily basis and never by accident, should be of great interest to anyone who already understands the connection between the pressure pattern and jetstream formation, along with the regulatory effect of jetstream activity on the movement of water vapor streams in the upper atmosphere, plus the surface warming effect of water vapor that has been allowed to migrate over broad swaths of territory in the higher latitudes. These processes create a feedback loop which could account for “stalled” weather patterns as well as major temperature anomalies. They surely deserve to be more deeply studied and ultimately made part of the basic curriculum of teachings in the climate and meteorological sciences.
Carl