Isentropic Surfaces - Science and Art Merges

#2065 "Summer Cirrus" Pixels Link

My first exposure to meteorology after nuclear physics was in terms of isobaric surfaces. Upper air charts and analyses of the atmospheric were all completed in the world of pressure. Aviation revolved around pressure surfaces and that encouraged meteorologists to do the same. Like the earthly view of the solar system and the universe, mankind first tried to make sense of everything else by placing themselves at the centre of it all.
The atmosphere does not work like that as I described in my very first post “Cloud Shapes and Lines in the Atmosphere”. The motion of air in the atmospheric frame of reference may be even further refined.

Like man, air parcels prefer the easy route that does not require spending any energy. Entropy is a thermodynamic quantity representing the amount of energy in a system (that is no longer available for doing mechanical work). For a parcel of air, entropy is a function of the air temperature, pressure and composition (water vapour, water, or ice).  The potential temperature of an air parcel is a conservative property as long as the entropy of the parcel does not change.  An isentropic chart shows these surfaces of constant entropy. Air parcels can move anywhere along these surfaces for free. I should have started my meteorological training on isentropic charts which are much more in the atmospheric frame of reference. It took me a while to figure this out…

The following graphic is a cross-section of the isentropic surfaces from Texas northward to South Dakota. The blue isentropic lines generally slope upward to the north. Of course weather is more complicated than this but simple truths are a good place to start. 

The pressure axis on the left gives a sense of height and those light grey lines are horizontal on the chart and approximately so in nature. The distance scale is on the bottom. Isentropic surfaces slope upwards towards cold air (when looking along a constant height or constant pressure surface). Alternatively, isentropic surfaces slant downward to the warmest air. Air parcels are restricted to move along these isentropic surfaces in the absence of latent heat release due to phase changes of water. The vertical motion of an air parcel can be easily diagnosed from its motion along the isentropic surface and as a meteorologist, this is something you really want to know.

Conveyor Belt Conceptual Model

Weather gets simpler when one applies these simple isentropic ideas to the Conveyor Belt Conceptual Model. Weather is much more of a creative ballet with this trio of air flow dancers than a battle. The WCB, DCB and CCB play a part in every weather system and I will introduce these cast members soon. My ongoing thanks to the COMET program for their support and encouragement!

Weather can be really beautiful. This is where I have been heading all along! Art and science merges.

Warmest regards,
Phil the Forecaster Chadwick