Back in December, I discussed a couple recent items in Physics Today of great relevance to fluid mechanics. I remarked that we don't get to see fluid mechanics on the cover of PT very often. Well, lo and behold, the last couple months have proven me delightfully wrong! The cover of the April issue featured "Fluid Dynamics of Dry Salt Lakes", referring to a "quick study" article by Beaume, Goehring, and Lasser. It discusses a theory of groundwater convection as a possible explanation of polygonal patterns on dry lake beds ("salt polygons"). The very next month, the cover features "A Shocking Start to Stars", referring to a piece by Ceccarelli and Codella, on the role of shock waves in the interstallar medium, especially in star formation, and even in synthesizing certain precursor molecules of life itself.
However, I was personally most enthralled by another piece that also appeared in the May issue by meteorologist Tim Palmer, "The real butterfly effect and maggoty applies". The title seems to have been taken from two earlier efforts, one coauthored by Palmer in 2019 on the real butterfly effect (published in Nonlinearity), and the other a 2002 PT Reference Frame article by Sir Michael Berry on singular limits. Both these papers seem to have had a strong influence on this article, which mainly deals with weather predictibility, and more generally the predictability of nonlinear systems, including the limits of artificial intelligence-based prediction. The article is very good, informative, and very readable. I'd come across the singular limit concept in discussions of aerodynamic lift, where viscosity is the parameter that when taken to exactly zero (and not just a limit approaching zero) results in the impossibility of lift. Reading Palmer's piece, and then Berry's, has helped me understand that there are other such examples in physics.
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