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A "jump" in the BHB distribution in the V, u-y CMD was recently detected inthe GC M13. It is morphologically best characterized as a discontinuity in u,u-y, with stars in the range 11,500<Teff(K)<20,000 deviating systematicallyfrom (in the sense of appearing brighter and/or hotter than) canonical ZAHBs.We present u, y photometry of 14 GCs obtained with 3 different telescopes(Danish, NOT, HST) and demonstrate that the u-jump is present in every GC whoseHB extends beyond 11,500K, irrespective of [Fe/H], mixing history on the RGB,and other GC parameters. We suggest that the u-jump is a ubiquitous feature,intrinsic to all HB stars hotter than 11,500K. We draw a parallel between theubiquitous nature of the u-jump and the problem of low measured gravities amongBHB stars. We note that the "logg-jump" occurs over the same temperature rangeas the u-jump, and that it occurs in every metal-poor GC for which gravitieshave been determined--irrespective of [Fe/H], mixing history on the RGB, or anyother GC parameters. Furthermore, the u-jump and the logg-jump are connected ona star-by-star basis. The two are likely different manifestations of the samephysical phenomenon. We present a framework which may simultaneously accountfor the u-jump and the logg-jump. Reviewing spectroscopic data for severalfield BHB stars, as well as two BHB stars in the GC NGC 6752, we find evidencethat radiative levitation of heavy elements takes place at Teff>11,500 K,dramatically enhancing their abundances in the atmospheres of BHB stars in the"critical" temperature region. Model atmospheres taking diffusion effects intoaccount are badly needed, and will likely lead to better overall agreementbetween canonical evolutionary theory and observations for BHB stars.Comment: ApJ, Main Journal, accepted. Contains several changes and update

Hot Horizontal‐Branch Stars: The Ubiquitous Nature of the “Jump” in Stromgrenu, Low Gravities, and the Role of Radiative Levitation of Metals