  We study the ``jump'' in the blue horizontal--branch (BHB) distribution first
detected by Grundahl et al. (1998) in the Galactic globular cluster (GC) M13.
On the basis of Stromgren photometry for a sample of fourteen GC's we show
that: 1) The jump is best characterized as a systematic shift, on a (u, u-y)
color-magnitude diagram, from canonical zero-age HB (ZAHB) models, in the sense
that the stars appear brighter and/or hotter than the models; 2) the jump is a
ubiquitous phenomenon, ocurring over the temperature range 11,500 < Teff <
20,000K; 3) An analogous feature is present in (log g, log Teff) diagrams --
indicating a common physical origin for the two phenomena; 4) The physical
mechanism responsible for the jump phenomenon is most likely radiative
levitation of iron and other
