  We study the emergent spectral fluxes of transiting hot Jupiters, using
secondary eclipses from Spitzer. To achieve a large and uniform sample, we have
re-analyzed all secondary eclipses for all hot Jupiters observed by Spitzer at
3.6- and/or 4.5 microns. Our sample comprises 457 eclipses of 122 planets,
including eclipses of 13 planets not previously published. We use these eclipse
depths to calculate the spectral fluxes emergent from the exoplanetary
atmospheres, and thereby infer temperature and spectral properties of hot
Jupiters. We find that an abrupt rise in brightness temperature, similar to a
phase change, occurs on the day side atmospheres of the population at an
equilibrium temperature between 1714K and 1818K (99-percent confidence limits).
The amplitude of the rise is 291 +/-49 Kelvins, and two viable causes are the
onset of magnetic drag that inhibits longitudinal heat redistribution, and/or
the rapid dissipation of day side clouds. We also study hot Jupiter spectral
properties with respect to metallicity and temperature inversions. Models
exhibiting 4.5 micron emission from temperature inversions reproduce our fluxes
statistically for the hottest planets, but the transition to emission is
gradual, not abrupt. The Spitzer fluxes are sensitive to metallicity for
planets cooler than approximately 1200 Kelvins, and most of the hot Jupiter
population falls between model tracks having solar to 30X-solar metallicity.
