  [Abridged] A key hypothesis in the field of exoplanet atmospheres is the
trend of atmospheric thermal structure with planetary equilibrium temperature.
We explore this trend and report here the first statistical detection of a
transition in the near-infrared (NIR) atmospheric emission between hot and
ultra-hot Jupiters. We measure this transition using secondary eclipse
observations and interpret this phenomenon as changes in atmospheric
properties, and more specifically in terms of transition from non-inverted to
inverted thermal profiles. We examine a sample of 78 hot Jupiters with
secondary eclipse measurements at 3.6 {\mu}m and 4.5 {\mu}m measured with
Spitzer Infrared Array Camera (IRAC). We measure the deviation of the data from
the blackbody, which we define as the difference between the observed 4.5
{\mu}m eclipse depth and that expected at this wavelength based on the
brightness temperature measured at 3.6 {\mu}m. We study how the deviation
between 3.6 and 4.5 {\mu}m changes with theoretical predictions with
equilibrium temperature and incoming stellar irradiation. We reveal a clear
transition in the observed emission spectra of the hot Jupiter population at
1660 +/- 100 K in the zero albedo, full redistribution equilibrium temperature.
We find the hotter exoplanets have even hotter daysides at 4.5 {\mu}m compared
to 3.6 {\mu}m, which manifests as an exponential increase in the emitted power
of the planets with stellar insolation. We propose that the measured transition
is a result of seeing carbon monoxide in emission due to the formation of
temperature inversions in the atmospheres of the hottest planets. These thermal
inversions could be caused by the presence of atomic and molecular species with
high opacities in the optical and/or the lack of cooling species. We find that
the population of hot Jupiters statistically disfavors high C/O planets (C/O>=
0.85).
