  Since their discovery three decades ago it has emerged that the physics of
high-$T_\mathrm{c}$ cuprate superconductors is characterised by multiple
temperature scales, and a phenomenology that deviates significantly from the
conventional paradigm of superconductivity. Below the "pseudogap" temperature,
$T^*$, a range of experiments indicate a reduction in the density of states.
Lower in temperature, $T_\mathrm{c}$ marks the onset of a bulk zero-resistance
state. Intermediate between these, numerous other temperature boundaries and
cross-overs have been identified, often postulated to be associated with
fluctuations of some kind. However, for the most part there is little consensus
either over their definitions or the physical mechanisms at play. One of these
temperature scales is the Nernst onset temperature, $T_\mathrm{onset}$, below
which thermoelectric phenomena characteristic of superconductivity are
observed. Depending on the material, the difference between $T_\mathrm{onset}$
and $T_\mathrm{c}$ ranges from almost nothing to over 100K. In this paper, we
identify $T_\mathrm{onset}$ from published experimental data according to a
consistent definition; this reveals a remarkable consistency of behaviour
across the whole high-$T_\mathrm{c}$ family, despite the appreciable variations
in other parameters. Our analysis suggests that in the cuprates there is an
inherent limit to $T_\mathrm{c} \sim 135$K. We compare this behaviour with
other strongly-correlated superconductors and propose a unified picture of
superconducting fluctuations close to the Mott state.
