To find 
, one uses the calculated adiabatic drop in 
across
the outer core. Braginsky and Roberts find a drop of 1300 K,
or 
. Boehler calculated the adiabatic
temperature across the outer core (starting from 
at the ICB), and, using his latest experimental results on the thermal
expansivity of iron at high pressure [ Boehler et al., 1990], obtained
at the core side of the
core-mantle boundary. This gives a temperature jump of about
1300 K across the core-mantle boundary. For the Poirier
calculation of 5000 K at the ICB, the temperature jump
at the CMB would be close to 1600 K.
With all these reservations, the current best estimate of the jump
in 
across the 
layer is about 1500 K.
But this must be considered as tentative and as a probable
upper limit. Nevertheless, current estimates
from mineral physics converge on the idea
that the thermal boundary layer is large at the
core-mantle boundary.