  Almost all parameterizations of turbulence in NWP models and GCM make the
assumption of equality of exchange coefficients for heat $K_h$ and water $K_w$.
However, large uncertainties exists in old papers published in the 1950s, 1960s
and 1970s, where the turbulent Lewis number Le_t $= K_h / K_w$ have been
evaluated from observations and then set to Le_t$=1$.
  The aim of this note is: 1) to trust the recommendations of Richardson
(1919), who suggested to use the moist-air entropy as a variable on which the
turbulence is acting; 2) to compute a new exchange coefficients $K_s$ for the
moist-air entropy; 3) to determine the values of the new entropy-Lewis number
Le_ts $= K_s / K_w$ from observations (M\'et\'eopole-Flux and Cabauw masts) and
from LES and SCM outputs for the IHOP case (Couvreux et al., 2005).
  It is shown that values of Le_ts significantly different from $1$ are
frequently observed and may have large consequences on the way the turbulence
fluxes are computed in NWP models and GCMs.
