There is a remarkable lack of understanding with respect to the sensitivity of the minimum temperature overnight to relatively slight changes in surface conditions. This post illustrates one example of this sensitivity. The implication of such influences is that the use of the minimum temperature as part of the construction of the global average surface temperature trend is seriously flawed, as we discussed in our paper
Pielke Sr., R.A., C. Davey, D. Niyogi, S. Fall, J. Steinweg-Woods, K. Hubbard, X. Lin, M. Cai, Y.-K. Lim, H. Li, J. Nielsen-Gammon, K. Gallo, R. Hale, R. Mahmood, S. Foster, R.T. McNider, and P. Blanken, 2007: Unresolved issues with the assessment of multi-decadal global land surface temperature trends. J. Geophys. Res., 112, D24S08, doi:10.1029/2006JD008229.
An example of this sensitivity is the strong effect on minimum temperature from the dry bulb temperature, T, and dew point temperature, TD, at sunset on clear nights with light winds and little horizontal temperature advection: see http://fawn.ifas.ufl.edu/tools/minimum_temperature/. This method uses the Brunt Equation as reported by Tom Oswalt, Larry Parsons and John Jackson of the University of Florida.
e.g. Putting a value of T = 80F and TD = 74F yields Tmim = 71.4
Putting a value of T = 80F and TD= 75F yields Tmin = 71.9F
which is a 0.5 difference. Such a change in dew point can occur just due to the growth of vegetation around an historical climate reference network site, as we have seen in quite a few of the photographs compiled at Watts Up With That. The presence of more vegetation can elevate the dew point temperature at sunset as a result of the transpiration of water vapor into the atmosphere. This effect on dew point is one of a number of surface and atmospheric influences on the surface air temperatures. I will be weblogging on more of these effects in upcoming posts.
Climate Science: Roger Pielke Sr. 