5. The assumption that radiometric surface 
temperature equals surface air temperature is not 
generally valid. In fact a considerable 
temperature discontinuity may exist in the lowest 
meter of the atmosphere during the day. In the 
present case satellite derived surface 
temperatures range as high as 55 C, while air 
temperatures, even in mid July, are mostly below 
40 C. This implies that the assumed temperature 
distribution T(p) = T (1 - k 6p/p) is 
substantially incorrect. 
Three potential sources of additional information 
may be considered: a) The sounder data from the 
HIRS may be used to extrapolate downward through 
the atmosphere (Davis and Tarpley, 1983); or b) 
one may use reported air temperatures from 
weather stations, with a resultant sampling 
problem in many areas of the world, or c) the 
estimated temperature of vegetated areas may be 
used to approximate air temperature at shelter 
height (Price, 1990). The last method uses only 
coincident AVHRR data, but also requires further 
analysis because few AVHRR picture elements 
represent pure vegetation cover in many areas 
such as the U.S. 
Further work is needed to establish the value of 
the procedure describe here for routine use in 
computations of the energy and moisture budgets 
at the earth's surface. 
CONCLUSION 
A procedure has been described for relating 
variability of AVHRR channels 4 and 5 to moisture 
and temperature. The method is an approximate 
procedure which is probably not competitive with 
conventional meteorological data in many areas. 
Its utility in practice will depend on the 
ability to screen the data for clouds, and to 
reduce errors associated with the various 
approximations. The principal advantage of the 
method is its capability to routinely process 
AVHRR data for the considerable information it 
contains about the earth's surface energy and 
moisture budget, and for possible correction of 
vegetation index calculations for atmospheric 
moisture. The method needs to be verified 
against an independent measure after the approach 
has been incorporated in a conventional sounding 
procedure. 
REFERENCES 
Davis, P.A., and J.D. Tarpley, 1983, Estimation 
of shelter temperatures from operational 
satellite sounder data, J. of Clim. and Appl. 
Met., 22:369-376. 
Price, J.C., 1984, Land surface temperature 
measurements from the split window channels of 
the NOAA 7 Advanced Very High Resolution 
Radiometer, J. Geophys. Res., 89, pp 7231-7237. 
Price, J.C., 1990, Using spatial context in 
satellite data to Infer regional scale 
évapotranspiration, IEEE Trans, on Geosci. and 
Remote Sens., in press. 
Kleespies, T. J., and L. M. McMillan, 1987, An 
Extension of the Split Window Technique for the 
Retreival of Précipitable Water: Experimental 
Verification, Contribution of NESDIS Authors to 
the International Workshop on Remote Sensing 
Retreival Methods, Williamsburg, VA., pp 335-345. 
Kneizys, F. X., E. P. Shettle, W.O. Gallery, L. 
H. Chetwynd, Jr., L. W. Abreu, J.E.A. Selby, R. 
W. Genn, and R. A. McClatchey, 1980, Atmospheric 
Transmittance/Radiance: Computer Code Lowtran 5, 
Envir.Res. Paper 697, pp 57-60, Air Force Geophys 
Lab., Hanscom AFB, Mass. 
Price, J. C., 1989, Estimating évapotranspiration 
over large areas, IGARSS'89/12th Canadian 
Symposium on Remote Sensing, 4, pp 2138-2141. 
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