Thermosonde data reveals a diurnal daytime shift in measured levels of C square (n) in the free atmosphere. The shift is manifested in two ways. First, an apparent offset in the smallest measured values of C square (n) exists. Secondly, the curve of the average profile shows an enhancement over nighttime profiles. Related optical and radar measurements have indicated that differences between day and night probably exist, but because of limited instrumental resolution and altitude capabilities those results are inconclusive. Several hypotheses have been put forward in this paper concerning possible instrumental or solar based sources of data contamination. We have examined the possibility that solar radiation causes probe heating with subsequent instrumental effects. Calculation, computer simulation, and direct measurements have shown that the sun heats the body of the probe sensor a couple of degrees above the ambient and that the level of heating depends upon the solar aspect angle and magnitude and direction of air flow over the probe. A small but insignificant AC type effect can result from improper probe geometry or probe mismatch together with a coupling of solar heating with velocity turbulence. Transient and DC type effects can occur, but measured, processed, and transmitted root mean square C square (n) information is not likely to contain instrumental contamination.