Microthermals are often used to test for local seeing contributions of sites and inside telescope buildings. There is however a degree of uncertainty in their use. The refractive fluctuations are proportional to the air density fluctuations. But for isobaric changes the fractional air density fluctuations are equal to minus the fractional absolute temperature fluctuations. Whereas for adiabatic changes the fractional density changes are equal to plus 1/(Gamma4) times the absolute temperature fluctuation. The image size contributions are equal to these changes to the 1.2 power. Thus adiabatic changes produce about three times as much image degradation as do isobaric changes. There is a particular susceptibility to adiabatic changes with wave motion affecting seeing in the presence of stable stratification associated with a nocturnal ground thermal inversion. It is recommended that microthermal measures of seeing contribution from sites and telescope facilities should be treated with distrust unless accompanied by simultaneous measurements of pressure fluctuations. In consequence a number of current indicators for the thermal design of large telescopes and their facilities must be considered moot.