It is shown that the medium-term frequency stability of four different 5-MHz oscillators controlled by two different types of AT-cut resonators is sensitive to changes in atmospheric moisture and possibly pressure. Stabilizing the pressure and humidity, in addition to the normal parameters in an otherwise controlled laboratory setting, improves the frequency stability for measurement times in the region of hours to days. The mechanism by which fluctuations in moisture and possibly pressure bring about frequency changes is speculated upon, noting that the effect could be due to changes in the thermal gradients, changes in the dielectric constants, or residual leakage between critical circuit elements. A simple test is introduced to determine the relative importance of thermal gradients within air-enclosed ovens.
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