[time-nuts] Cavity frequency air filled vs vacuum?

[Hal Murray]

kb8tq at n1k.org said:
> And for major fun, you can get into altitude and humidity corrections on the
> exact number. Since your altitude does not change day to day, humidity is
> often the thing that drives you a bit nuts.  

Is it altitude or air pressure (density)?

There is a semi-joke about good enough pendulum clocks turning into 
garvitometers.  How accurately can you measure the frequency of a cavity?  
Can a good setup be used as a barometer?

How does humidity compare with air pressure?



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