Here’s a plot of the error between the measured frequency of WWV and a temperature controlled crystal oscillator (not NIST-traceable, but temperature stable and verified within 200 MHz). WWV transmits a clock frequency, set by a cesium fountain (a bank of cesium clocks), which is accurate to within 1 part in 10^16. We receive the transmission here in Cleveland, via several ionospheric hops, and the path length changes with time and space weather. We can see these changes in path length by measuring the difference between the received clock from WWV and our local oscillator. When we plot it clearly shows sunrise and sunset here and in Colorado. Graph by Skylar KD9JPX.