Figure 6. Spectrogram of natural frequencies as observed at MIK, May 2001 through November 2003. Spectrogram composed of 1-hr time windows, each scaled so maximum is 1 and plotted with linear color bar. Weather data are from JPL weather station ( = 8.5 km). Vertical breaks in data are due to days with data glitches or no recorded data. Tick marks on x axis correspond to first of the month labeled underneath. The peaks in natural frequencies are observed to wander over the course of the 2.5 years. No long-term correlation with temperature is observed, though rain causes temporary lengthening of natural frequency.