Statistic to describe variability of a time series value This statistics newbie could use some help with time series statistics.
I am looking at sunlight (irradiance) over the course of a day. One day got cloudy, so the values went down, mid-afternoon. The curve looks like this:

The next day, was clear, so the curve has only 1 clear apex:

Is there a statistic to describe each day as "steady" or "changeable"?

You could evaluate the Energy spectral density integrated over a range of frequencies which you consider relevant to your idea of variability of irradiance .  The energy spectral density is the squared absolute value of the Fourier transform of your time-series measurement (in your case, irradiance). If you integrated over frequencies between (say) 10 times per day and 100 times per day then a day with a sunny morning and cloudy afternoon would have a low variability but a day with the same level of change happening half-hourly would have a high variability.
