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"?

I'd start with looking at the roughness type of metric, e.g. the sum of squared second differences $$\sum_i (x_i-2*x_{i-1}+x_{i-2})^2$$ or the variance of these differences.
