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"?

Suppose you use only data from sunrise to sunset.  With no obstruction, it appears that a quadratic trend in time adequately models your data.  When an obstruction occurs, the quadratic fit would be less adequate.  Hence, one can test the discrepancy between the quadratic fit and a nonparametric fit (LOESS, local linear regression, etc.) to a dataset.  Perhaps simpler, one could test the linear model vs the quadratic model.  If the quadratic fit does not hold, then obstruction has been detected and you can mark it as "changeable."
