# How to measure change in time series?

I have some weekly data about Apple app store ranking of the top apps listed (also of the sub categories such as games, weather, etc.). For example:

               week1   week2   week3   week4   ...
Angry Birds    15      13      1       5       ...
Weather Pro    ...


I am thinking about trying to estimate a parameter of "turbulence" or "change" for each category and compare them to each other. So, how can I measure change in time series?

-