I am looking at different airports and have data on the amount of passengers per month for years 2012-2016. I need to be able to compare the extent to which traffic fluctuates from month to month, over a year. I am looking for a formula and/or method that will produce a value indicative of a market's vulnerability (how much it changes) to seasonality.
try Facebook's Prophet model. I think this will be best suitable out of many time series Models(AR/MA/ARCH/GARCH/ARIMA/ETS/ARTX etc).
refer Facebookincubator for detailed information.