annual (log) rainfall seasonality index

Annual rainfall seasonality is an index derived from two ratios. The ratio of warm (Oct-Nov-Dec-Jan-Feb-Mar) to cool (Apr-May-Jun-Jul-Aug-Sep) season log-rainfall totals (minus 1) are assigned positive values when rainfall in the warm season is greater than rainfall in the cool season. The ratio of cool to warm season log-rainfall totals (plus 1) are assigned negative values when rainfall in the cool season is greater than rainfall in the warm season.