Spring or autumn (log) rainfall season

Spring or autumn rainfall seasonality is an index derived from two ratios. The ratio of spring (Sep-Oct-Nov) to autumn (Mar-Apr-May) log-rainfall totals (minus 1) are assigned positive values when rainfall in spring is greater than rainfall in autumn. The ratio of autumn to spring log-rainfall totals (plus 1) are assigned negative values when rainfall in the autumn is greater than rainfall in spring.