The annual number of passengers on flight Global Air $01$ from Boston to Bermuda has been tabulated for four years.

$\backslash$table$[[$Season$,$Year,$],[2018,2019,2020,2021],[$Winter$,41,40,45,49],[$Spring$,31,27,20,15],[$Summer$,42,37,41,42],[$Fall$,37,43,37,43]]$

What is the seasonal index for Spring?

Round to two decimal digits.