Here is a simple example.
library(lubridate)
DF <- data.frame(Year=seq(1991,2000,1),Rain=runif(10,40,150))
DF
Year Rain
1 1991 87.86195
2 1992 133.48462
3 1993 49.38801
4 1994 145.06642
5 1995 110.06277
6 1996 136.55426
7 1997 111.44178
8 1998 95.93573
9 1999 130.38815
10 2000 63.17043
DF$AvgRain <- ifelse(leap_year(DF$Year),DF$Rain/366,DF$Rain/365)
DF
Year Rain AvgRain
1 1991 87.86195 0.2407177
2 1992 133.48462 0.3647121
3 1993 49.38801 0.1353096
4 1994 145.06642 0.3974422
5 1995 110.06277 0.3015418
6 1996 136.55426 0.3730991
7 1997 111.44178 0.3053199
8 1998 95.93573 0.2628376
9 1999 130.38815 0.3572278
10 2000 63.17043 0.1725968