[R-sig-Geo] Calculate percent cover in a raster

[Thiago V. dos Santos]

Hi all,

I have a raster at 0.5 degree resolution with monthly totals of rainfall, which range from 50 mm to 600 mm:

require(raster)

## scratch a raster and fill some random values
r <- raster(nrows=22, ncols=20, xmn=-58, xmx=-48, ymn=-33, ymx=-22)
r[] <- round(runif(22 * 20, min=50, max=600), digits=0)


How can I calculate the percentage area of the raster covered with some rainfall ranges? 

Let's say for example 0 to 100, 101 to 200, 201 to 300, 301 to 400, 401 to 500 and finally 501 to 600 mm? 
Thanks in advance,
 -- Thiago V. dos Santos

PhD student
Land and Atmospheric Science
University of Minnesota