title: Column means over a finite range of rows
body: <p>I am working with climate data in New Mexico and I am an R novice. I am trying to replace NA with means but there are 37 different sites in my df. I want the means of the column for which the DF$STATION.NAME (in column 1) is unique. I cant be using data from one location to find the mean of another... obviously. so really I should have a mean for each month, for each station.</p>
  
  <p>My data is organized by station.name vertically in column 1 and readings for months jan-dec in columns following, including a total column at the end (right). readings or observations are for each station for each month, over several years (station name listed in new row for each new year.) </p>
  
  <p>I need to replace the NAs with the sums of the CLDD for the given month within the given station.name, how do I do this?</p>