A data frame containing a variable named "dates" in chron format and different climatic variables for each time step and for each site
var.name
Character object specifying the name of the climatic variable for which we will perfom spatial average
nstat
Numeric vector to define the number of sites used to obtain climatic series
na.proc
A logical value indicating whether NA values should be stripped before the computation proceeds
Value
An additional column will be added in the data frame introduced to contain the average of the variable of interest over the different sites for each time step