def add_VAR_DESC(cdf):
    """
    add the VAR_DESC global attribute constructed from variable names found in the file

    :param object cdf: netCDF file object
    """
    varkeys = cdf.variables.keys()
    dimkeys = cdf.dimensions.keys()
    varnames = []
    for key in varkeys:
        varnames.append(key)
    dimnames = []
    for key in dimkeys:
        dimnames.append(key)
    buf = ''
    for varname in varnames:
        if varname not in dimnames:
            buf = '%s:%s' % (buf, varname)
    cdf.VAR_DESC = buf
