我的数据是
"V1" "V2" "V3" "V4" "V5" "V6" "V7" "V8" "V9" "V10" "V11" "V12" "V13" "V14" "V15" "V16"
"1" "区站号" "纬度" "经度" "观测场拔海高度" "年" "月" "日" "平均气温" "日最高气温" "日最低气温"
"2" "59287" "2308" "11319" "66" "1981" "1" "1" "15.6" "23.6" "9.4" "156" "236" "94" "0" "0" "0"
"3" "59287" "2308" "11319" "66" "1981" "1" "2" "15.5" "19.7" "13.2" "155" "197" "132" "0" "0" "0"
"4" "59287" "2308" "11319" "66" "1981" "1" "3" "13.7" "19.8" "9.4" "137" "198" "94" "0" "0" "0"
"5" "59287" "2308" "11319" "66" "1981" "1" "4" "13.4" "18.8" "8" "134" "188" "80" "0" "0" "0"
数据没有弄完，一直到15年12月31日的。这些数据是保存在X这个数据框中。
现在我想用R求每个月平均温度，我要怎么求？
作为一个R语言新手，请大神指点，谢谢！