average with multiple stations

[simoes]

hi all, i have seven stations with following time series:

#1 - 20 years - 1966 to 1985
#2 - 27 years - 1963 to 1989
#3 - 27 years - 1963 to 1989
#4 - 24 years - 1968 to 1991
#5 - 27 years - 1963 to 1989
#6 - 20 years - 1963 to 1982
#7 - 30 years - 1963 to 1992

i have one value observed per year (station #1, 20 values; station #2, 27
values...). i like to do a simple average with the available values for each
year. examples:

t = 1963
average1963 = (#2-63 + #3-63 + #5-63 + #6-63 + #7-63) / 4

t = 1991
average1991 = (#4-91 + #7-91) / 2

please, someone helps me?

thanks,

--
simoes