I need an aggregate with 2 parameters

Hello:

I have a table with pluviometrical data

meteo (rain float, day date, oid station)

I want to select the the day of maximum value for each year for a given
measurement station. It should look like :

select max(rain),day from meteo group by date_part('year', day) where
station=7654765;

though it  obiously doesn't work.

I think that an aggregate function would be the best way to do this,
because the table is quite large and I need indexes for date and also
for stations, so adding a new one for the rain is too much. But the
aggregate should look like:

max_rain(rain, day) and return the day corresponding to the maximum
rain, So the query would be something like:

select max(rain), max_rain(rain, day) from meteo group by
date_part('year', day) where station=47654765;

I've tried to define a composite type for rain and day and the neccesary
functions to make the aggregate, but I cannot find the correct way. Can
someone help me? Is this sollution possible? 

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