[R] Please help me on a simple avg question

[facehappywy]

Thanks every one first.

I have a dataset like this

Year	Month	Day	Hour	DIR	SPD (m/s)	SKYCOVER	TEMP (C)	RH(%)
2009	1	1	0	310	13.85824	3	-5	23
2009	1	1	1	330	14.30528	3	-7.222222222	26
2009	1	1	2	320	10.72896	3	-7.222222222	24
2009	1	1	3	320	10.72896	3	-6.666666667	23
2009	1	1	4	320	10.28192	3	-7.222222222	26
2009	1	1	5	330	10.28192	3	-7.222222222	26
2009	1	1	6	330	10.28192	3	-7.222222222	25
2009	1	1	6	320	12.96416	3	-7.222222222	26
2009	1	1	7	330	11.176	3	-7.222222222	26
2009	1	1	8	330	10.72896	3	-7.777777778	28
2009	1	1	9	330	10.72896	3	-7.777777778	27
2009	1	1	9	320	11.176	3	-7.222222222	26
2009	1	1	10	320	12.51712	3	-7.777777778	24
2009	1	1	11	310	9.83488	3	-7.777777778	24
2009	1	1	12	310	9.83488	3	-8.333333333	25
2009	1	1	12	310	10.72896	3	-8.888888889	26
2009	1	1	13	320	9.83488	3	-7.777777778	26
2009	1	1	14	310	9.83488	3	-7.222222222	23
2009	1	1	15	310	9.83488	3	-7.222222222	23
2009	1	1	15	320	10.72896	0	-7.222222222	23
2009	1	1	16	320	8.04672	0	-6.111111111	21
2009	1	1	17	300	9.83488	0	-3.888888889	18
2009	1	1	18	300	9.83488	0	-4.444444444	19
2009	1	1	18	310	9.38784	0	-2.777777778	16
2009	1	1	19	320	7.59968	3	-2.777777778	16
2009	1	1	20	310	6.25856	3	-2.777777778	18
2009	1	1	21	310	6.25856	3	-2.777777778	18
2009	1	1	21	310	7.15264	3	-2.777777778	16
2009	1	1	22	310	7.59968	3	-2.777777778	16
2009	1	1	23	300	5.81152	3	-3.888888889	16


It is a one year (2009), 12 month, every day, every hour data. Notice that
there are sometimes several readings  within one hour. I want to calculate
the hourly avg for each day in each month. Anyone could help me on this. 
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