[R] how to divide a data frame of half-hourly wind observations into subset of three hours

[Stefano Sofia]

Dear R-users,
from a data frame of half-hourly wind observations (direction and speed, three years data), I need to evaluate the first main direction for each period of three hours (h03, h06, h09, h12, h15, h18, h21, h24 of each day).

The command "windRose" of the package "openair" crates a data frame that contains the information I need.
Can I ask you which is the most efficient way to run the windRose command for each subset of three hours?

The initial data frame is called wind_df is like

Date Direction Speed
2015-06-30 00:30:00 315 2.9
2015-06-30 01:00:00 338 3.5
2015-06-30 01:30:00 23 4.8
2015-06-30 02:00:00 338 4.1
2015-06-30 02:30:00 135 1.1
2015-06-30 03:00:00 45 2.7
2015-06-30 03:30:00  1.7
2015-06-30 04:00:00 293 1
2015-06-30 04:30:00 45 1.5
2015-06-30 05:00:00 45 2.3
...

The command for the wind rose creation is
output1_df <- windRose(wind_df, ws="Speed", wd="Direction", layout=c(1,1), ws.int=4, angle=22.5, type="default", bias.corr=TRUE, cols="default", grid.line=NULL, width=2, seg=NULL, auto.text=TRUE, breaks=c(0,0.3,1.5,3.4,5.4,7.9,10.7,13.8,17.1,20.7,24.4,28.4,32.6,40), offset=5, normalise=TRUE, max.freq=NULL, paddle=TRUE, key.header=NULL, key.footer="(km/hr)", key.position="bottom", key=TRUE, dig.lab=2, statistic="prop.count", pollutant=NULL, annotate=TRUE, border=NA, par.settings=list(fontsize=list(text=10)))

and I evaluate the first main direction through
output2_df <- output1_df$data
output2_df$wd[output2_df$freqs == max(output2_df$freqs)])


Thank you for your attention and your help
Stefano Sofia


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