For the data first posted--
ix=find([mod(wdir(:,4),3)==0 & wdir(:,5)==0]).'
ix =
1 19 37 55 73 91 109 127 145
>> wdirkeep=wdir(ix,:)
wdirkeep =
2009 1 1 0 0 0 20
2009 1 1 3 0 0 27
2009 1 1 6 0 0 67
2009 1 1 9 0 0 104
2009 1 1 12 0 0 90
2009 1 1 15 0 0 92
2009 1 1 18 0 0 106
2009 1 1 21 0 0 340
2009 1 2 0 0 0 25
>>
ADDENDUM
NB: To just do the selection, note that you don't need the find operation; that just returns the actual locations which is easier to read than the full-length logical vector. If simply need the result, then use the logical-addressing vector as
ix=(mod(wdir(:,4),3)==0 & wdir(:,5)==0).';
wdirkeep=wdir(ix,:);
or, unless there's reason to address this same subset again, there's no real reason to even make the temporary array at all (other than for legibility of code, I'll often write the indexing expression first as above and then do the substitution into the other function to keep parenthesis-nesting straight; writing it all in one pass can get to be too much to keep track of at one time)--
wdirkeep=wdir((mod(wdir(:,4),3)==0 & wdir(:,5)==0),:).';
