function wn=cutoff(sp,cop)
%CUTOFF compute the cutoff frequency for time-series filtering.
%
%  Input: sp - sampling period in hours
%         cop - cutoff period in hours
% 
% Output: wn - cutoff frequency in rad/sec
%
% Call as: wn=cutoff(sp,cop);
%
% wn can be input into BUTTER to build a digital Butterworth filter.
%

sf = 2*pi/(sp*3600);
cof = 2*pi/(cop*3600);
wn = cof/(sf/2);