function psi = psi_wvar(zet)
% computes MOST vertical velocity sturcture function
% sig-w = 1.25 * usr * psi_wvar(zet)
% Kaimal and Finnigan eq. 1.33, p.16

psi = (1 + 3*abs(zet)).^(1/3); % unstable
ii = zet>0;
psi(ii) = 1 + 0.2*zet(ii); % stable

end