Atmospheric boundary layer motions are statistically non-stationary, and therefore, it is necessary to transform them into statistically stationary (statistical properties do not change in time) time series. This transformation is performed by the mean removal process, which defi nes an integral time scale for turbulent fl uctuations. The present study compares two methods for estimating physically relevant time scale in a stable, wintertime nocturnal boundary layer. This time scale is used to remove the unsteady mean component of the instantaneous time signal for any meteorological variable. In this way, turbulent fluxes that appear statistically stationary can be yield. The fi rst method employs the Fourier analysis in order to remove the contamination by mesoscale motions in calculation of eddy correlation flux. The second method used for determining a relevant turbulence avenging time scale represents a cumulative integral of the kinematic heat flux or momentum flux cospectrum: the ogive method. The time scale is evaluated using three-dimensional sonic anemometry data acquired at five levels of a meteorological mast (20, 32, 40, 55 and 62 m a.g.l.) located in the industrial part of Kutina, a town in Croatia. The results indicate the existence of the spectral gap scale in the spectrum of streamwise velocity component at all levels except at 32 m. This gap scale can be found for periods between 30 and 7 min for 20 m level and between 22 and 7 min at other levels. The time scale estimated by the second method is based on the location of the local maximum value of the ogive function of the kinematic heat fl ux and momentum fl ux cospectra. The ogive method gives time scales that defi ne the high-frequency end of the spectral gap.Gibanja u atmosferskom graničnom sloju su statistički nestacionarna te ih se stoganužno mora transformirati u statistički stacionarne vremenske nizove. (Statistička svojstva stacionarnih nizova se ne mijenjaju u vremenu.) Ova transformacija se izvodipostupkom uklanjanja srednjaka, čime se definira vremenska skala turbulentnih fluktuacija. U ovoj studiji uspoređujemo dvije metode pomoću kojih ocjenjujemo fizikalno relevantne turbulentne vremenske skale u stabilnom, zimskom noćnom graničnom sloju. Ovavremenska skala se koristi za uklanjanje nestacionarne srednje komponente u vremenskomnizu bilo koje od mjerenih meteoroloških varijabli. Prva metoda uključuje primjenu Fourierove analize, kako bi u računanju vertikalnih tokova turbulentnih kovarijanci utjecajmezoskalnih gibanja bio isključen. Druga metoda korištena za određivanje relevantne turbulentne vremenske skale usrednjavanja predstavlja kumulativni integral kospektrakinematičkog toka topline ili toka impulsa (eng. ogive metoda). Za ocjenu ove vremenskeskale koriste se trodimenzionalni podaci soničnih anemometara postavljenih na pet visinameteorološkog tornja (20, 32, 40, 55 i 62 m iznad tla), koji se nalazi u industrijskom dijelugrada Kutine. Rezultati upućuju na to da na svim visinama, izuzev 32 m, postoji procjep uspektru komponente vjetra usmjerene niz srednje strujanje. Spektralni procjep se uočavana periodima između 30 i 7 minuta na visini od 20 m, te između 22 i 7 minuta na ostalimvisinama. Vremenska skala turbulentnog usrednjavanja dobivena pomoću druge metodese temelji na pronalaženju frekvencije, pri kojoj se javlja lokalni maksimum u kumulativnom integralu kospektra kinematičkog toka topline i toka impulsa. Druga metoda dajevremenske skale koje definiraju visokofrekventni kraj spektralnog procjepa
