The evolution of wind waves and swells in an opposing and following winds is investigated through laboratory experiments. The fluctuations of air pressures and water surface elevations under the coexistence of wind waves and swell were measured to evaluate an energy flux from the wind to the waves. The power spectra and the cross spectra between the wind-induced pressure and the surface elevation were calculated using the laboratory data. It was found that the energy flux from wind to swells in an opposing wind were negative for the period of swell longer than 1.2 s, but positive for the periods shorter than 1.2 s. For a following wind, the swell with the shorter period can be easily amplified. The significant wave heights of an opposing swell were found to be amplified when the wind waves effectively grew.
