Flow visualization experiments and film thickness measurements were conducted in air-water upward annular flow in a tube of 31.8 mm i.d. As the liquid flow rate was increased at a constant gas flow rate, first disturbance waves and then ephemeral waves were found to occur on the liquid film. A model to determine the film Reynolds number has been developed, and is used to show that the onset of disturbance waves and of ephemeral waves happens at a substrate film Reynolds number of about 500. It is proposed that the ephemeral waves are none but developing disturbance waves.