A humidifier (4) for a respiratory apparatus (1) for delivering a humidified flow of breathable gas to a patient includes a humidifier chamber configured to store a supply of water to humidify the flow of breathable gas. The humidifier chamber includes a first heating element configured to heat the supply of water. The humidifier (4) also includes a relative humidity sensor (52) to detect a relative humidity of ambient air and generate signals indicative of the ambient relative humidity； a first temperature sensor (48) to detect a temperature of ambient air and generate signals indicative of the ambient temperature； and a controller (40) configured to determine an absolute humidity of the ambient air from the signals generated by the relative humidity sensor (52) and the first temperature sensor (48) and to control the first heating element to provide a predetermined relative humidity to the flow of breathable gas. A method of humidifying a flow of breathable gas to be provided to a patient includes determining an absolute humidity of ambient air used to form the flow of breathable gas； and controlling a temperature of a supply of water that humidifies the flow of breathable gas to provide a predetermined absolute humidity corresponding to a predetermined temperature and a predetermined relative humidity of the flow to be delivered to the patient.
