The invention provides a heart rate monitor, a method and a computer software product. The method determines from the user's electrocardiogram a reference value of a heart rate variable characterizing the heart rate; determines at least one environmental parameter value obtainable from air pressure using air pressure measurement; associates the reference value of the heart rate variable with at least one environmental parameter value; and records in a register at least one environmental parameter value and the reference value of the heart rate variable associated with the at least one environmental parameter value.