The present invention proposes a portable barometric altimeter (10) including a pressure sensor (14), a calculation unit (16) which determines an altitude value (H) as a function of the pressure (P) measured by the sensor (14) and as a function of a reference temperature (T.sub.0) corresponding to an estimate of the temperature of the air mass at sea level, and a time measuring unit (18) that supplies calendar data (D), characterized in that it includes a correction circuit (32), which selects the reference temperature value (T.sub.0) from among a series of values (T.sub.S) contained in a database stored in the altimeter (10), as a function of the date (D) and as a function of a predefined climate value (C). The invention also proposes a method of determining altitude (H).