The average temperature of the air as indicated by a properly exposed thermometer during a given time period, usually a day, a month, or a year. For climatological tables, the mean temperature is generally calculated for each month and for the year. For charts, the observed mean values at station level are reduced to sea level by adding a correction for elevation, usually taken as 0.5°C for each 100 m (1°F for 360 ft), but in some mountainous countries different rates are used, based on local observations.