A correction of the recent air-temperature record at the historical meteorological station of the National Observatory of Athens (NOA) due to instrument change

Two discontinuities were detected in the air-temperature time series at the meteorological station of the National Observatory of Athens. The first discontinuity reflects the instrumental change, which took place in June 1995 and the second discontinuity (and most pronounced) the application of a correction factor to the temperature values (in January 1997), after a calibration of the new thermometers. As a result, a cooling bias was observed after June 1995 and a warming bias after January 1997. The magnitude of bias exhibited a seasonal variability being more pronounced and reaching up to 0.67A degrees C during the warm period of the year. The common period of operation of the 'old' and 'new' instrumentation was used for the estimation of monthly correction factors and the removal of the bias. The application of the new correction factors, restore the continuity in the air temperature record after June 1995.