DAILY AIR TEMPERATURE AND PRESSURE SERIES ... - BALTEX

200 ANDERS MOBERG ET AL.
Figure 12. Difference in annual mean sea level pressure between Stockholm and the gridpoint 60 ◦ N
20 ◦ E (Jones et al. 1999b). (a) Before homogenization. (b) After homogenization. The grey curve in
(a) shows the correction constants given in Table VI as a time series (but with opposite signs).
using a linear regression relation. The remainder (26) was filled in through linear
interpolation between the pressure for neighbouring days.
5.5. COMMENTS TO THE HOMOGENEITY TEST RESULTS
Concerning the period 1756–1858, the assumed period for the Ekström barometer,
there are several sub-periods with different biases. The results suggest that the bias
originally was only –0.6 hPa, indicating that the instrument was well made and that
our assumptions concerning reductions (Section 5.3) were essentially relevant. An
abrupt change to a bias of +1.8 hPa occurred in March 1774. The reason for this
is entirely unknown. The bias was thereafter between +1.8 and +1.4 hPa until November
1839, except for two sub-periods (1803–1808, 1821–1824) with markedly
different biases. The latter sub-period began with two weeks of missing data in October
1821 and ended with two days in July 1824 having double observations. This
suggests that the Ekström barometer (still assuming this one was used) went out of
order in October 1821 and was sent for restoration. Another barometer (with bias
+4.5 hPa) was probably used until July 1824. The situation may have been similar
1803–1808, although a different barometer (with bias +0.5 to –1.4 hPa) could have
been used. From November 1839 to August 1843 the bias was +4.3 hPa (nearly