DAILY AIR TEMPERATURE AND PRESSURE SERIES ... - BALTEX

178 ANDERS MOBERG ET AL.
Figure 2. The astronomical observatory in Stockholm 1784. The arrow marks the probable thermometer
position, outside a window on the north-facing wall, which was used from 1754 to
1875.
position. This position, at 5.8 m above ground (Figure 2) was used until the end of
August 1875, when the building underwent major changes and a new section was
built on its north side (Hamberg, 1906). From September 1875 through 1960 the
thermometer was placed on the new north-facing wall at 1.5–1.9 m above ground.
A metal screen, placed in front of a window, (Figure 3) was introduced in January
1878 for protection against radiation. Modén (1963) reported that the screen was
painted white, although it is not known if it was painted already when it was new.
In 1947, when the observation hours changed, the window screen became exposed
to sunlight at the time of observations in summer mornings. Therefore,
a psychrometer-thermometer was placed in a small free standing screen, about
10 metres northeast of the building. The morning observation was made on this
thermometer during May–September from 1947 to 1960, whereas all other observations
were made in the window screen (Modén 1963). In August 1960 a standard
SMHI screen (Stevenson type, thermometer height 1.5–1.7 m) (Figure 3) was introduced.
Since 1961, all temperature readings are made there. Inspection reports
from 1970 onwards reveal that the screen repeatedly has become dirty because of
air pollution. This may have caused temperatures to be positively biased. Furthermore,
the position of the screen, between a tall and dense hedge and the building,
is not optimal for temperature measurements as the ventilation is poor.