Geoscience Reference
In-Depth Information
the Geological Society) was one of the few dealing with palaeometeorology that
had been published in a geological journal up to that time, which, in Harmer's
case, indicated how certain problems in geology could possibly be resolved by
means of meteorological methods. He believed that advances made in meteorology
during the late 19th century in synoptic weather analysis could be adapted to
resolve such questions (Harmer
1901
).
Winds are an important factor in determining the distribution of climatic zones. Deviations
of the isotherms from the normal are generally connected with the direction of the pre-
vailing winds. The influence of marine currents is indirect rather than direct. Changes of
wind cause marked and sudden changes in the weather, though the general direction of
ocean currents remains the same. Permanent alterations in climate during past epochs
would have equally resulted from permanent changes in the wind. Anomalous weather is
due to some unusual arrangement of high and low pressure areas. Former cases of
anomalous climate can only have occurred when the meteorological conditions were
favourable.
Continental areas tend to be cyclonic in summer and anticyclonic in winter, while the
reverse is broadly true of the oceans. During the Glacial Period ice-covered areas would
have remained more or less anticyclonic throughout the year, while low pressure areas
must have prevailed in regions to the south of them and over the adjoining oceans. This
would have altered the prevalent direction of the winds and the distribution of rainfall;
thus the anticyclone of the European ice-sheet may have caused cyclonic storms to pass
farther south than at present, bringing oceanic winds over the Sahara, which formerly
enjoyed a humid climate. Dead shells are rarely found now on the eastern shores of
Norfolk and Suffolk, though they are driven on to the Dutch coast by westerly gales. Shell-
débris in the Upper Crag-beds of East Anglia shows that easterly gales were common at
that period. This may have been due to the altered path of cyclones, caused by the glacial
conditions which were becoming established in regions to the north of Great Britain
(Harmer
1901
).
To highlight his approach, Harmer published selected charts illustrating his
reconstruction of circulation patterns for the winter and summer seasons during the
maximum glaciation of Europe about 18,000 years ago when northern and central
parts of the British Isles were covered by a thick and extensive layer of ice
(Kington
2010
). By means of these charts he inferred that travelling depressions,
which at present generally move west-east over the Atlantic towards Europe, must
have taken a southeasterly track more frequently than today due to the blocking
effect of persistent and strong anticyclonic patterns over Scandinavia and adjacent
areas. Also, he showed that the sub-tropical high pressure belt, which today pre-
vents moist winds from the Atlantic blowing over the Saharan desert region,
moved equatorwards. Consequently, with presumably increased rainfall occurring
in lower middle latitudes and the subtropics, a humid climate prevailed over
regions now affected by drought. It had long been known, as acknowledged by
Harmer, that the Saharan region, which today has a virtually rainless climate,
formerly experienced comparatively humid conditions during the Pleistocene
epoch. Harmer's charts also corroborate the present concept of an expanding and
contracting circumpolar vortex of the Northern Hemisphere in cold and warm
periods respectively during and since the Pleistocene Ice Age.
