Part I: Seals teeth and whales ears - Scott Polar Research Institute ...

seldom below -12°C. In terms of human susceptibility it is effectively colder if
the wind chill factor is applied. However, we did experience temperatures of -
40°C on a winter journey and this was very cold, particularly when a wind was
blowing. An air temperature of -40°C with a wind of only [25 knots] is
equivalent to a wind chill of - 60°C and much colder in a gale). On a still sunny
day in summer though, the solar radiation allowed us to go shirtless when
working outdoors in the sun, but it remained cold in the shade. (The solar
maximum temperature was typically 45°C on a fine February day when the
shade air temperature was 2°C). The coldest month is usually July, the warmest
February. The prevailing westerly winds blow from southwest to northwest and
frequent warm northerly winds are experienced due to föhn effects, as clouds
pour from the north over the mountain barrier of Coronation Island. Gales occur
on about 60 days a year and at times the wind strengths were off the scale of our
anemometer, more than [100 mph]!
In fact the South Orkneys lie in a zone with the highest annual average cloud
cover in the world, also the strongest winds at sea producing the highest waves!
The day to day weather is determined by a succession of depressions travelling
around the Antarctic from West to East, which bring changeable weather as the
fronts pass through with monotonous regularity. They also bring rapidly
changing sea conditions, as we were to find on occasion to our alarm. In winter
when the sea is frozen well north of the islands the depression tracks tend to
pass further to the north and in general, though colder, the winter climate is
preferable.
So we experienced frequent heavy cloud cover and lack of sunshine and
frequent, often heavy, precipitation as snow or rain, which may fall in any
month. In an average year, from the records, there are about 280 days with
snowfall and drifting snow occurs on about 120 days! Signy experiences 60-80%
cloud cover in winter and 80% in summer; a common weather condition known
to us as ‘mank’ is when the island is shrouded in low cloud. Conversely ‘dingle
days’ occurred once or twice a week, more often in winter. The average total
annual sunshine is 520 hours (a mere 14% of that theoretically possible!).
The seasonal temperature changes are also influenced by the presence of sea
ice - as pack ice and icebergs move up to or away from the islands - producing
falling and rising temperatures. When the sea freezes to form fast ice the
temperature plummets, rising again when the ice breaks up and moves away.
Such fluctuations may happen many times during the summer and occasionally
during the winter, under the influence of sea swell and wind sheer. Over some
forty years of records the pack was usually sighted first in April/May (83%),
otherwise in June/July (17%). Sea ice thick enough to walk upon has been first
recorded over this period in April (7.5%), May (40%), June (37.5%) and July
(15%). The ice has moved out of Factory Cove in June (3%), in August (8%), in
September (26%), October (26%), November 21%) and December (l6%). In our
two years (l948 and l949) the pack was first sighted on 4 and 5 April; it could
safely be walked on by 7 May and 13 April respectively, and went out on 29
October and 19 November. These were therefore years with relatively long
periods of fast ice and lower than usual air temperatures, as we were to find out.
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