Weather Definitive

Weather Definitive

Two-hundred and fifty years ago Captain James Cook landed at Possession Bay and took possession of the island, in the name of King George III and his heirs. In this anniversary year we celebrate South Georgia and its change from a little known and little valued island to a thriving UK Overseas Territory which is globally renowned for championing science, conservation and sustainable management. Each set of stamps issued this year will focus on a different element of South Georgia and shine a light on both its history and its future.

This second issue is a definitive set and focuses on weather - an element of South Georgia which is as captivating and challenging now as it was 250 years ago.

The weather on South Georgia is principally driven by the oceans which surround the island. As South Georgia is south of the polar front and directly in the path of the Antarctic circumpolar current the cooling effect of the ocean is key to the islands cold, wet, windy weather. Average temperature on South Georgia in the winter at sea level is 0oC to -1oC but the moisture laden air means that in this period precipitation falls as snow which frequently covers the entire island down to sea level from May to October. The maritime climate means that in summer temperatures at sea level range from an average of 3oC to 5oC although there can be significant variation locally. On the sheltered north coast temperatures in the summer months regularly exceed 10oC and the highest temperature recorded was from King Edward Point which once reached 28.8oC.

More than any other feature it is the wind, and its interaction with the high peaks in the centre of South Georgia, which drives local weather. The prevailing westerly wind and stream of weather systems brings frequent gales, low cloud, rain and snow. The parts of the island exposed to the full force of this weather on the southern coast and both easterly and westerly extremes are typically cooler and damper than the central northern coastline which is sheltered by the towering peaks of the Allardyce Range. However, whilst ordinarily providing a degree of shelter, being in the lee of these mountains means the bays experience two unusual wind phenomena. Katabatic winds occurs when air flowing from the west comes into contact with the heavily glaciated interior of the island, cools rapidly and becoming denser, drains away down slope. Föhn winds occur when the westerly airstream is partly blocked by towering peaks in the centre of the island causing it to be forced around the lower areas. However, a small amount ‘spills’ over the crest of the mountains and drops down the lee of the slope warming rapidly as it decants causing a strong, warm off shore wind. Waves within the föhn winds sometimes trigger violent downdrafts of up to 100 mph known as williwaws.

It is the wind and its interaction with the mountains that creates the striking lenticular cloud formations. Lenticular clouds are lens shaped and form when there is a stable flow of moist air over mountains and creates a standing wave. Lenticular clouds are often mistaken for UFOs because of their lens-like shape and smooth, saucer-like appearance.

Although the day-to-day variation in the weather is pronounced and often dramatic, it is the long-term average changes to weather patterns, or climate, which are having the most impact on the South Georgia environment. In the 250 years since Cook first landed the climate has changed profoundly. The impacts of climate change are multiple. The glaciers which 250 years ago flowed from the islands interior out to the sea have retreated dramatically, at a rate of anything from around 12 m per year on the cooler south side of the island to 384 m per year on the Neumayer glacier on the north coast. The retreat of glaciers has meant in some cases where glaciers once terminated in the sea, they now terminate on land leaving bare ground which is open for colonisation by plants and invertebrates.

It is not just retreating ice on the land which is affecting South Georgia. The warming waters of the Southern Ocean mean that in some years, the extent of the sea ice around the Antarctic continent is significantly reduced which can alter the distribution of krill and other prey species of the marine mammals and penguins that breed on South Georgia. The pattern of these changes and consequences for the ecosystem are complex and not fully understood making it a priority for research in the region.

Technical Details

Design Bee Design

Photography 5p - Anna Carter

10p, 70p, £1.25, £1.40, £2 & £3 - Dr Tom Hubbard

£1 & £2.05 - George Gittins

£1.85 & FDC - Carrie Gunn

£1.55 - Andy Black

£5 - Vikki Foster

Printer Cartor

Process Stochastic lithography

Perforation 13 ¼ x 13 ¼ per 2cms

Stamp size 60 x 24mm

Sheet layout 10

Release date 31 January, 2025

Production Co-ordination Creative Direction (Worldwide) Ltd