WEATHER IN THE ANTARCTIC
Antarctica is a huge continent over 50 times the size of Great Britain. It rises steeply from sea level at the coast to a vast ice plateau lying above 2000 metres and covering more than half the continent. The mean altitude of the continent is 2300 metres and the plateau reaches above 4000 metres at its highest. The Vinson Massif in the Ellsworth Mountains, which rises to 5140 metres, is the highest point of the continent. The five British bases are at coastal sites, primarily so that stores can easily be transported ashore. This means that in some ways their weather is not a true representation of the continent as a whole, as they are much milder due to the influence of the sea.
The weather varies enormously over this large area. Depressions cannot penetrate far inland and the interior receives the equivalent of only 50 mm rainfall each year - about as much as the Sahara desert. The high altitude of the polar plateau and the long winter night make it the coldest place on Earth. The northernmost coastal regions have mean summer temperatures just above freezing, whilst summer temperatures at the American Amundsen-Scott station at the South Pole are around -25° C. Even the warmest temperature ever reached there is only -14 C. Winter temperatures for the Antarctic peninsula stations are highly variable depending on where the weather has come from. Northerly winds bringing down air from mid-latitudes can push temperatures above freezing, whilst southerly winds from the continental interior can send the temperature plunging to -40 C. The South Pole, at an altitude of 2800 m, has a long, cold winter lasting from March to October with a mean winter temperature of -59 C. The Russian Vostok station (78°S, 107°E), located near the South Geomagnetic Pole and 3500 m above sea level, has an annual mean temperature of only -56 C. The lowest air temperature ever recorded on the Earth's surface, -89.2 C, was measured at this station.
The air temperature can be a misleading indicator of comfort when working outdoors. On a sunny summer's day it is quite possible to work outside in shirt sleeves and shorts, even with the temperature well below freezing, provided the air is calm. Sunburn can be quite a problem as the snow surface reflects over 90% of the sunlight falling on it, thus nearly doubling the exposure to solar radiation. By contrast a strong gale can cause such a large "wind chill factor" that even air temperatures near freezing can feel the same as 20 degrees below zero in calm air.
Strong katabatic winds, caused by the flow of cold air off the plateau, make some coastal sites the windiest places in the world. Port Martin (67°S 141°E) is an especially windy site with an annual mean wind speed of 17 m s-1 (33 kt - nearly gale force). The station has recorded a monthly mean wind speed of 28 ms-1 (54 kt - storm force 10) and a daily mean of 46 ms-1 (89 kt). On average it has only 22 days a year with mean wind speeds less than 13 ms-1 (25 kt). Winds at the South Pole are mostly controlled by the slope of the snow surface and are a relatively uniform 6 m s-1 (12 kt) from a direction of longitude 22° East. There are few calms or gales.
Meteorological observations are made regularly throughout the day at each of the British bases. Surface temperature, humidity, sunshine, pressure, wind speed and direction are largely measured by automated instruments but an observer is needed to estimate the visibility and the amount, type and height of clouds. He also needs to keep note of the weather: rain, snow, fog, gale etc. as well as more unusual phenomena: diamond dust, halos, mirages and the aurora australis. The observations are expressed in a numeric code and sent via geostationary satellites to meteorological centres in the northern hemisphere where they join thousands of other observations from all over the world. They are processed by super-computers and used to forecast the weather.
The cold atmosphere of the Antarctic makes it a good place to see some of the more unusual meteorological phenomena. Halo phenomena are caused by reflection and refraction of light from the sun or moon by ice crystals in the atmosphere. They may take the form of rings, arcs, pillars or bright spots. Haloes can also be seen in the UK and are normally formed by high, thin clouds. In the Antarctic another phenomena called diamond dust also produces them. Here, because the air is so cold, ice can crystalize out of clear air. As the crystals tumble slowly to the ground they catch and reflect the sunlight like thousands of tiny diamonds.
Stations near the Antarctic coast are quite cloudy because of the frequent passage of depressions and the influence of the sea. The further a station is inland, the less cloudy it becomes. Signy has an average cloud cover of 86%, Halley 66% and the South Pole an average of 41%. The Antarctic atmosphere is very clear, as there are few sources of pollution. On a fine day it is possible to see mountains well over 100 km away. In these conditions, estimating distances can be very deceptive. Objects may appear to be close by, when in fact it would take many hours of travel to reach them.
Every day at Halley a balloon is launched, carrying meteorological instruments. The instrument package signals back the temperature, humidity and pressure to an altitude of over 20 km. Special ascents are sometimes made to help study the lower part of the atmosphere called the troposphere, where weather systems are active. These include flights to investigate very stable conditions in the lowest layer, which mainly occur during the winter and other flights to study depressions forming in the Weddell Sea.
Ozone has been measured from Faraday and Halley for over 30 years using an ultraviolet spectrophotometer. The measurements show that a seasonal decrease or "hole" now appears in the Antarctic ozone layer each spring. The ozone is being destroyed because of the release of chlorofluorocarbons (CFCs), mostly in the northern hemisphere. These diffuse into the stratosphere, where they are broken down to release chlorine. the Antarctic stratosphere becomes very cold during the polar night and thin clouds form, altering the balance between chlorine and other gases. When the sun returns the chlorine is able to take part in complex catalytic chemical reactions which destroy ozone and create the ozone hole. When the stratosphere warms up again during the summer, these reactions cease and the ozone hole disappears.
With the onset of autumn, ice starts to form on the surface of the sea. As the temperature drops, this pack ice spreads round the continent and effectively doubles its size. It is the growth of this ice which prevents ships travelling to the Antarctic during the winter. The pack can grow to several metres in thickness, particularly if it survives through the following summer. The pack is not a solid mass of ice, but forms floes which are moved about by the wind. Leads form between floes and these can be seen from a distance because they make clouds above them appear darker, a phenomenon known as water sky. In the nineteenth century sealing ships used this knowledge to assist in navigating through the ice. Occasionally large ice-free areas called polynyas form in the pack. These are thought to be caused by ocean currents pushing up warmer water.
The largest iceberg ever sighted was a section which had broken off an Antarctic ice shelf and was more than 160 km long. Many Antarctic icebergs are fragments which have broken off coastal ice shelves. Because they are usually flat topped and of considerable area they are known as tabular bergs. Some have drifted northwards as far as 26° S. The Lambert Glacier in the Australian Antarctic Territory is the world's longest glacier: it is 400 km long and 65 km wide. Each year 35 km3 of ice is discharged from it. Icebergs from glaciers are usually much more irregular in shape than those from ice shelves.
The Antarctic ice sheets, which reach a maximum thickness of about 4700 m, contain about 90% of all the world's ice - enough to raise sea level by about 65 metres if it all melted. Climate models predict eventual temperature rises of up to 10°C in Antarctica if atmospheric carbon dioxide levels were doubled (which will happen by 2030 if the burning of fossil fuels etc. continue as at present). This would leave most of the continent with temperatures well below freezing, but the warming of the surrounding oceans could cause some of the floating ice shelves to disintegrate. Icebergs would then break off more frequently from the continental ice sheets. This increased ice loss would be partially compensated by greater snowfall in a warmer and hence moister climate. It is estimated that the net loss of ice would lead to global sea level increases of up to 1 metre as the ice sheets slowly responded over 500 years to the warming of the atmosphere and oceans.
A Table giving the conditions found in Antartica
A typical day for a meteorologist at Faraday would start at 8 am local time (11 am GMT). After breakfast, he would carry out the 12 GMT meteorological observation. On an average summer's day it would be daylight and the weather might be slight snow or sleet, with overcast skies, light winds and the temperature near freezing. By contrast, during the winter it would be dark and with clear skies there would be a hard frost and temperatures around -30°C. He would contact neighbouring bases on the radio, collect their observations, check them and then enter them into a data collection platform for transmission via satellite to the UK Meteorological Office. The next task might be to make a measurement of the amount of ozone with the Dobson spectrophotometer. Both meteorological and ozone observations are done regularly throughout the day. In addition to his scientific work, the meteorologist has to help with the general base work, which can range from washing up to painting walls. In the evening after working up the days observations there would be time to relax in the lounge, before going to bed.
We have installed Dartcom HRPT receivers at Halley and on RRS Ernest Shackleton and RRS James Clark Ross. These false colour images show the Larsen Ice Shelf in December and in March , the Wilkins Ice Shelf and the Ronne-Filchner Ice Shelf