Antarctica is the coldest, windiest and driest continent on Earth.
The average annual temperature ranges from about −10°C on the Antarctic coast to −60°C at the highest parts of the interior. Near the coast the temperature can exceed +10°C at times in summer and fall to below −40°C in winter. Over the elevated inland, it can rise to about −30°C in summer but fall below −80°C in winter. The lowest temperature yet recorded on the Earth’s surface was −89.2°C at Vostok station on 21 July 1983.
Surrounding Antarctica there is normally a belt of low pressure, the circumpolar trough, containing multiple low centres. The continent itself is dominated by high pressure but meaningful analysis of surface pressure data is difficult because of the elevated nature of much of Antarctica.
Radiative cooling over the Antarctic ice sheet produces very cold, dense air that flows away from elevated areas and is replaced by subsiding air from above. The resulting katabatic winds accelerate downhill, enhanced by the confluence of glacial valleys. Katabatic winds blow with great consistency over large areas. At the coast they lose their driving force and soon dissipate offshore.
Low-pressure systems near the Antarctic coast can interact with katabatic winds to increase their strength. Resulting wind speeds can exceed 100km/h for days at a time. Wind gusts well over 200km/h have been measured.
Air over the interior of Antarctica is usually subsiding and dry, resulting in little cloud there. Around the coast more moisture is available and low-pressure systems have a greater influence, so cloudy conditions are more common, particularly on the Antarctic Peninsular.
Though rain is observed at times near the coast, most precipitation over Antarctica is in the form of snow or ice crystals. It is difficult to accurately measure snowfall under windy conditions, but the average accumulation of snow over the continent as a whole is estimated to be equivalent to about 150mm of water per year. Over the elevated plateau the annual value is less than 50mm. Generally near the coast it exceeds 200mm, the heaviest being over 1000mm for an area near the Bellingshausen Sea.
Under windy conditions loose snow can be picked up and carried along. When the snow is still below eye level it is called drifting snow but when raised above eye level it is called blowing snow. In the latter case visibility is generally very poor. Wind speeds of over 30km/h can lead to drifting snow, while wind speeds over 60km/h are more likely to produce blowing snow.
Blizzards and whiteout
Blizzards are said to occur when wind speeds are gale force or stronger for at least an hour, the temperature is less that 0°C and visibility is reduced to 100m or less. These conditions are dangerous and disruptive for outdoor activities. Sometimes blizzards persist for days.
Whiteout is an optical phenomenon in which uniform light conditions effectively make it impossible to distinguish shadows, landmarks or the horizon. This can occur when the snow cover is unbroken and the sky is overcast. Whiteout is a serious hazard as it causes a loss of perspective and direction.