George, Monday 19 April 2010
A fortnight ago I saw some people looking out of the lounge window of Scott Base, New Zealand’s Antarctic base. Given the spectacular views, this is not unusual. However, this time when I followed their gaze I saw something pretty peculiar. The usual view of Black Island and White Island was strangely different. A long strip of new ‘land mass’ had settled at their base, with a discernable boundary above and below, and it extended right along the horizon.
Pegasus Air Strip, which can usually only be observed as small dots in the distance on a clear day, towered high and wide, as though brought many miles forward across the ice field. What we were seeing was an immense superior mirage.
The temperature had taken a sudden and dramatic dip from the minus twenties to minus 30. The following week we saw a mirage almost everyday. Sometimes it was a smooth, stable magnified image, or else a distorted mixture of topsy-turvy pictures mirrored along the horizontal axis creating dazzling long symmetrical patterns.
Superior mirages occur due to temperature inversion, when the air below the line of sight is colder than that above and are common in polar regions. The refractive difference between bands of cold and warmer air acts as a lens, bringing forward objects on or beyond the horizon, making them appear larger - an optical effect much like looking through a glass of water. Turbulence in air density may also cause a mirage to dance and change rapidly, a kind known rather poetically as a Fata Morgana, named after the magical shape-shifting sister of King Arthur; but we have yet to see this type.
With the long nights rapidly drawing in, these spectacular displays will soon be invisible to us – but then the Aurora Australis should keep us distracted!