Victoria University Antarctic Research Expedition Science and Logistics Reports 1980-81: VUWAE 25
As seen from Scott Base and New Harbour, the weather at Erebus summit appeared to be good from 22 November to 3 December 1980. The Fang was in cloud and mist most days between 12 and 24 December. When helicopters could get in they came bearing vapour trails.
The summit area was unusually free of snow when we .arrived there on 17 December, testifying the previous good weather. Fog and snow-fall began on 23 December and from 25 December on, fog and blowing snow became general as shown in Appendix VI. It appears that the horizontal wind shear boundary, often evidenced by the Erebus plume rising vertically for a short distance and then spreading out horizontally, had descended below the summit level and was creating an orographic cloud cap.
The deterioration in the weather experienced on Erebus after Christmas in 1974/5 and 1978/9 may have been a similar phenomenon.
In my opinion, the best four week period on Erebus is usually 24 November to 22 December.