280
A. Pellegrini et al.
of dense fog.and snow plumes can rise hundreds of métrés even over level ground
[27]. 'Blowing snow' indicates [9] a much more vehement type of wind-driven
snow. Under extreme conditions the horizontal visibility can be strongly reduced,
to less than 10 m.
For the area of Victoria Land, not having measurements of précipitation and
least of ail of drifting or blowing snow, the winds blowing on Nansen Ice Sheet
recorded by station 7350 were considered. Speeds ranging from 8 to 14 m s’1 are
assumed to be related to drifting snow and speeds greater than 14 ms1 to blowing snow épisodes. Station 7350 has been considered as représentative of the
whole area, since it is located at the central-northern part of Nansen Ice Sheet;
therefore it is often subject to flows originating in the interior of the continent
and blowing onto Nansen Ice Sheet from the west along Reeves Glacier and from
the north along the Priestley Glacier (Fig. 6). This is a conservative hypothesis:
if drifting or blowing snow épisodes take place at station 7350, they would be
more probably occurring at Inexpressible Island and on other sites of Nansen
Ice Sheet more exposed to intense winds. However, station 7350 has a more
complété set of data.
The number of consecutive observations in which station 7350 reported a wind
speed between 8 and 14 ms1 is shown in Fig. 7, while Fig. 8 reports consecutive
épisodes with wind speed greater than 14 m s’1.
As can be seen from Fig. 7, épisodes in which wind speed is high enough to
cause drifting snow hâve a short duration, as a great many of these do not last
more than 6 h, and only nine cases lasting more than 1 day were recorded.
Typically, these épisodes of drifting snow hâve a duration ranging from 1 to 11 3hourly consecutive data (less than 3 h to 33 h).
The cases in which wind speed is compatible with blowing snow (wind speed
greater than 14 m s1; Fig. 8) hâve an occurrence from 1 to 39 consecutive cases.
The longest épisode occurred in winter 1992 and lasted about 5 days, from 03:00
on 12 August to 21:00 16 August. It was characterised by dry air masses that blow
with a speed of about 90 km h1. To hâve an indication of the origin of these air
masses, also in this case the direction of winds was considered.
To individualise the air masses affect invest the station 7350 blowing from the
Plateau and having a speed greater than 14 m s1, wind direction greater than 255°
and less than 45° was analysed; the remaining angular sector was considered représentative of the flows from the sea. From this analysis it was shown that except
for one épisode in 1987 and a few in 1996 (considered until June), the other
épisodes always occurred when Terra Nova Bay is affected by extraordinary katabatic winds.
7 Visual Observations of Précipitation
It has been pointed out [28] that prevailing conditions on the coasts of Victoria
Land are alternatively determined by: East to south-easterly moist and relatively
warm air flow of oceanic origin; and westerly dry and cold winds blowing down
to the coasts from the Plateau. The weather associated with the former is typically cloudy with relatively important snowfall along the coasts and weak to moder-
A. Pellegrini et al.
of dense fog.and snow plumes can rise hundreds of métrés even over level ground
[27]. 'Blowing snow' indicates [9] a much more vehement type of wind-driven
snow. Under extreme conditions the horizontal visibility can be strongly reduced,
to less than 10 m.
For the area of Victoria Land, not having measurements of précipitation and
least of ail of drifting or blowing snow, the winds blowing on Nansen Ice Sheet
recorded by station 7350 were considered. Speeds ranging from 8 to 14 m s’1 are
assumed to be related to drifting snow and speeds greater than 14 ms1 to blowing snow épisodes. Station 7350 has been considered as représentative of the
whole area, since it is located at the central-northern part of Nansen Ice Sheet;
therefore it is often subject to flows originating in the interior of the continent
and blowing onto Nansen Ice Sheet from the west along Reeves Glacier and from
the north along the Priestley Glacier (Fig. 6). This is a conservative hypothesis:
if drifting or blowing snow épisodes take place at station 7350, they would be
more probably occurring at Inexpressible Island and on other sites of Nansen
Ice Sheet more exposed to intense winds. However, station 7350 has a more
complété set of data.
The number of consecutive observations in which station 7350 reported a wind
speed between 8 and 14 ms1 is shown in Fig. 7, while Fig. 8 reports consecutive
épisodes with wind speed greater than 14 m s’1.
As can be seen from Fig. 7, épisodes in which wind speed is high enough to
cause drifting snow hâve a short duration, as a great many of these do not last
more than 6 h, and only nine cases lasting more than 1 day were recorded.
Typically, these épisodes of drifting snow hâve a duration ranging from 1 to 11 3hourly consecutive data (less than 3 h to 33 h).
The cases in which wind speed is compatible with blowing snow (wind speed
greater than 14 m s1; Fig. 8) hâve an occurrence from 1 to 39 consecutive cases.
The longest épisode occurred in winter 1992 and lasted about 5 days, from 03:00
on 12 August to 21:00 16 August. It was characterised by dry air masses that blow
with a speed of about 90 km h1. To hâve an indication of the origin of these air
masses, also in this case the direction of winds was considered.
To individualise the air masses affect invest the station 7350 blowing from the
Plateau and having a speed greater than 14 m s1, wind direction greater than 255°
and less than 45° was analysed; the remaining angular sector was considered représentative of the flows from the sea. From this analysis it was shown that except
for one épisode in 1987 and a few in 1996 (considered until June), the other
épisodes always occurred when Terra Nova Bay is affected by extraordinary katabatic winds.
7 Visual Observations of Précipitation
It has been pointed out [28] that prevailing conditions on the coasts of Victoria
Land are alternatively determined by: East to south-easterly moist and relatively
warm air flow of oceanic origin; and westerly dry and cold winds blowing down
to the coasts from the Plateau. The weather associated with the former is typically cloudy with relatively important snowfall along the coasts and weak to moder-