7.3.9                                Ski Hi/Ski Blu  

7.3.9.1                          Orography and the local environment

Sky–Blu and Sky–Hi are depots located near the Sky–Hi Nunataks (see Figure 7.3.1). During the summer season a British forward field camp is set up to make use of the blue–ice runway situated at Sky–Blu. The Sky–Hi depot is located some distance from the Nunataks and has been little used in recent years as operations have moved to Sky–Blu, using wheeled aircraft on the blue–ice runway.

The Sky–Blu blue–ice runway (74º 51.14´ S, 71º 33.6´ W) is located very close to the Sky–Hi Nunataks, which are to the north of the (see Figure 7.3.9.1.1). The area of blue ice is probably caused by the precipitation shadow of the high ground to the north and the scouring effect of winds enhanced by the local orography. The ground to the south of the runway drops away towards the Merrick Mountains.

7.3.9.2                          Operational requirements and activities relevant to the forecasting process

The British Antarctic Survey maintains a summer only forward field camp and depot at Sky‑Blu to assist with transport of fuel and scientists into the deep field. A previous depot, called Sky–Hi, some 12 km away is now no longer used operationally. The blue–ice runway at Sky–Blu is used, during the summer season, by wheeled aircraft; however, the diversion options available for a wheeled aircraft operating this far south are very limited.

The blue–ice runway has different operating criteria from the normal skiway used in Antarctica. On the blue ice wheeled aircraft can only handle quite low cross winds while taking off and landing and wind from any direction can make taxiing difficult. However, low contrast is less important when the blue ice is exposed. During the flying season drifting snow often covers the blue–ice runway and this has to be cleared by hand and using light machinery before the runway can be used.

A ski–way, for use by ski–equipped planes, is located some distance from the blue–ice runway for use when the blue–ice runway cannot be used.

A small hut is located near the blue–ice runway and is occupied by two or three staff during the summer. Staff with only basic training make basic meteorological observations in support of air operations here.

7.3.9.3                          Data sources and services provided

Meteorological observations are only made at Sky–Blu in support of air operations. These are either passed to Rothera or to aircraft on route to Sky–Blu. An AWS, which feeds data into the GTS via ARGOS, is situated near the runway. No forecasts are available from Sky–Blu, although Rothera Station will issue forecasts for this area during the summer season.

7.3.9.4                          Important weather phenomena and forecasting techniques used at the location

General overview

Sky–Hi nunataks are located at the base of the Antarctic Peninsula some 200 km from the coast. This means that synoptic–scale lows associated with the circumpolar trough are less important at this site than at sites further north on the Peninsula. However, occasionally frontal system associated with slow–moving low–pressure systems near Alexander Island can give periods of low visibility and blowing snow.

 Figure 7.3.9.1.1     A map of the Sky–Blu blue–ice runway and camp.

These notes on weather phenomena and forecasting techniques are written for the Sky–Blu depot and the blue–ice runway and are only applicable to the Ski–Hi depot in a general way. The Ski–Hi depot is further from steep orography, being about 1 km from the nearest nunataks.

Surface wind and the pressure field

Mean–monthly wind speeds and directions at the Ski–Hi AWS are shown in Table 7.3.9.4.1 (in Appendix 2) while mean–monthly station–level pressures at this AWS are shown in Table 7.3.9.4.2 (in Appendix 2).

Quite often orographic effects will dominate the weather experienced at Sky–Blu. The general surface wind is often defined by katabatic flow from the high ground to the north, while the local wind is controlled by the presence of the steep orography of the Ski–Hi Nunataks.

Forecasting the occurrence of a katabatic flow can be difficult in this location with so little in situ data available. In general katabatic flow is enhanced when the large–scale geostrophic flow is northerly. Also the katabatic flow tends to vary diurnally, being at a maximum during the morning and slackening slightly during the afternoon. Unfortunately, the katabatic wind often disregards even these simple rules.

The close proximity of the Ski–Hi nunataks means that the local wind is extremely difficult to forecast. The accommodation hut is some distance from and higher than the blue–ice runway and the wind speed and direction may be different between the two sites.

Upper wind, temperature and humidity.

No radiosonde data are available for this region and so upper–air condition are predicted at Rothera using UKMO model fields. These tend to be reasonable although need to be used with caution this far south.

Clouds

The distribution of clouds at Sky–Blu is strongly controlled by the orography. Clouds are mostly stratiform and normally at least 300 m (~1,000 ft) above the station. Cloud is most prevalent during northerly winds when moisture is advected south from the coast. The high orography just to north of the blue–ice runway tends to prevent the formation of lower cloud.

Low–lying stratus does sometimes form in the low–lying areas to the south of the runway some 5 km away. The base, and sometimes the tops, of this stratus are often below the height of the blue–ice runway.

Visibility: blowing snow and fog

Fog is rare at Sky–Blu as it is too far south and distant from the coast for there to be sufficient moisture for fog to form. Low cloud to the south of the runway can reduce the visibility to a few kilometres in that direction.

The major cause of reduced visibility at Sky–Blu is blowing snow. This can occur when the wind is greater than 7–10 m s–1 (~15–20 kt) although the exact threshold and the total flux of blowing snow will depend on the state of the snow surface. Blowing snow can reduce the visibility to less than 100 m. Blowing snow can also effect flying operations by causing snowdrifts to form across the blue–ice runway.

Surface contrast, including white–out

The presence of the blue–ice runway means that surface contrast is less critical at Ski–Blu for air operations, than at other sites in Antarctica. When the blue–ice runway is clear of surface snow, wheeled aircraft can land in conditions that would normally be described as poor contrast. Wheeled aircraft can use the blue–ice runway if there is a thin (<50 mm) uniform layer of snow covering it. When the runway is in this condition it necessary for the contrast to be moderate to good before aircraft can land.

Horizontal definition

The presence of the nearby nunataks and the blue–ice runway means that horizontal definition is less important at this site than at other sites on uniform featureless ice sheets.

Precipitation

The precipitation at this site is very low as the site is in the precipitation shadow of higher ground to the north. What little snow that does fall blows away and the net accumulation is zero or negative that allows the blue–ice areas to form. Even a few kilometres further south, away from the direct effect of the nunataks, stake measurements show that the annual accumulation is only a few centimetres. Sky–Blu is too far south for rain ever to have been recorded.

Temperature and chill factor

The temperature at Sky–Blu is normally below –10ºC during the summer, although occasionally warm air pushing south will raise the temperature towards, but not above, zero. Mean–monthly temperatures at the Ski–Hi AWS are shown in Table 7.3.9.4.3 (in Appendix 2).

Icing

The clouds found at Sky–Blu tend to be thinner and contain less liquid water than at sites further north on the Peninsula. Therefore airframe icing at Sky–Blu itself is not a great problem, although light icing may occur in some conditions. However, further to the north, closer to the coast, cloud liquid water is likely to be higher and icing could become moderate to heavy and this could affect flights to and from Sky–Blu.

Turbulence

As Sky–Blu is well south of the track of the jet streams most turbulence in this area will be driven by orography. The northern end of the blue–ice runway is very close to a nunatak and turbulence can affect both take off and landing. This turbulence is very difficult to forecast, although observations of blowing snow flowing around the nunatak may give some idea of the presence of turbulence.

Hydraulic jumps

Hydraulic jumps have not been reported at Sky–Blu.

Sea ice

Not relevant at this location

Wind and swell

Not relevant at this location.