For four days now, we've been trying to get a helicopter flight back to CTAM from our remote camp here on Mt. Hope. It's not that the weather has been exceptionally nasty, but it has not been good flying weather. I had always thought that for flights to be suspended in Antarctica, it meant that there was a howling blizzard outside. While that may happen, during the summer research season, more often than not, it is low clouds and fog that make flying hazardous.
We did have two days where it snowed off and on, but we never saw much accumulation - maybe about a quarter of an inch. During this snow storm, the wind even hardly blew; I don't think we've had winds here over 15 mph. The weather factor that has held us pinned in camp is fog and overcast skies. Most of the time we have not been able to see the summit of Mt. Hope, only about 2,200 feet above us. When we walk out to a vantage point just east of camp and look up the Beardmore GlacierA mass of ice that persists for many years and notably deforms and flows under the influence of gravity. toward the South Pole, we can only see the bases of the mountains we've come from: Cloudmaker Peak and Mt. Kyffin. Sometimes we can see the faint disk of the sun attempting to shine through the clouds, but it doesn't break through. Helicopters won't fly in this weather because they cannot see the ground through the fog. When they do have areas where the fog has thinned, the lighting is flat and doesn't allow them to distinguish surface features on the snow and ice.
When out in the field and requesting flight information, it is important to be able to describe the weather conditions to the flight center. There are eight key points to be reported: wind direction, wind speed, visibility, cloud height, cloud type and cell appearance, cloud cover, surface definition, and horizon definition. The following descriptions come from the Field Manual for the United States Antarctic Program.
1) Wind Direction: expressed to the nearest ten degrees and is based on the Grid North system unique to Antarctica. (Since lines of longitude converge at the poles, small distances on the ground can equal large degree changes. This makes the expression of headings with respect to true north unreliable.) In this grid system, east and west meridians as well as north and south lines run parallel to each other.
2) Wind Speed: expressed in knots/hour (a knot is about 1.2 mph). Prevailing wind speed is reported as well as speed of wind gusts, if there are any.
- Visibility: given in meters and dependent on the geography of where you are. Maximum visibility on a clear day is seven miles for a flat ground horizon (at that point, you cannot determine surface conditions any longer).
4) Cloud height: expressed in feet and estimated. If you're in an area with geographical features of known elevations, use these as a reference. Cloud heights are reported as Above Ground Level (AGL).
5) Cloud type and cell appearance: help determine the height of a cloud layer. The atmosphere is shallower here than at the equator, so cloud types occur lower in the sky. There are three types: Low clouds (stratus and stratocumulus) commonly found at the surface and up to to 6,500 ft AGL; Mid-level clouds (altostratus and altocumulus) generally found from 6,500-13,000 ft AGL; High clouds (cirrostratus and cirrus) found from 12,000-16,000 ft AGL.
6) Cloud cover: expressed in eighths of the sky. When reporting, start and the ground and proceed upward. This category can be broken into six types: Clear and Few - trace to 2/8ths of sky cover; Scattered - 3/8ths to 4/8th sky cover; Broken - more than 5/8ths, but the entire sky is not covered; Overcast - the entire sky is covered; Partial Obscurity - sky is partially obscured by snow or blowing snow, some clouds may be discernible; Total Obscurity - sky is totally obscured by snow or blowing snow.
7) Surface Definition: ease of identification of surface features. There are four breakdowns in this category: Good - snow surface features such as sastrugi [waves of snow, similar to frozen water waves], drifts, and gullies are easily identified by shadow; Fair - snow surface features can be identified by contrast, no definite shadows exist; Poor - snow surface features cannot be readily identified except from close up; Nil - snow surface features cannot be identified, no shadows or contrast.
8) Horizon Definition: this identifies how clearly the horizon can be seen. There are also four categories: Good - horizon is sharply defined by shadow or contrast; Fair - horizon may be identified, but the contrast between sky and snow is not sharply defined; Poor - horizon is barely discernible; Nil - total loss of horizon; snow surface merges with the whiteness of the sky.
Here's how our flight-related weather report has been recently:
Wind from grid south (down glacier), 5 knots/hour, visibility 5,000 meters all directions, cloud height 1,000 feet AGL, low clouds, overcast, surface definition fair, horizon definition poor.
Wind from grid north, variable up to 5 knots/hour, visibility 5,000 meters both directions, surface definition poor, horizon definition poor, 4/8th mid-level clouds, 4/8th low-level grid north, base 2,000 feet AGL