Flat Snow Show
The snow surface at the Recovery Lakes is the flattest we’ve ever seen in Antarctica. But, if you look carefully, it tells a story about the weather, and the dryness of this continent.
Location: Recovery Lake “Z”, 81º 42’ S, 8º 34’ E
Weather: All clear, -23C, wind 26 kts
Every one of us has commented now about how unusually flat this area is. Over most of Antarctica, small snow dunes, called sastrugi, dot the surface like waves on a choppy lake. Here, the surface is almost perfectly flat, with no bumps higher than a few centimetres (an inch or two) for many miles.
But there is a pattern, if you look closely. Every few tens of meters there is a little patch of whiter snow, with small ripple dunes that face the wind. The patches are not very big; one of our vehicles would cover them completely; but its our hunch that these little ripples, on very still water, represent the current ‘active’ accumulation for this year.
It’s not much: probably not even an inch, if the patches were spread out evenly. It’s quite likely that, at this low level of snowfall, you don’t see a separate layer for each year in all places.
But the story doesn’t end there. These little patches have older siblings all around them, other patches that are changing in response to the wind and the sun and the frequent cold snaps. First, it seems, after a snowfall, for a brief period there is a thin uniform layer of cold fine snow. But as soon as the wind picks the next time, it moves the snow to the low spots in the surface – just barely lower than the rest of the surface, there are shallow hollows where the ripple patches form. Each successive wind-storm, transporting a stream of blowing snow right above the surface, shapes the patches over time. The first step is to sandblast the tops of the ripples off – the troughs are still there, but the tops are flattened. Then later storms sand the ripples completely away, and the patches become flat, blank, rounded patches looking a bit like white craft paper. But then, with more exposure to cold and sunlight and the dryness, the wind slowly creates small pits in the oldest patches. This is the most common thing on the surface – at Recovery Lakes, the whole area looks ‘etched’, with thousands of small pits swept out by the dry, relentless wind.
The only ‘excitement’ in this surface is the occasional spot where the shape of the hills and plains conspire to slow the wind down a bit, and drop out a bit more snow. Here, instead of flat, there are some of the largest sastrugi in Antarctica (see photo from January 7). These too, have a story to tell, because the large menagerie of structures is built by winds from two directions. One direction, the ‘snowfall’ storms, lay down long rounded ridges of snow, like submarines about to surface. The other direction, the ‘dry cold’ katabatic storms, erode these hills into the fantastic shapes. At Recovery lakes, the two directions were about 60 degrees apart in bearing; low pressures from the far-off Weddell Sea bring the moisture, and dry cold from the higher Plateau, sculpts the raw material into… dragons, porpoises, sitting toads, Rolling Stones logos, and … surfboards.
The flat and seemingly timeless snow surface at the Recovery lakes tells a many-layered story. Photo: Stein Tronstad/NPI