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Just Average

The value of 6 mm of water per year is an average, and it's an average over both space and time.

The Dry Valleys cover a region that is about one and a half times the size of Rhode Island and exhibit a gradient in precipitation. Areas closer to the coast with its comparatively moister air receive more precipitation than areas further away. John and I saw some evidence of this when, after a snow storm, we were helicoptered down Taylor Valley. The Lake Fryxell basin, which is the basin that opens onto McMurdo Sound, was covered with a thin layer of snow. In contrast, it appeared that the area surrounding Lake Bonney, which is further inland, was snow-free.

The 6 mm of precipitation per year is also an average over time. This year has above average snowfall.

So, to sum up, an average 6 mm of water per year is just that - an average.

When John and I hike around the Dry Valleys, we might cross desert pavement, negotiate our way around rocks and boulders, or make a path through the sandy remains of what was a lake bottom. Then we might hit a drift of snow.

Snow drifts tend to form in areas that are low compared to their surroundings. Wind velocities decrease in such areas causing the wind to drop some of its load. For instance when wind hits a hill, snow will accumulate on the ground on the hill's leeward (downwind) side.

When crossing snow drifts, John and I have been up to our ankles or even our knees in snow. But if we took all the snow in that drift and spread it out evenly over areas that didn't have any snow, it would form a pretty thin layer--making a low spatial average.

Cont'd