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Calculating probability of opportunity

A project log for Ice Station Thuban Prime

Another temporary enclosure made entirely of water

Kenji LarsenKenji Larsen 12/04/2014 at 04:040 Comments

The large snowstorm that hit my area in November reminded me to check my stock of hexagonal ice forms, get them prepped for an opportunity. The snow accumulated about 30 cm in my area but it was wet snow, dense, very difficult to shovel due to weight. It has since turned completely into groundwater.

What I need is a stretch of cold temperatures, below freezing for approximately ten days to two weeks (forecasted) to begin deployment. I've played this game before - the weather forecasts are sensitive to initial conditions, and long term forecasts are more or less meaningless. So it is a probability game. Short term forecasts are very good, however. In the 10 day range, the idea is to estimate, or more properly bet, that temperatures will stay below freezing, with a few excursions above freezing tolerable, as long as they are short in duration, perhaps one or two hours (usually around 2 pm - lagging the sun).

This is a tough game. How fast can one freeze two to three tons of water? The answer depends on a few things, like how much below freezing the weather is (delta T determines the efficiency of heat transfer); the surface are to volume ratio of the item to be frozen; the characteristic minimum distance (from surface to centroid) of the item; ... and more practical things, like - how fast does the water flow out of the hose, and how many hours do I have to fill the buckets! Also, ground temperature, which dominates the temperature of the water source, and affects freezing rate through contact patch. But generally, it's a long time. 15 hours or more for a 1 gallon brick in typical just-below-freezing conditions.

This determines the number of bricks required to be in progress at once, since about 3000 bricks are needed. It's a race against the weather. Ice Station Thuban had relatively few panels, but they were large. But they were flat, so minimum distance was small. But the large area meant they were spread over the ground, so that meant that ground temperature dominated their freezing schedule. The new bricks have a larger height to ground cover ratio, so they should stick out like heat sink fins in the cold air (in comparison to the flat panels).

Will we get enough days in a row to deploy this year? I hope so. The snow made me hopeful. The lack of snow now, however, reminds me of the statistics.

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