Rights: The University of Waikato Published 21 June 2007 Download

Dr Katja Riedel of NIWA talks about how snow turns into ice and how the air gets trapped in the ice during this process.



The ice is formed mainly out of snow, and between snowflakes there are little gaps of air, and you know that snow can be really fluffy, but when it settles down on the ground it kind of compacts more and more, and you see it’s not fluffy any more after a day.

But after a week or so when the snow is exposed to the environment to more sun to warmer temperatures, all these snowflakes, they get very round, so instead of fluffy they go more and more icy. We have channels between these ice balls now and they are in exchange with the atmosphere. And when more and more snow falls on top of that these ice balls get more and more compressed, and because of this, these channels at some stage close off, and that is what is very important for us because we call it bubble close off time. And that is a time when the atmosphere can’t exchange with the air any more and that is when we really get a snapshot of the atmosphere. The time it takes for firn to become ice is very much depending on how much snow is falling on top of this piece of ice at the time during the year. You have some regions where there is a lot of snow. We call that snow accumulation, sometimes it can be up to a metre per year. And then you see that this process happens much faster. And there is this one place in Antarctica which is called Law Dome and there it happens within 50 years. So in 50 years the firn becomes ice and all the bubbles are closed.

But then there are other regions in Antarctica where there’s little, little tiny bit of snow falling every year, and then it takes a long time to really the bubble closure happens and the firn becomes ice.

Dr David Etheridge, CSIRO Australia