Posts Tagged ‘frostflowers’

Frost Flowers Revisited

September 17, 2009

Somebody asked about frost flowers (thanks, MeghanC!) This gives me an excuse to pontificate upon them a bit. Here’s a great picture of frost flowers from a New Scientist gallery:



Those of you who haven’t spent a lot of time hanging out in the polar regions may not be familiar with the sequence of events involved in the formation of sea ice (apologies if I’ve explained this one before…) As the winter begins and the ocean cools down, little bits of ice begin to form at the surface of the water; these are called frazil or grease ice. Eventually they stick together into a flat sheet, called nilas ice. Pockets of seawater (brine) get trapped between the crystals as they freeze.

As the ice gets colder, water freezes onto the walls of the brine pockets. When water freezes, it tends to exclude any non-water substances, so the brine within the pockets gets saltier. Water also expands as it freezes, so some of the salty brine is pushed out of the ice. Part of it goes downward, and the sinking of this cold, dense, salty water has interesting effects on ocean currents. Part of it goes upward through whatever channels it can find:



Experimenting with hand-drawn diagrams this time, as you can see.

The ice continues to freeze, and the brine being pushed out of the pockets forms a thin layer across the ice. This layer is very salty, so it stays liquid even at very low temperatures:


Frost flowers form at temperatures below -15C (or lower, depending on who you ask.) Seawater, of course, can never get colder than -1.8C. This means that under frost-flower-forming conditions, the ocean is quite a lot warmer than the air. The brine layer on the surface of the newly-formed ice is also comparatively warm, and it gives off water vapor. The vapor re-condenses and forms crystals on top of the briny “bumps” we saw earlier:


The flowers continue to grow, and some of the brine travels up them via capillary action, making them very salty compared to most sea ice:


That, anyway, is the short version of how frost flowers form. They may have some effects on the atmosphere because salts from the ocean are caught in their delicate fronds where the salts can easily be picked up by wind. They may also, as we are now seeing, have some effect on ice albedo both by themselves and by capturing snow that blows across the ice.

Curse You, Frost Flowers

September 5, 2009

Another sea ice day on Friday. We admired some Fata Morgana mirages on the way out:




Some nice sastrugi (snow dunes) with a band of mirage in the background



A mysterious cyclopean edifice in the distance

We returned to our previous sample spot and attempted to get better measurements by removing the snow:



Janitorial duty on the sea ice

The crust of snow was pretty well stuck on to the ice, requiring us to shovel, sweep, and then kneel down and scrape with spatulas and ice axes until the ice was more or less clear. We tasted the snow and found it was salty, which means that a lot of it actually started out as frost flowers. Frost flowers are fluffy, rather ethereal-looking crystals that form on new sea ice; they are pretty fascinating, scientifically. If anyone requests it in the comments I’ll write up a post about how they form.

Nifty as frost flowers are, they are making our life difficult. As you may recall from my previous post Salt, Sea Ice and Science we are looking at the way the albedo of sea ice changes when the salt in brine pockets forms crystals. Unfortunately, it’s very hard to see anything useful through a layer of frost flower remains. Hence our janitorial activities.

I am reminded that I promised equipment pictures in that post I just linked. You’ve seen the field equipment, of course, but I shall have to do a post about lab technique and some pictures of my impressive array of beakers.