Derek – Friday 7th January 2022

It was -2ºC in the early hours and the pond is still frozen when I come into the garden at 10 am. The ice isn’t smooth but has ribs in places, rather like a church ceiling. I suspect it has something to do with the fact that ice has more volume than the same weight of water, and in trying to fit into the same space, ribs are pushed up.

We take it for granted that ice floats on water. This insulates the water beneath, so it doesn’t get so cold. If ice were heavier than water, it would form at the bottom, making the pond more likely to freeze completely. In this case, fish and bottom feeders would have a hard time, eventually being forced out of the water by the rising ice.

Hope’s apparatus demonstrates how ice forms at the surface of water. The apparatus consists of a tube of water, which has outside and around the middle a cooling mixture of ice and salt. There’s a thermometer at the top and another at the bottom. Beginning at room temperature, the warmer water is at the top, as we might expect. When the top water cools down to 4ºC, it is now at its maximum density and sinks. Then the top water gets cooler and freezes while the bottom water remains at 4 degrees.

Lucky for the fish.

If ice formed at the bottom first, then icebergs wouldn’t float but would be at the bottom of the sea. They would cause less trouble to shipping, but ice wouldn’t cover the surface in the Arctic and Antarctic. Ice round the poles reflects heat through its whiteness. No ice cover, which is the way we are heading, and the world would be hotter. The heat-reflecting power of ice is called the albedo effect.

London is at the same latitude as Calgary in Canada which has temperatures down to -40ºC in winter. The Gulf Stream makes the difference for the UK. One possible effect of climate change is the shutting down of the Gulf Stream. A thought which makes me shiver. Incidentally, -40 degrees Celsius is equal to -40 degrees Fahrenheit, the only equality in the two temperature scales.

You might wonder why plants don’t freeze in the cold. Herbaceous plants die off with only their roots alive in winter. These are protected from frost by the surrounding soil. In some exposed plants proteins in their cells act like anti-freeze to stop them freezing up. Deciduous trees lose their leaves in autumn and so avoid frost damage to the foliage. Conifers, such as pines, have needles and these have less surface area with fewer stomata which close to keep out the cold. Added to which, pine resin acts as anti-freeze.

Walking down to the garden along Earlham Grove, I noted lichen on the pavement, mostly by the walls. It gets scuffed away in the middle of the pavement. We have very little round our fence. Partly because of our activity in putting up and painting the fence. That certainly killed off any lichen at the time. Since then (2016) it is likely that keeping down weeds on the outside of the fence has culled lichens too. Or even assiduous sweeping.

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