Frozen lochs – what lies beneath?

In recent weeks as we experienced wintery conditions, many people will have noticed their local ponds and lochs froze over for a period. In today’s blog our freshwater advisory officer Ewan Lawrie takes a closer look at what’s happening below the surface.   

A frozen Loch Morlich in the Cairngorms ©Catriona Webster/NatureScot

In the recent cold snap many of our lochs and ponds have frozen over, with a thin layer of ice forming on the surface. Beautiful to look at, but have you ever wondered why lochs freeze from the top down? For most substances, the cooler they get the denser they get with less space between the molecules. Warmer, less dense substances tend to float; without this property hot air ballooning would be much less popular!

Ice starting to thicken at Loch Leven National Nature Reserve (NNR) ©Simon Ritchie/NatureScot

A quirk of water is that it is at its densest at around 40 C. Below this temperature the hydrogen bonds are not able to pack the molecules as tightly. So, as lochs cool in winter denser water drops to the bottom of the loch. This in itself is important ecologically as it creates mixing throughout the whole of the loch. But, as the temperature drops below 40 C it becomes less dense, floating on the surface and eventually forming ice.

Frozen Loch Kinord at Muir of Dinnet National Nature Reserve (NNR) ©Catriona Reid/NatureScot

The ice and snow provide a layer of insulation on the top of the loch and because they are on the surface are more strongly affected by the heat of the sun and melt more quickly. Ten years ago Loch Leven froze in the harbour to a depth of 9 inches, but only relatively shallow lochs freeze completely. If lochs froze from the bottom it would have an important effect on the ecology with only specialised plants and animals being able to survive.

Ice forming at Muir Of Dinnet at Muir of Dinnet National Nature Reserve (NNR) ©Catriona Reid/NatureScot

Historically there has been a sport and culture in Scotland based around the ice. In the 19th Century there were special trains and platforms for curlers and many now sadly neglected shallow curling ponds across the country. However even if ice looks thick, it can still be thin in areas and venturing out on it can be very risky for you or your pet. If in any doubt stay safe, stay off the ice and just enjoy the view!

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