How did people make ice before freezers? © Getty Images

How did people make ice before freezers?

Whisky on the rocks? Ice in your refreshing lemonade? The Persians used a clever bit of physics to keep their drinks cool.

Asked by: Danielle Brandon, Doncaster 

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For millennia, those rich enough got servants to gather snow and ice formed during the winter and stored it in straw-lined underground pits called ‘ice houses’. But the ancient Persians stumbled across a neat bit of physics that allowed them to create ice from water even during the summer.

At night, the Earth stays warm through the famous ‘greenhouse effect’, in which gases in the atmosphere help to trap the Sun’s warmth. But on clear nights with low humidity, this effect is weaker, and objects can radiate their heat directly into space – a process called ‘radiative cooling’. The Persians found that this allowed thin layers of water in purpose-built trenches to drop to a low enough temperature to freeze – even after hot days in the desert.

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