QotW: Ice on the windshield

If you wipe dew of a windshield at below-zero, why does it suddenly freeze solid?
29 October 2019


A frozen car windshield.



When the outside temperature is hovering around the freezing mark, the condensation - or dew - on my automobile windshield is in a liquid state. But if I wipe the windshield, the liquid water changes to ice. Why is that?


Adam Murphy is answering this super-cool question from listener Mike...

Adam - It can be a real pain. One moment you’re just wiping your windshield on a cold morning, the next… you’ve got to go about taking all the ice off. But what’s going on? I reached out to Liz Thomas, paleoclimatologist at the British Antarctic Survey here in Cambridge, to find out.

Liz - This is the result of a phenomenon known as supercooling: the process of lowering the temperature of a liquid below its freezing point without it becoming a solid. This happens because they lack something to start the freezing process called a seed crystal.

Adam - This can happen to water bottles to. You can put them in your freezer and they won’t freeze! That is, until you give them a knock.

Liz - On a cold morning when the temperatures are close to zero, the water vapour in the air - or the dew on your windscreen - is in this supercooled state. The action of spreading the supercooled droplets over the cold surface of the windshield turns the water vapour into ice. This is because the windshield is generally colder than the surrounding air.

Adam - Sloshing the water around on your windshield then makes a point the water can freeze from. So you’re a superhero! You can turn water into ice with just your hand! But it’s not all so chill.

Liz - Another example of supercooling is freezing rain. In this case the rain droplets become supercooled in the upper atmosphere, but remain as supercooled droplets until they reach the Earth’s surface where they turn into ice. This can be a real problem for airplanes, where the ice can build up rapidly on the wings and make flying incredibly dangerous.

Adam - And as alancalverd pointed out on the forum, supercooled water can freeze in seconds. So thanks to Liz Thomas for giving an answer to such a n-ice question. Next week we’re answering this one from Vinnie…

Vinnie - I have read mosquitos have a preference for blood type and prefer people with Type O blood over those with Type B, or prefer Type B over Type A. Is this true, and how do they know the difference between types?

Izzie - If you can answer Vinnie’s question, email chris@thenakedscientists.com, find us on Facebook, tweet @nakedscientists or join in the debate on the forum - that’s the naked scientists.com/forum.

Chris - That’s actually a very good question, isn’t it. How does a mosquito know your blood group before it bites you?

Izzie - I definitely know want to know, and like, what is my blood type? Oh my goodness, I’m overthinking everything!


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