Weather science: Make frost at home!

It's been unseasonably mild this February, so if you're craving a piece of winter, why not try making frost at home!

Half-fill your tin can with ice, fill a quarter more of the can with water, then liberally sprinkle with salt. Cover your tin with a lid and shake gently to make your solution.

Wait a few minutes (though it can take up to ten) and watch your frost form! Generally the more salt, the quicker the process, while too much water can slow the process.

Frost is a thin layer of ice crystals which form on surfaces below zero, and below the dew point. This means moisture is present, so as air condenses in a sub-zero environment, it forms ice crystals on a surface rather than dew.

In our experiment, putting ice inside your can makes it very cold and the outside surface below freezing. Adding salt lowers the freezing point of water, causing the ice to melt. The water vapour in the air then condenses as ice crystals on the outside.

This process of using salt to lower the freezing point of water is exactly how gritting our roads works (you can read more on that here).