HE­LEN CZ­ER­SKI ON … FROST

BBC Earth (Asia) - - Comment & Analysis - Dr He­len Cz­er­ski is a physi­cist and BBC science pre­sen­ter

“THERE ARE CLEAR AND FROSTY MORN­INGS, WHEN THE OUT­SIDE WORLD HAS TRANS­FORMED IT­SELF INTO A TWIN­KLING DIS­NEY SET”

Win­ter can get a bit grim par­tic­u­larly in places like the UK: grey and cold with a gen­er­ous dash of sleet. But there are clear and frosty morn­ings that make up for it, when the out­side world has trans­formed it­self into a twin­kling Dis­ney set. Yet when you look at the frost, it isn’t evenly dis­trib­uted. Plants and wooden benches are often cov­ered in it, but metal rail­ings and the patches of ground be­neath trees aren’t. It looks as though frost is an indi­ca­tor for some­thing, but what is it?

The beauty of frost comes from the mo­ment of its for­ma­tion. When we think of wa­ter cool­ing, we tend to as­sume that wa­ter vapour will con­dense to form liq­uid wa­ter, which will then freeze to form ice. But frost is as­sem­bled from thin air and by­passes the mid­dle stage com­pletely. Mol­e­cules of wa­ter in the air bump into an ice crys­tal and just freeze di­rectly onto it, drop­ping into place on the ex­ist­ing crys­tal struc­ture. It turns out that a float­ing wa­ter mol­e­cule is more likely to join the frozen crowd if there’s a space for it on a rough sur­face, so lumps and bumps get filled in and nice, smooth crys­talline facets form. But that doesn’t an­swer the ques­tion about where frost is most likely to hap­pen.

For frost to form, a solid sur­face needs to be be­low 0°C, and there needs to be suf­fi­cient wa­ter in the air. When it’s close to freez­ing, the air is gen­er­ally very dry al­ready, but if there are more than five wa­ter mol­e­cules in ev­ery thou­sand air mol­e­cules, the air is of­fi­cially su­per­sat­u­rated, and frost can be wa­ter’s route out. This is part of the puz­zle – pock­ets of cold, hu­mid air are where you will find frost. For ex­am­ple, some dark sur­faces ra­di­ate their heat away quickly, and so they will cool more than their sur­round­ings, drop­ping be­low the tem­per­a­ture needed for frost. This ex­plains why there’s often a frost-free patch un­der a tree. The tree is in­su­lat­ing the ground around it, pre­vent­ing the soil from drop­ping be­low the crit­i­cal tem­per­a­ture.

But there’s one ex­tra con­di­tion for frost for­ma­tion. Even if the tem­per­a­ture and hu­mid­ity are right, frost may still not form. And that’s be­cause, for that first float­ing wa­ter mol­e­cule to freeze when it bumps into a solid sur­face, that sur­face has to have the right struc­ture for it to lock on to. If there’s al­ready ice there, that’s per­fect – the new mol­e­cule can just slot into its place in the ice crys­tal. If not, you need a nu­cleus. This is a start­ing point that pro­vides the right struc­ture, like a flat Lego base­plate that lets you po­si­tion the first bricks. Plants are often beau­ti­fully frosted be­cause bac­te­ria on their sur­face play the nu­cleus role. There are a few types of bac­te­ria that do this, and they’re ex­tremely com­mon. Without this coat­ing of bi­o­log­i­cal ice nu­clei, plants would stay frost-free to lower tem­per­a­tures. But na­ture is full of bac­te­ria, and frosted lawns and hedges are the re­sult.

So that cheer­ful frosty morn­ing can be ap­pre­ci­ated on two lev­els. There’s the white sparkle it­self, and there are the in­vis­i­ble pat­terns of tem­per­a­ture, hu­mid­ity and ice nu­clei that are re­vealed in the twin­kling. But frost ap­pre­ci­a­tion is a chilly hobby, and the sort of thing that earns you a hot drink when you get back in­side. Brrr!

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