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Storm in a teacup

Bénard cells in a beverage 

As a teacher I relished taking unconventional steps to the delight and edification of my students. I’d be the rare teacher that would allow pupils to rush to the window to watch a rainbow, catch snowflakes, measure the acidity of rain and hurl leaves in a vortex. Why not, after all I taught weather and climate. It didn’t always produce awe. I recall taking 13-year-olds on the school field. I stood in front of them such that the sun was behind them. I knew it was raining on the hills behind me so I said to them, “You should be able to see a rainbow. Can you?”

“Yes,” they chorused. “So what?”

I tried to explain that I couldn’t see it because the sun has to be behind the viewer and the angle of sunlight to the raindrops and back to the eye has to be 42 degrees. Oh well.

Another natural phenomenon that got me into trouble once in a café concerned a storm in a tea cup. I was with my Geography head of department in a café in Church Stretton, Shropshire. We had been measure streams, vegetation and microclimate on the Long Mynd and relaxing. To my delight convection cells revealed themselves on the surface my cup of tea.

I gathered the student around and showed them. Cells of paler tea (microbubbles) were pushed up to the surface (convection) while darker lines appear where the cooled tea sinks under gravity. Cells are hidden but exist inside the cup. This phenomenon is known as Rayleigh-Bénard convection. Henri Bénard (1874-1939) described them first, Raleigh did most of the physics equations later.

Thing is, Bénard cells can occur in any hot beverage including coffee and hot chocolate, but is best observed in hot coffee. By chance, through the window were altocumulus clouds, demonstrating the same phenomenon ie the white stuff is rising moisture when the gaps between is sinking air.

One of the students said, “What’s the point?”

What’s the point????? And I’ll add !!! even though as an editor multiple exclamations are a no no.

So with heavy heart I said, “You are observing something hardly anyone notices. There’s a delight in that, yes? And there’s science to this beautiful phenomenon. It’s a show put on just for us for the cost of a drink. Not everyone can see it. You need a light – even from a window – shining over the surface of the tea – and don’t drink it too soon.

Furthermore, you can instantly wipe the Bénard cells by striking a match over them. The sudden burst of particles destroys the static status. So, I ask one of the smokers among the pupils for matches. I poise over the Bénard cells and strike a match.

It was at this point that the waitress asked me to leave.

Other Nelder News

What a turn up for the books – well, short story. I have a preposterous tale shortlisted for the prestigious BSFA Awards 2018. If you are a member or are attending the FollyCon / EasterCon in Harrogate this Easter I would be enormously grateful if you would consider voting for ANGULAR SIZE in the shorter story category. Link is here


I can recommend this new venture. It is a web-based writing and showcasing site that has already run one successful competition. I am the next honorary judge for the suspense category. If you are hard at work with #NaNoWriMo or just finishing off a novel or long story of at least 10k words and want to know what to do with it, why not enter it for

Free to enter, huge prizes

worth a look

tweets @FicFun

XAGHRA’S REVENGE  released this summer. Pirates abducted 5000 people off the tiny Mediterranean island near Malta in 1551. Nearly 500yrs passed until their spirits enacted revenge with my authorial help. 

I am to do a signing of Xaghra’s Revenge at the Preluna Hotel, Sliema in April 2018. I stayed at this hotel when I did the research and even wrote some chapters there.


Run, hide! alien apocalypse.
Infectious amnesia. Free on KindleUnlimited or
99 pence/cents ARIA

My other books can be found on the Amazon Author page

Or if you fancy a children’s picture book about Timmy the Tornado – a kind of social story to help children grow up and be kind. ebook 99 pence



The post Storm in a teacup appeared first on Geoff Nelder - Science Fiction Writer.

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