What would happen if, rather than five minutes taken noticing the surroundings of a café, you were to look closely at the coffee you brewed in the morning? Different roasters, different coffees, an opportunity to notice something new in each brew. And so it was that a couple of weeks ago a package arrived in the post from the coffee subscription site “Dog and Hat“. Together with a note (in answer to a question I had sent them) ‘Recycled box, paper, mail bag’, came two coffees. An Ethiopian honey processed coffee from Hundred House and a Mexican washed coffee from Coatepec via Quarter Horse coffee.
Each time I moved the bag from Hundred House, a lovely aroma was released. So I moved it around quite a lot. While brewing a V60 with it, the morning light poured through the window producing beautiful lensing effects through the bubbles on the coffee surface and reflections from the coffee itself. The brewed coffee had such a sweet, fruity aroma reminding me of cherries that gave way to plums on tasting. What I took as toffee seemed to be described on the tasting notes as “dates” or “molasses”. Close enough I think. A lovely coffee to enjoy slowly.
Printed onto the bag was a star with extra lines coming out of it, suggestive of a twinkling star at night. Although each star is massive, they are all at such a great distance from us that they appear to us as point sources of light. And since all light gets refracted when it goes from one medium to another (think about the appearance of that paper straw in a glass of water) the star will appear to twinkle from our position on the Earth below our turbulent atmosphere. Although on a clear night we may not notice it directly, regions of relative hot and cool air in the atmosphere are constantly moving. Layers of air move over each other creating waves much like you see on the seashore and it is this turbulent environment that refracts the light from the stars in such a shimmering way. We can see a similar effect in tea (though not so easily in coffee*): When we pour hot tea into a cold cup, the convection in the cup leads to there being areas of hotter and cooler tea. The refractive index of water is temperature dependent and so the light incident on the tea gets refracted (bent) by different amounts depending on whether it encounters a cool region or a warm region. This leads to the lines of light that we see dancing on the bottom of the cup¹.
Although atmospheric turbulence is inferred by the twinkling of stars, a beautiful visualisation of that turbulence can be seen in the form of the Kelvin-Helmholtz instability. Named after Lord Kelvin and Hermann von Helmholtz, this instability manifests as a string of waves on a cloud. It occurs when a fast moving layer of air flows over a slower moving one. The phenomenon is fleeting. If you are lucky enough to see it, the pattern manifests only for a very short time. They are definitely worth watching out for.
Depictions of atmospheric turbulence can also be seen in some paintings. It is said that Vincent van Gogh’s depiction of turbulence in his painting “Starry Night” is extraordinarily accurate. Certainly it is striking that the turbulence depicted by van Gogh does look like the turbulence in a coffee cup. However apparently it goes much deeper than this. In a numerical analysis of the turbulent patterns in a few van Gogh paintings, researchers showed that van Gogh’s depiction was very close to the mathematical (Kolmogorov) description of turbulent flow.
On their website, Hundred House discuss their aim of being a “collective space, where conversation, art and industry meet, over a cup of coffee”. Pouring a coffee, and watching the turbulence in the cup, perhaps pause a while to consider these points of connection and maybe add a bit of science to the mix. This week if you are in the Northern hemisphere, the Perseid meteor shower offers a particularly great time to reflect on turbulence in the atmosphere and the twinkling of the stars. If you locate the “W” of Cassiopeia (currently in the north east viewed from London) and watch, slightly underneath it towards Perseus, you should see a few meteors of the Perseid meteor shower (perhaps 60-70 per hour during the peak of 11th-13th August). While watching for the shooting stars, it is worth looking at those that twinkle. Which twinkle more, the stars of Cassiopeia or the stars toward the horizon? Why do you think this is?
Whether you watch the stars or just prepare your coffee, take the time. Enjoy your brew.
You can find out more about the coffee subscriptions at Dog and Hat coffee, here and more about Hundred House coffee, here. Do get in touch (email, Twitter, Facebook or comments) if you notice anything you want to share.
*We don’t see this so often in coffee because coffee, generally, absorbs more light than tea and so it is harder to see the bottom of the cup.
¹Another effect that can lead to these patterns in swimming pools and similar large bodies of water is caused by waves on the surface of the water. Where waves form on the surface of the pool, the curved surface acts as a lens focussing the light to the floor of the pond. As the waves move on the surface, the pattern on the pool floor will change similarly to that in the tea cup.