Predicting the weather with a cup of coffee?

What do the bubbles on the surface of your coffee tell you about the weather?

weather, bubbles, coffee, coffee physics, weather prediction, meteorology

There is a lot of physics going on with the bubbles on this coffee, but can they be used to predict the weather?

You have just poured a cup of freshly brewed coffee into your favourite mug and watched as bubbles on the surface collect in the middle of the cup. It occurs to you that it is going to be a good day, but is that because you are enjoying your coffee or because of the position of the bubbles?

There are a large number of sayings about the weather in the English language. Some of the sayings have a basis in fact, for example the famous “red sky at night, shepherd’s delight, red sky in the morning, shepherd’s warning“. Others though seem to verge on the superstitious (“If in autumn cows lie on their right sides the winter will be severe; if on their left sides, it will be mild”), or unlikely (“As August, so the next February”).  In 1869, Richard Inwards published a collection of sayings about the weather. “Weather Lore” has since undergone several new editions and remains in print although Inwards himself died in 1937. Amongst the sayings contained in the book is one about coffee:

When the bubbles of coffee collect in the centre of the cup, expect fair weather. When they adhere to the cup forming a ring, expect rain. If they separate without assuming any fixed position, expect changeable weather.

A quick search on the internet shows that this example of weather lore is still circulated, there is even a ‘theory‘ as to why it should be true. But is it true or is it just an old wives’ tale? Although I have consumed a lot of coffee I have never undertaken enough of a statistical study to find out if there could be an element of truth in this particular saying. The number of bubbles on the surface of the coffee is going to depend, amongst other things, on the type of coffee, the freshness of the roast and the speed at which you poured it. While the position of the bubbles will depend on how you poured the coffee into the mug, the surface tension in the coffee and the temperature. It would appear that there are too many variables to easily do a study and furthermore that the mechanism by which coffee could work as a weather indicator is unclear. It is tempting to write off this particular ‘lore’ as just another superstition but before we do that, it is worth revisiting another old wives tale which involves Kepler, Galileo, the Moon and the tides.

tides, old wives legends, Kepler, Galileo, Lindisfarne, bubbles in coffee

The pilgrim path between Lindisfarne and the mainland that emerges at low tide is marked by sticks. But what causes the tides?

Back in the mid-17th century, Newton’s theory of universal gravitation had not yet been published. It was increasingly clear that the Earth orbited around the Sun and that the Moon orbited around the Earth, but why exactly did they do that? Gilbert’s 1600 work De Magnete (about electricity and magnetism) had revealed what seemed to be an “action at a distance”. Yet the scientific thought of the day, still considerably influenced by Aristotelianism, believed that an object could only exert a force on another object if it was somehow in contact with it. There was no room for the heavenly bodies to exert a force on things that were found on the Earth. Indeed, when Kepler suggested that the Moon somehow influenced the tides on the Earth (as we now know that it does), Galileo reproached him for believing “old wives’ tales”: We should not have to rely on some ‘magical attraction’ between the moon and the water to explain the tides!

The point of this anecdote is not to suggest that a cup of coffee can indeed predict the weather. The point is that sometimes we should be a little bit more circumspect before stating categorically that something is true or false when that statement is based, in reality, purely on what we believe we know about the world. We should always be open to asking questions about what we see in our daily life and how it relates to the world around us. It will of course be hard to do a proper statistical study of whether the bubbles go to the edge or stay in the centre depending on the weather (whilst keeping everything constant). Still, there are a lot of people who drink a lot of coffee and this seems to me to offer a good excuse to drink more, so perhaps you have some comments to make on this? Can a cup of coffee predict the weather? Let me know what you think in the comments section below.

 

Weather legends taken from “Weather Lore”, Richard Inwards, Revised & Edited by EL Hawke, Rider and Company publishers, 1950

Galileo/Kepler anecdote from “History and Philosophy of Science”, LWH Hull, Longmans, Green and Co. 1959

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