According to the weather lore, coffee supposedly can foretell what type of weather is in store for you. There is a belief that if the bubbles of coffee collect in the center of your cup, you can expect fair weather. If they adhere to the cup, forming a ring, you should expect rain. If they separate without assuming any fixed pattern, changeable weather should be on its way.
The theory behind this is that generally speaking, weather tends to be better during times of increased atmospheric pressure, and when the pressure is low we’re more likely to get rain. What this means for your morning brew is that high pressure will cause the surface of your drink to be slightly concave, causing the bubbles to amass in the centre of the cup, and during low pressure the surface would be convex, causing the bubbles to move to the edge.
Confusingly though, depending on who you listen to you may hear differing opinions on which means good weather and which means poor weather. Does this mean that the result of pressure on weather is not fully understood, or does it just mean that the whole theory is nonsense?
Most people believe that the theory isn’t really true, or at least that it isn’t reliable enough to plan your day around. The reason for this is 2-fold, firstly that it doesn’t seem to be clear what the effect of atmospheric pressure on the weather actually is. It’s traditionally thought that low pressure means rain is more likely, but this isn’t reliably accurate. Secondly, it’s generally thought that the changes in air pressure would have such a minimal impact on the surface of a cup of coffee that the direction of the bubbles is most likely to be completely random!