From someone's website
There's always something new to learn! According to the Farmer's Almanac, your coffee cup can help you predict the weather:
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.
I don't think I'd ever noticed bubbles in my coffee cup, but when I read about it I started looking for them. Sure enough, there they were! On inclement days the bubbles headed for the sides of the cup, but on the nice days they did stay in the middle.
So, the coffee barometer worked. But why? The bubble's movement is caused by changes in humidity and air pressure.