weather glass

A mystery I can't understand.  Blown glass and coloured water can predict the weather!
This weather glass was purchased at Home Hardware for $14.95, and it has given me endless enjoyment.  It is a sealed glass globe with a tube open at the top.
It's all about barometric pressure.  When the weather is turning fair, barometric pressure is high, and it pushes down on the water, making it retreat in the tube.

When the pressure is low, the water rises in the tube, in this case, flowing over the top.
It means bad weather is on the way.