When a storm on earth is approaching and the elevation increases, the number of molecules decrease and the density of the air becomes less and less which causes a drop in or decrease in air pressure.
That's one reason why when the air pressure decreases a storm is usually approaching and the further the air pressure on the barometer decreases the faster the storm is approaching.
The lower the air pressure the worse the storm is likely to be and the closer the storm is to your location where the barometer is located as well.
When the barometer shows the air pressure increasing it usually means that the weather for that day is gonna be nice and no storms are present.
But as soon as the barometer shows a drop in air pressure again the storm is likely on it's way again.