Why low barometric pressure indicates bad weather and vice versa?

531 views

I’ve noticed that when barometric pressure drops this is usually followed by bad weather and vice versa. I’ve always thought that when rain is approaching this means humid air which has water vapour molecules in it (so it must be heavier than dry air?) thus higher pressure reading on the barometer because heavier air is exerting more force on the Earth’s surface. I’m clearly wrong here but couldn’t find a clear explanation as to why.

In: Physics