Why do swallows have good weather when they fly high?

There is an old folk saying: "Swallows fly low in the snake gallery, and heavy rain is coming", which shows that swallows are very sensitive to air pressure, and the air pressure will affect their free take-off to varying degrees when bad weather comes. On the contrary, the air pressure in good weather has no negative effect on their free take-off, so the weather can be judged from the flying height of swallows. I remember when I was a child, when swallows were flying freely, I often saw a group of swallows flying around at low altitude before it rained, and sometimes I saw snakes crawling on the road. At the same time, the sky is overcast and it will rain soon.