Here's why:

* Barometric pressure indicates the weight of the air above a particular location.

* Falling barometric pressure generally means that an area of low pressure is approaching.

* Low pressure systems are associated with rising air.

* Rising air cools, leading to condensation and cloud formation.

* Further cooling in the rising air can lead to precipitation, or rain.

However, it's important to note:

* A falling barometer is not a guarantee of rain.

* Other factors, such as humidity and atmospheric stability, also play a role.

* A rising barometer usually indicates fair weather.