Why does low air pressure and high relative humidity cause rainfall?

Question

Why does low air pressure and high relative humidity cause rainfall?

Answer 1

High pressure does allow for much or any moisture and low pressure does.   If that volume of air is saturated (high humidity) then it will rain.

Answer 2

In areas of low pressure the atmosphere is being forced to rise up, rising air cools and the moisture in it condenses to form clouds and then rain when the water droplets get big enough.

Relative humidity is a measure of how much moisture is in the air, high RH more moisture.

More moisture combined with rising cooling air means greater condensation, more droplets and more rain.