Air conditioning works just fine unless it rains?

Sounds like you are low on Freon. It is not uncommon for when you are in high humidity situations that the A/C is not able to keep up. Basically, the A/C running is what controls moisture so when there is more than it can handle the A/C stops performing well.

I would suggest we start with checking the Freon levels because that will tell us if it is low of the compressor is failing.

https://www.2carpros.com/articles/re-charge-an-air-conditioner-system

There is a chance we have a sensor issue but the pressures would be correct at that point, so we need to start with the pressures. Plus, on this older vehicle, I doubt there is a humidity sensor in the system.

Please see the attached info below for a chart on pressures and possible causes.

Let us know what you find. Thanks