So your car is sitting outside on a rainy day and you go to start it, but it wont start and if your car is sitting outside on a sunny day, and you go to start it and it starts.
You did not say if this happens when you first try and start your car,,,or that it happens,,after you have been driving the car for awhile,,then stop the car,and then try and re-start the car.
What is the difference between a sunny day and a rainy or damp day ?
Water, moisture, humidity.
Here are a few ideas,,try and isolate where and how, water, moisture and humidity are affecting your cars ability to run.
A. On a rainy or damp day when this happens, take a blow dryer, you know the kind you use to dry your hair after a shower, plug it in with an extension cord, place it on a box or something so it won't be lying on the floor,,turn it on so the inside of your car stats warm and comfy.
Then when you go and start the car,,and it starts right up and runs fine,,then you will know that moisture in the air is affecting,,something, INSIDE your car.
Now lets say that there is no change,,then the problem is OUTSIDE of the car.
The next thing to do would be to place the car under a shelter that exposes the car to the damp air on a rainy day, but not exposed to the rain,,this is to determine if the problem is from a water leak or from humidity.
My guess is that the problem is a faulty Air Temperature sensor, Mass air Flow Sensor or Manifold Absolute Pressure sensor.
All 3 of these can be affected by barometric pressure and humidity changes. These changes can fool sensors that temperature or barometric pressure has changed.