I will buy an umbrella when it rains.

I will buy an umbrella when it is going to rain.

What is the difference between the first one and the second one?

Besides, can I change "when it is raining" in the first sentence?

Thanks a lot!

1. I will buy an umbrella when it rains.

2. I will buy an umbrella when it is going to rain.

What is the difference between the first one and the second one?

Besides, can I change it to "when it is raining" in the first sentence?

Thanks a lot!