What is the converse of "hence"?

Imagine I have two sentences. The second one causes or precedes the first one, like:

It is raining. There are a lot of clouds.

You can say:

There are a lot of clouds. Hence, it is raining.

I want to know if there is one word to obtain this structure:

It is raining. ..., there are a lot of clouds.

Is there a word I can put on the ellipsis? I know I could use a description (for instance "This is because"). Starting the sentence with "Because" is not allowed in my case.

What about since and for?

There are a lot of clouds, hence it is raining.

Since there are a lot of clouds, it is raining.

It is raining, since there are a lot of clouds.

It is raining. For there are a lot of clouds.

I've seen hence and since forming a pair quite often. Furthermore, it is not at all unusual or frowned upon to begin a sentence with "For, ..." (even though I personally do not like it).

As I understand it, you're trying to show the fundamental connection between the first state and the second. The second state caused the first, so it should follow that its present when the first is. You're not looking for the "opposite", (an imprecise term usually meaning inverse) you want the converse. Using that understanding, we could have:

Its raining. Naturally, its cloudy.

Where its refers to the local weather condition by context, and the lack of change in context implies that the two are related, so naturally implies that there's a fundamental relation. You could also use:

Its raining. Of course it's cloudy.

Hinting that raining should inherently imply cloudy.

Its raining. Its obviously also cloudy.

Explicitly saying that there's a known relation there.

Alternatively to all of that, you could join the two sentences, resulting in the smoother:

Its raining, so naturally its cloudy.