These tenses are very hard in english, even for native speakers. These both sound a little strange because it sounds like you are claiming to control the weather, so I'd use "We" instead of "It" "We will be very hot if we go to the US next month." Everything sounds good, and your tense is right. Maybe I'd flip the sentence so it starts with if, but I think that's just taste. For the second, the way the sentence is structured makes it sound like the past, so you'd want to use the past perfect if so. I think the issue is that the going sort of has to have happened in the past if it would have occurred. "We would be (or would have been if it's all in the past) so (sounds more natural than very) hot if we had gone to the US today" sounds more natural. "We would be so hot if we were in the US right now" sounds normal. So in some sense, your tenses are again correct in this example, I think the verb of motion just made it a little weird.

They aren’t natural. The weather doesn’t depend on your traveling, and we don’t have to say that we will be hot if we are somewhere that is hot. That’s understood. It’ll be (too) hot in Florida next month. Let’s go to Iceland. It’s hot in Florida now. (I’m glad we went to Iceland. , I’m glad we’re in Iceland.) Examples of the tenses: If we go to Florida next month, I’ll complain about the heat. If we had gone to Florida, I’d be complaining about the heat. (It’s hot there now)

The first sentence is good and its meaning is reasonably clear. There is some ambiguity about where it will be hot, and the sentence would have been more accurate if you had said "We will find hot weather if we go to the US next month" but it is not necessary to say this. The second sentence is grammatical but I do not know what it means. It would be very hot WHERE? Are you saying it would be very hot where you are, or very hot in the U.S. It could mean either one. Another problem is that your sentence makes it sound like you are still considering the possibility of going to the U.S. today. Since you are talking about a decision that you did not make in the past, it is necessary to use past tense. "Went", in your sentence, is not past tense. It is subjunctive mood. Say it this way: "We would be very hot today if we had gone to the US." "We" is better than "it" because the weather does not depend upon what you do.