Translation:Is it hot today?
is this true for all weather, or just for things like temperature? I've seen some cases where duo accepted future tense responses in English with "です” at the end of the sentence. For instance, the "preferred" translation for "今日は雨ですか?" was "will it rain today?", but I'm not sure if that's the common usage, or if it's just Duo being goofy about translation conventions.
You can use でしょう with any type of weather. If you use です in a statement about weather, you're saying with 100% certainty that it is going to happen. We can't usually know with that much certainty what the weather will be, which is why many Japanese people avoid using です.
今日は雨ですか is a question rather than a statement of fact, so I think you can be a little more flexible.
If you say "It's hot today, isn't it?", you think that it is hot and are looking for the other person to agree. In Japanese, you would say 今日は暑くないですか (kyou wa atsukunai desu ka) or 今日は暑いですね (kyou wa atsui desu ne) to capture that same nuance.
今日は暑いですか (kyou wa atsui desu ka) is just a straightforward question, where the speaker is genuinely asking if it's hot today.