Translation:But it is hot today.
It's actually helpful that people write out the kanji because duolingo doesn't and it is necessary..
FYI there are two kanji for "atsui" the one used here is correct as it's for climate (暑い), the other is 熱い for hot objects, etc. I just put the two kanji in google and found an explanation. This is a similar case to what I encountered when they started talking about "warm" in Japanese (暖かい - warm climate).
Nice, finally taught me how to say "but"! That seems like a very useful word, and it comes up in the weather section of all things.
"Hot" is more extreme. You might say 95 degrees Fahrenheit or above is hot, but 65-70 degrees Fahrenheit is just warm.
32°c & 18-21°c, converted to Celsius so that metric users like me don't have to google it.
Could be. Or in reply to something like "finally, it's fall," or "let's make hot coffee," or anything like that.
Is it not also acceptable to say "But today will be hot."? I was marked wrong for that, but since です can indicate either present or future tense, I think either one should work.