Translation:Isn't it hot?
Yes, but we're taking about the weather and you pretty much never say "boy, it sure is thick out today!" Even if you're taking about snowfall or fog, the only two weather phenomenon I would ever call "thick," it would be part of a longer phrase like "the fog is thick today," rather than on it's own.
Either way, when you learn kanji you will never have this problem (while reading anyway) because 暑い (あつ-い) means hot and 厚い (あつ-い) means thick. They are different words.