"Jak chladný je dnes?" could mean:
- How cold is the juice served today? I've been having it every day, sometimes it's very cold, sometimes it's lukewarm. (or "salad", or another masculine-gendered thing)
- How distant/reserved is he today? Perhaps when you're visiting your uncle in a retired home and asking one of the caretakers what his mood is today. (a bit unlikely)
In other words, people would wonder what cold thing you're referring to, and few would figure out that you're talking about the weather.
Otherwise what would the adjective "chladný" actually point to? The air? The wind? The feeling you have on your skin when you're outside?
Good question, the short answer is: here it's an adverb, but it can be a noun (neuter).
So the adjective "chladný" can form adverbs in two ways:
1) chladně -- meaning "coldly", in a cold way, often used metaphorically, for example: "Přijali mě chladně" (They received me coldly, i.e. not in a friendly way.)
2) chladno -- only used in a predicate (i.e. with "být"), for example: "Bylo chladno" (It was cold. = the weather was cold) or "Je mi chladno" (I am/feel cold). As an adverb, this never changes form.
And then we can derive a noun, which looks the same: "chladno". It can mean cold weather, for example "noční chladno" (the cold of the night), and we can even use it in plural: "Přišla noční chladna" (literally "Night colds have come", meaning cold temperature at night). It can also mean "a cold environment", so you can read on a lot of labels on products: "Uchovávajte v chladnu a suchu" (Keep in a cold and dry place) - where both "chladno" and "sucho" are nouns referring to "environment" or "conditions" rather than "place".
And then we can form another noun, less common: "chladnost", meaning "coldness", where the "-ost" suffix has the same function as English "-ness".
You can apply the same principles to many other adjectives, for example:
- teplý (warm) -> teple (adverb), teplo (adverb and noun)
- vlhký (damp/humid) -> vlhce (adverb), vlhko (adverb and noun)
- smutný (sad) -> smutně (adverb), smutno (adverb and noun)
"Díval se na mě smutně." (He looked at me sadly), "Bylo mu smutno." (He felt sad) -- both are adverbs, but the "-o" one is used predicatively (is, was...).