I think it may be chilled.
The rarely used adverb form is frostily. It means in a frosty or cold manner. Metaphorically, it can mean in a haughty or disdaining manner.
Only bitterly is an adverb, modifying cold which here is an adjective referring to weather, temperature, or wind.
Cold
It can be, when it is used idiomatically, e.g turned down cold, cold sober. Otherwise it is an adjective and the adverb form is coldly.
Despite ending in -LY, chilly is an adjective (somewhat cold). The adverb (chillily) is virtually never used.
The woman spoke huskily, as if she had a cold.
So is an adverb in that example.
winters are very cold
An adverb used to give force or emphasis, for example, ( without intensifier) My feet are cold (With intensifier) My feet are extremely cold
Pretty is primarily an adjective (attractive, nice-looking, for a female), but can also colloquially be an adverb meaning "considerably"-- e.g. "It was pretty cold last night."
"Although" is an adverb, to be used at the beginning of an adverbial clause: Although it is very cold, he did not want a coat.
Adverb