Q: coldness と coolness はどう違いますか?
A: "Coolness" is less extreme than "coldness". So an autumn day might be cool, but it probably isn't cold. In the middle of winter in Finland, it would probably be cold (not cool).

Because coolness isn't as extreme, it can be quite pleasant. You will often hear the word "cool" when people are describing pleasant sensations. For example:
It was a hot day, but a cool breeze blew off the sea.
The water was cool and refreshing.
Her hand felt cool on my forehead.

"Cold" tends to express a more unpleasant or extreme temperature. For example:
The water was cold, so I got out quickly!
My hands were cold when I came in from the snow.


Q: The coldness came back again today. この表現は自然ですか?
A: The cold came back today again.
Q: The coldness of winter is severe. It is about time that I want to go to a hot spring. この表現は自然ですか?
A: 'The coldness of winter is severe. It is about time that I go to a hot spring." ✔

But if you say this sentence to someone, it will sound awkward and unnatural.

'The coldness of winter is severe.'
This style of writing is usually used in novels because of the exaggeration and tone. When you say this in a daily conversation, it will sound strange and grim.

You use 'severe' when something has a bad, dangerous, or serious effect. Usually it is used to describe diseases and sickness.

'She has a severe fever.' = She has a very bad fever.

'It is about time that I go to a hot spring.' sounds more casual. So when you combine the two sentences, it sounds unnatual.

So, the better way to say it is:

The winter is getting too cold. I should go to a hot spring.
Q: The coldness recorded in the Showa Era has come back. この表現は自然ですか?
A: "The freezing temperatures that were recorded in the Showa Era seem to have returned." if you want to sound really fancy when you talk 🤓🤓
Q: What's this coldness outside today? この表現は自然ですか?
A: "What's with this coldness today?" A bit colloquial, but it has the meaning, 外でなんでこんなに寒いの、今日は?

"How cold is it outside today?" 「外で今日の寒さはどれくらい?」か「今日は外で何度?」