das Wetter without an article

Question

Why do we say "es ist schönes Wetter" and not "es ist ein schönes Wetter"?
Do adjectives replace articles, or is this just some exception?

Answer 1

Same reason why there is no article in the english translation "there is nice weather": Because there is only one weather. There are not two or many of them.

The German word Wetter can have four meanings:

1. Das Wetter (neuter, no plural form, only used in singular)
   in English: the weather
   Current state of the climate (Is it raining? Is the sun shining? How strong is the wind? What's the temperature of the air?)

2. Das Wetter (neuter, can be used in singular and plural)
   German synonyms: Gewitter, Unwetter
   in English: storm, tempest, thunderstorm, windstorm

3. Die Wetter (no singular form (and therefore no specific gender), only used in plural)
   in English: mine air
   Gases in a mine

4. Der Wetter (masculine, both singular and plural)
   A person who is betting (to bet = wetten)
   in English: bettor, wagerer, backer

The Wetter you are talking about is #1 in my list. And when we talk about this Wetter, then there is only one. You can use it with a determining article

Das Wetter ist schön.
The weather is fine.

You can also compare the actual weather with the weather form another day:

Heute haben wir ein wärmeres Wetter als gestern.
Today we have a warmer weather than yesterday.

But in fact (in the logic of German language) today is not a different weather than yesterday. In fact the state or the manifestation of the only one weather that exists has changed. So, when you talk about any weather, you still talk about the one weather, and so it doesn't make much sense to use an indefinite articel:

Heute haben wir schönes Wetter.
Today we have nice weather.

You also could omit the attribute. The sentence still would be grammatically correct:

Heute haben wir Wetter.
Today we have weather.

You also can add an indefinite article:

Heute haben wir ein Wetter.
Today we have a weather.

But with or without an article, this sentence doesn't express any interesting meaning. You just say, that weather exists. But this something everybody already knows.