Learn English – Contraction for ‘are’ with nouns

Certainly, ‘dew’ is normally uncountable, but the writer precedes it here with the indefinite article for literary effect. Other meteorological phenomena, such as fog and mist, can be treated in the same way. It's not at all a stupid question.

Like most pedagogical grammars, your book gives simplified advice that does not reflect the complexity of English as it is actually used.

The key point here is not whether the noun is countable or not but whether it is countable and concrete or countable and abstract. If the noun is a concrete noun such as girl then it is probably better to say: Is there a girl in the room? or Are there any girls in the room?

Constructions such as:

• Is there any girl in the room?
• Is there any dog on the street?
• Did you borrow any book from the library?

may be considered suspect by some careful grammarians.

However, there are many countable but abstract nouns where any is perfectly acceptable:

• Is there any difference between .. ?
• Is there any way I can .. ?
• Is there any reason why .. ?
• Is there any chance .. ?
• Is there any need to .. ?