Questions & Answers about Luften er tørr.
You’d say:
• Luft er tørr.
Here luft stands in the indefinite form (“air” generically). Note that you don’t add en/ei because luft is normally an uncountable noun when used this way.
luft belongs to the common gender class (formerly feminine, now often just called “common”).
• Its indefinite article is en (though with mass nouns you often omit it).
• Its definite singular ending is -en, giving luften.