"The fall is hot."
Because saying الخريف الحار would change the status of the adjective حار from being "predicative adjective" to "attributive adjective"
Predicative Adjective: an adjective that tells the status or information about the thing we are talking about. In English, you usually separate this adjective by putting "is" or "are" (or any form of the verb to be) before this adjective; e.g. the fall IS hot. In Arabic, predicative is typically not defined with "AL".
Attributive Adjective: These are adjectives that attach themselves to the nouns in English. Like the hot fall. in Arabic, attributive adjectives come after the noun BUT they mimic the noun in everything: number, gender, and definition.
Thus, if I change حار to الحار then my sentence in English would be (the hot fall) instead of (the fall is hot).