Learn Arabic (Modern Standard): الشمس حارة جدا في الصحراء.

Some Arabic nouns are inherently feminine by convention, even without the typical ـة (taa marbuta) ending. Nouns related to nature, like الشمس (the sun) and النار (the fire), as well as paired body parts, are usually treated as feminine. Thus, the adjective describing the sun must be the feminine حارة.

Why does جدا (very) come after حارة (hot) instead of before it?

In English, adverbs of degree usually come before the adjective (like "very hot"). However, in Arabic, جدا almost always directly follows the adjective it modifies. You are literally saying "hot very" (حارة جدا).

Is there a missing verb for "is" between الشمس and حارة?

No, this is a complete sentence! In the present tense, Arabic doesn't use a verb for "to be" (like "is" or "are"). Instead, it uses a nominal sentence, where the subject (الشمس) is simply placed right before the information describing it (حارة جدا) to mean "The sun is very hot."

Why is the "l" sound in ال (the) skipped when pronouncing الشمس and الصحراء?