At the time, no one could explain why the weather of 1816 was do unusual. Is it adjective, adverb, or noun clause

The correct answer is; adjective clause.

Explanation:

The adjective clause refers to an independent clause that modifies a noun as an adjective. This type of clause provides the necessary information about the word it modifies. Adjective clauses usually start with words like: who, where, what, why, etc.

In this example, the adjective unusual modifies the noun weather and provides additional information about the weather of 1816.