Learn Spanish (Latin America): El clima estuvo muy húmedo en el valle ayer. - The weather was very damp in the valley yesterday.

We use estar for weather when it is followed by an adjective describing the condition (like húmedo or nublado). We use hacer when followed by a noun (like hace calor). We use the preterite tense (estuvo) rather than the imperfect (estaba) because the word ayer frames the weather as a completed, closed event that happened yesterday, rather than a continuous background description.

I thought "the weather" was el tiempo. Is el clima interchangeable with it?

Yes, for the most part. While el clima technically means "the climate" (the long-term pattern), in Latin America it is very commonly used in everyday conversation to mean "the daily weather," just like el tiempo. Both are perfectly fine here.

Can húmedo be used for wet objects, or just the weather?

It can be used for both! It translates to "humid" or "damp." Just as the weather can be húmedo, a towel that hasn't fully dried can be described as una toalla húmeda.

Could ayer go at the beginning of the sentence?