Learn Spanish (Latin America): La temperatura habrá bajado un grado. - The temperature must have gone down by one degree.

This is a special use of the future perfect tense in Spanish! While habrá bajado literally means "will have gone down," it is frequently used to make a strong guess or express probability about an event in the recent past. English usually does this by saying "must have [done something]."

Where is the word for "by" in the Spanish sentence?

English says "gone down by one degree," but Spanish doesn't require a preposition here. When a verb expresses a change in quantity, like bajar (to go down) or subir (to go up), you just state the amount directly afterward: habrá bajado un grado.

Could I use the verb disminuir (to decrease) instead of bajar?

Yes, you could say habrá disminuido un grado. However, bajar (to go down, drop) is much more common in everyday speech when talking about the weather or temperature, whereas disminuir sounds a bit more formal.

What is another way to say "must have gone down" if I forget to use this future perfect trick?

You can use the regular present perfect tense and add a word that means "probably." For example, Seguramente ha bajado un grado (Surely it has gone down a degree) or Probablemente ha bajado un grado.