Learn Finnish: Se oli nimenomaan sää, joka pilasi matkan. - It was specifically the weather that ruined the trip.

This is a cleft sentence, used to put strong focus on one specific part of the sentence—in this case, sää. It translates perfectly to the English "It was the weather that...", heavily highlighting that the weather, and not something else like the hotel or the food, was the culprit.

Does the relative pronoun joka refer back to se or to sää?

It refers back to sää (the weather). Even though the sentence starts with the demonstrative pronoun se, the focused noun is sää. Because sää is a regular, specific noun, the correct relative pronoun is joka. If the relative pronoun referred to the pronoun se itself or to an entire preceding clause, you would use mikä instead.

Why is the object matkan in the accusative case (with an -n ending) rather than the partitive?

The verb pilata (to ruin) typically takes a total object because ruining something is seen as a completed, bounded action. Here, matkan is the object of the relative clause (joka pilasi matkan). Since this subclause has a standard grammatical subject (joka, acting on behalf of the weather), the total object takes the normal -n ending.

What does nimenomaan mean here, and could it be placed elsewhere in the sentence?