When does Sirocco do damage

I suppose this question applies to all persistenet damage spells, heres the one in particular
https://www.d20pfsrd.com/magic/all-spells/s/sirocco/

Does this do damage inutially on casting?
At the start of anyone insides turn? At the end? A combination?

Thanks guys!

They take damage on the first turn you cast the spell and take the damage again on your turn each subsequent round they remain in the area. If a creature was outside the area at the time of your initial casting and then moved inside the area at a later round, they would take damage as soon as they enter, and then continue to take subsequent damage on your turn like normal thereafter.