Priestly Control Weather can, if Weather is a major sphere, move the weather by 2 steps, which would let you take a typical Athasian Hot, Clear, Calm day and turn it into Cool, with a light rain and a strong wind. It lasts for 4d122 hours (average of 52), and covers 4d42 square miles.
So, does that note about “if Weather is a major sphere” apply if someone has Major access to Air (the only way to cast it on Athas)? And, more fun for gaming, what if someone starts doing this? Just a high-level templar, cleric, or druid who just casts this every other day or so?
On the other hand, psionics can potentially accomplish this or similar without any issues. I will have to check the Master List to remind myself about psionic weather effects, if any, but there are ways and means even if not.
By the rules, an Air Cleric, Druid, and Templar can accomplish this. However, they would have to be 13th level, which is pretty high in 2nd edition. In addition, they would have to want to do it. I can see an air templar or druid doing it for the communities they protect, but not for a city-state. Templars might be willing to do it, if ordered to do so by their master, but they certainly wouldn’t do so out of the goodness of their black hearts
In DSS2 (“Earth, Air, Fire, and Water”) [don’t ask me why they didn’t make the title alphabetical], Control Weather is included in the spell list for Clerics of Air (p 94), which I think could imply it’s a major (for Druids, it’s explicitly so). Which is just what @Rajaat99 is noting above (though I think it’s L14 for 7th level spell access for the druids and air clerics, L15 for templars). I think Control Wind doesn’t get you the same ability to adjust humidity – it has an ability to up/down regulate the speed of the wind.
I do like @LibraryOgre’s broader meta-gaming point though that if you had a high enough priest/templar do this every other day, you could have a local cool spot. I think as a DM, I’d play this as you’d have to rob Peter to pay Paul here – that is, if you did this you’d be drawing that favorable weather from somewhere else (up on the Ringing Mountains?) and so you’d need some sort of justification that tied to the pact with the Elemental Plane of Air granting the power. If a Templar did this (even from their black-heart) for one of the SKs, it feels like it’d be a great adventuring set up for some clerics/druids to try to stop this weather manipulation.
What if this is how Lalai-Puy maintains the Crescent Forest? A couple high-level templars who cause enough rain that the forest can survive, but the population of Gulg is so small that there’s seldom more than one or two who can do it?