I guess for greater context I'll post some relevant stuff
EL 16 (Strong Sign): The sun is barely visible in the sky, and snow and ice cover the land. Entire villages are empty, and terrible ice creatures roam the land. In one final desperate measure, the local lord entreats the PCs to solve this problem, giving them whatever resources he can muster. There is only one place to go: the mountain. While the characters climb its snowbound slopes, all manner of alien ice creatures try to slow their ascent, leading to a terrible conflict with the keepers of the summit: a tribe of converted frost giants (encounter 3―1, page 42).
EL 17 (Overwhelming Sign): As the PCs reach the mountain's peak, the sun no longer rises and the temperature keeps dropping.
There is no sunlight anymore, and those who don't have darkvision are practically blind. At the summit, the PCs encounter the most powerful representatives of Father Llymic's brood, including a transformed dragon. After braving these last challenges, Father Llymic's icy form finally stirs. His gigantic alien bulk shatters the last sheet of ice and surveys the perfect darkness he has created (encounters 3―2 and 3―3, pages 44 and 46).
Only after Father Llymic's defeat might the sun rise again and the world begin to thaw. What world is left, however, is for you to decide.
The Overwhelming Sign is important as stated here
Overwhelming: The sun vanishes altogether. Spells and spelllike abilities that have the light descriptor, as well as alchemical or magic items that generate light effects, cease to function. Darkness spells continue to be enhanced as under the strong sign. The average temperature plunges another 10 degrees and drops by 1 degree further every day thereafter.
So on his release the global temp drops by 10 degrees (presumably F but I guess C can be used as an alternate end). Then drops 1 degree every day until the entire planet is frozen over.