The dark night sky matches the aura of two certain individuals - one specific Severus Tobias Snape and one specific Harry James Potter. While the Potion Master's mood is darkened by the prospect of the brat's arrival, the said brat's soul is darkened by the prospect of having to face a certain Headmaster.
As he sets foot in the Great Hall, Harry's soul darkens to the point of insanity, but he reigns it in. Nobody must know what I hide. And hence, he tries to calm his ever growing hate and hides behind feigned innocence. A name is called, a hat is placed and a decision is made - one which rocks the Wizarding World to it's very core and confuses the hell out of a certain greasy-haired man.
The Golden Boy is placed in Slytherin.