If you look up to the ceiling and wonder 'why the sky?'? Here is the answer: the ceiling is enchanted. It can use what it knows to create a replica of the weather outside. But, no one wants it to rain on them while they're having dinner so the weather stops six feet above the ground. Why is it enchanted? No one really knows for sure. But in the daily prophet there was a great entry about the topic: 

Hogwarts enchanted ceiling 

At Hogwarts School of Witchcraft and Wizardry in the Great Hall you can find you self in wind, sleet, snow, hail, or any other weather you can find outside. When it is raining, the ceiling is a darker blue with gray clouds up above. The rain comes down up until the 6 foot line. The same for snow. In a wind storm, the wind blows food into the air! As the children eat their food they can experience the outdoor weather.