The upper deck at Bill Snyder Stadium is a single deck of seating on the East side of the field. These seats are completely exposed to the elements and are among the last seats in the stadium to get shaded.
As long as you bring your sunblock, you'll find that the upper deck is actually situated at a good height for watching a football game. Seats in the lowest rows of Sections 425-427 offer one of the best views in the entire stadium. You'll just want to avoid the actual first row, as the railing provides a bit of distraction when trying to see the field.