The Ringworld is an object around 1 million miles wide and with a diameter of approximately 2 AU, encircling a Sun-like star. From what I can tell the surface of the Ringworld is "flat", as in the base of the ring is not curved.
The Sun however has a diameter of roughly 864.938 miles in diameter. Even on our modestly-sized planet of 7.901 miles in diamter around the poles (25 more around the equator) there are rather big differences in the climates in the various regions of Earth: wet, dry, warm, cold and so on.
But when you are talking about an object that has a million... well, "vertical" miles compared to Earth's mere 3950,5 miles, and that the northern and southern parts of Earth are very cold, should the norther and southern parts of the Ringworld's surface (Belts? Wall borders?) not be extremely cold as well?
One of the reasons that the poles are cold because of the slanted angle sunlight goes through the atmosphere because of the Earth's shape. The Ringworld hower is flat, but because of its diameter is so massive (bigger than its star, even!) this effect might be in place as well.
So what I wonder is, is there any description of the Ringworld having such massive ice belts near its rim walls? Is it even possible for them to be there? Or is the inside of the Ring curved, so that all of the Ring's surfaces are the same distance and at the same angle of the sun?