The light seems to be brightest on the western side of the main continent, and dimmest to the eastern side. The light's almost gone as you wrap around Gehenna and Niflheim far to the south. There are no seasons in Hell. In fact, there's never anything in the environment to indicate temporal change except, perhaps, home-made hourglasses or clocks. Those work fine.
Time progresses in what appears to be a natural fashion.
In fact, Hell's Timekeeper, in the Council Complex in the Upper City, has as a job the monitoring of Hell's timepieces. See 'news time' on-game. Hell's ambient temperature is almost inconsequential to a Fallen, though Hell is hot by any standards in Pande and north Gehenna and gets cooler by degrees as one moves away from the realms of Pande and upper Gehenna until it freezes 'water' to the extreme south. At its hottest, Hell is not hot enough to melt metal. You need a forge for that.