The fertile farmlands of central Cyrodiil, around Lake Rumare and the Nibenay Valley, the region commonly known as "The Heartland," are temperate in climate, supporting the crops and livestock that feed all of central Tamriel. Rain and thunderstorms are frequent, but the region is free from the sandstorms of Hammerfell to the west or the monsoons of Black Marsh to the southeast.
Much has been made of Heimskr's classical description of Cyrodiil as a jungle or rainforest. My studies indicate that the use of the phrase "endless jungle" to describe Cyrodiil appears to be an error in transcription. Close study of the original, badly faded manuscript reveals that the phrase was miscopied, and should be more accurately rendered as "extensive uplands." The adjectives "an equatorial rain" as applied to the Nibenese forest do not appear in the original manuscript at all, and I would posit were added by the scribe in support of his previous erroneous use of "jungle." Lady Cinnabar of Taneth, of course, takes issues with this exegesis, but the flaws in her methods of scholarship have been well-documented elsewhere.