As we mentioned in the "Narrator Point of View" section, Le Guin presents the novel as Genly Ai's field report to the Ekumen. It naturally follows that the tone should also have a field reporty feel to it. Ai's attitude is very exact, even in some of the pretty extreme circumstances he finds himself in. Here's a quick example:
One day about noon, Odorny Nimmer, the sixty-first day of the journey that bland blind nothingness about us began to flow and writhe. (18.107)
At this point, Ai is fatigued both emotional and physically and in extreme danger traveling the Ice. But since this report is being written after the fact, he still manages to be exact on the time, the day, and even gives the Gethenian calendar names for the month and day. Of course, Le Guin doesn't sacrifice flare for exactness (notice the awesome alliteration in "bland blind" and the great use of the word "writhe"). It's still a story after all.
Of course, we should note the field report tone is used exclusively with Ai's report, the bulk of the story. Estraven's chapters take on a journalistic tone since they are journal entries, and the hearth-tale chapters have a folk tale tone because they're just that. It just goes to show how masterful Le Guin's prose is that she can manage to juggle all these tones and still keep a coherent story. Bravo, Le Guin, bravo!