Fogs and mists become especially prominent as autumn nights cool the moist atmosphere of heavily vegetated areas. While fogs are most common in many areas through the winter, in those same areas they first become prominent in autumn ...
From Haiku World, by William J. Higgenson.
This entry shows an interesting facet of Japanese season words. Kiri literally means fog or mist. But in haiku it became traditional to associate it with autumn, so in English translation the word autumn is usually added. Particular words are usually placed in a season by when they first occur, or by when they are most noticeable. So "long day" is a spring season word because, although the days are longest in summer, you first notice the day lengthening in spring.
This kind of shorthand fills the world of the haikuist, and is part of the reason so much can be fit into so few words. Many, many words have meanings in haiku that are fully understood only to someone familiar with this shorthand.