The forces of nature affect us. Just like the day-night cycle (circadian rhythm) has an impact on our physiology, the changes in season impact us as well. The qualities (gunas) which are the most fundamental way of understanding this impact change throughout the day, night, month, and year…
Depending on the geographical location, the seasons bring different qualities. In temperate areas like most of the United States, the fall season is considered a Vata Season — the sharp intense heat of the summer shift as a crisp, dry coolness bring the season of Fall.
The transition between two seasons (typically the final 15 days of one season and the first 15 days of the next) is a time to adjust diet, lifestyle and also clothes and attitudes from the old season to enter afresh into the next.