Fall often surprises me with how fast it comes. Of course, I'd never be ready to finish with summer. But it does seem that in one day, all of a sudden, the summer garden turns into a fall garden. Whether I'm ready or not.
Its mostly something to do with the lighting I think. I'm trying to figure it out. When I take a photo in the summer, the colors are warm and yellow highlighted, but now shadows are too dark, highlights too bright, blues and yellows tend to be missing. When I look around, the sun is glaring in my eyes and I can't see into the shadows.
Of course the angle of the sun is lower, accounting for the glare in my eyes and the deeper shadows. Probably less moisture in the air since its a bit cooler, which may account for some changes in light quality. And the light now travels through more atmosphere, attenuating shorter wavelengths: UV, blues, some green. Some mammalian seasonal clocks respond to these wavelengths. Plants are very sensitive to these changes. The garden rapidly changes its character as the fall light quality changes.
Someone used the word pallet. The fall pallet in the garden is unique. The greens of chlorophyll are fading, replaced by reds, oranges, golds. The dry air helps to fade the greens. My pumpkins are now bright orange, the gardens are full to bursting with tall yellow sunflowers.
I read somewhere that perception of fall relates back to our hunter-gatherer ancestry. Back when we had to anticipate winter with instincts and stock up on food. I can't say I feel an urgency to go hunting, more of a sadness. Another garden is winding down. Another winter is between us and the next garden.