Autumn color is often attributed to the first cold snap, but the trigger is the lengthening night. Weather determines how vivid the display becomes and how long it lasts, not when it starts.
What the trigger actually is
Deciduous trees track the duration of uninterrupted darkness, which lengthens on a fixed astronomical schedule that repeats identically every year.
Once the nights pass a threshold specific to the species, the tree begins forming a barrier of cells at the base of each leaf stalk and winds down chlorophyll production.
Because the cue is astronomical rather than thermal, the sequence in which species turn is remarkably consistent from year to year even when temperatures are not.
Why green disappears before other colors arrive
Chlorophyll is the pigment that captures light for photosynthesis, and it is continuously broken down and rebuilt through the growing season.
When rebuilding stops, the existing chlorophyll degrades within days and the green masking effect is removed.
Yellow and orange pigments were present in the leaf all along, serving supporting roles in light capture and protection. They become visible only because the green has gone.
How the reds are different
The red and purple pigments are not revealed but manufactured, produced in the leaf during autumn itself from sugars trapped behind the developing barrier.
That production requires sunlight and takes place most readily when warm sunny days let the leaf make sugar and cool nights slow its export to the trunk.
The dependence on current weather is why red display quality varies sharply between years while yellow display is comparatively stable.
Why a dry summer changes the outcome
Drought stress can push a tree to shed leaves early, cutting the display short before pigments develop or simply turning foliage brown.
Excess rain and heavy cloud reduce sugar production and mute the reds, producing a flatter display even when timing is normal.
A moderate growing season followed by dry, bright autumn weather with cool nights and no hard freeze produces the strongest color, which is a narrow set of conditions.
What ends the display
The barrier at the leaf base continues to develop until the connection is severed and the leaf falls, a process that proceeds regardless of how the color looks.
Wind and heavy rain strip leaves mechanically well before that point, which is why a single storm can end a peak that had days left.
A hard freeze kills leaf tissue outright and halts pigment production, turning a developing display brown in a night.