What to wear for 40-degree temperature swings
Some climates do not have a temperature so much as a schedule: 38 at sunrise, 72 by mid-afternoon. Single garments cannot span that. What works is treating the day as three separate forecasts and dressing in pieces that hand off to each other.
The morning layer that disappears
The first layer to go should be the most packable: a light insulated vest or a compact puffer that compresses into its own pocket and lives in a tote by 10am. Warmth you can carry in one hand is worth more than warmth you cannot take off.
The mid layer that does the work
A mid-weight knit or overshirt is the real answer layer, warm at 50 with the shell on, comfortable at 65 with it off. Wool and wool blends regulate across a wider band than cotton, which is why they dominate this slot. This is the layer you are photographed in, so pick it like an outfit, not underwear.
The base that stands alone
By 2pm the base is the outfit, so it has to look intentional: a real tee, fine knit, or shirt, not a thermal unless the day never warms up. Breathable natural fibers or merino handle the swing better than synthetics, which feel clammy once the temperature climbs.
Accessories as the adjustment dial
A beanie, light scarf, and gloves cost almost no bag space and shift your comfort range by 10 degrees in each direction. On swing days, accessories do the fine-tuning that layers do too coarsely. The full system is three layers plus small stuff, and it covers 38 to 75 without a wardrobe change.