The jacket should be chosen based on the primary use and the minimum expected temperature, not as an "all-purpose" garment.
As a practical reference:
+5 °C to 0 °C → Light jacket, designed for active use or thin layers during short stops.
0 °C to −5 °C → Medium insulating jacket, suitable for stops, camping, and occasional static use.
−5 °C to −10 °C → Jacket with greater insulation, intended for real cold and longer static periods.
< −10 °C → Specific layering system or a winter jacket; an ultralight jacket alone is not sufficient.
Factors that adjust this range:
Wind and humidity, which greatly reduce thermal performance.
Type of activity (moving vs. standing still).
Complementary layers (windbreaker, waterproof, base layer).
In ultralight hiking, the correct approach is not to oversize the jacket but to adjust the insulation to the real range of use and complement it with the rest of the system.
The jacket offers a better fit, wind control, and backpack compatibility, ideal for technical routes or strong winds.
The poncho is lighter and ventilates better, but provides less protection in the wind and may limit mobility. In ultralight hiking, the choice is made based on terrain and exposure, not fashion.
Because when hiking, the main problem is not getting wet from the outside, but becoming soaked from internal condensation. Ultralight solutions prioritize:
