Imagine spending a stormy night out in the hills with a broken ankle. Unless you are carrying a tent, sleeping bag and spare clothing it's likely to be a chilly experience.
Air temperature isn't the biggest problem! I'd rather be in a blizzard and several degrees below freezing than +5'C and raining with 100% humidity. That's because when the air is saturated it is very difficult to keep condensation and rain out of your insulation and, as this post shows, just a small amount of water will dramatically reduce your ability to stay warm. For this reason, and the fact that mountain weather can be severe at any time of year, I'd suggest carrying the same spare insulation, summer or winter, whenever storms are likely (during the colder months you will already be wearing more insulation).
The Blizzard products offer emergency insulation that is far less effected by saturation and this forms the core of my spare kit. I also use a 2 person shelter, sit mat, and a synthetic insulated jacket (with hood) that gets used for lunch stops. This provides between 8 and 11Togs of damp resistant insulation in addition to the garments I use while on the move... be sure to pack some spare food to fuel the shivering!
Click here for a comparison of some insulated jackets.
Why it feels colder when it rains.
In Britain rain occurs in cool summer weather and warm winter weather. This link explains.
Please browse the Insulation section on the left, for more information.
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