Cold Weather Stove Performance
For long winter trips where you’d be forced to cook many meals below zero I would personally use a liquid fuel stove running on white gas. This is because white gas isn’t as affected by the cold and these stoves are more efficient at melting lots of snow, plus performance remains constant. Nevertheless if you’re doing a short winter trip, or a winter climb with two or three bivvies most people will opt for a cartridge stove.
At sea level Butane vaporizes at a temperature above -1°C. This decreases with altitude gain (as air pressure is reduced) so it’ll vaporize at -11°C at 10,000ft (3,050m) and -19°C at 20,000ft (7,000m). This might not sound too bad, but this temperature is not only dictated by the ambient temperature, but more importantly the temperature within the cartridge itself. As the liquid fuel vaporizes it expands 270 times its volume, causing cooling (like in a fridge). You can see this when ice builds up on a cartridge even in mild weather. This means that unless you’re high on K2 Butane is a poor mountaineering fuel for general use.
Propane on the other hand vaporizes at -42°C at sea level and is obviously an awesome cold performer. Unfortunately Propane must be stored under much higher pressures, making it impossible to manufacture easily portable, lightweight, pure Propane canisters.
A way around this is to blend the two fuels (80/20 or 70/30 butane/propane) to increase cold weather performance while maintaining a portable cartridge. When the cartridge is full this works great for cold conditions, unfortunately the more volatile Propane burns off faster (even if you shake it up). Add to this the fact that performance drops by 50% as the pressure drops and you see why cartridge stoves seem amazing when first fired up on a fresh cartridge (in the shop), but seem less effective on their 10th brew on the Walker Spur.
MSR Iso Butane is another blend that has been introduced, which increases the amount of Propane to its maximum level in order to increase the stove’s mountaineering performance.
If cooking in an enclosed space like a tent porch in winter this isn’t a great problem as the ambient temperature soon rises and all you need to do is insulate the cartridge from the ground. Due to the unstable nature of tower cookers you need to be careful if propping the stove on top of something, but a small square of wood or, better still, something like Primus’s folding support (30g/£4) is a good option.
If you are camping or bivvying keep the next morning’s canister in your sleeping bag so it’s warm for the first brew and swapping between two canisters (putting one in your jacket) can also reduce the effect of cooling. Some people mistakenly think that insulating the canister with foam will improve its performance but this only increases the rate at which it cools as no ambient heat can warm it (even if it is super cold outside it’ll be colder in the canister).
If forced to cook outside on an open bivvy then you need to stop the cartridge freezing. As mentioned above a windshield can help warm the cartridge, or you can pop the cartridge into the water every now and again as you’re cooking. Another option is to place the canister in the pan lid and let it stand in half an inch of cold water, topping it up as you heat the water in the pan above (you’ll need a second lid which can just be made of tin foil). Many climbers in the past played around with copper tubing that ran through the flame and back down to the canister, but this has largely fallen from favour, with climbers preferring to use the above methods or just use white gas stoves if it’s that bad. Hand warmers can also be used very effectively, but this is either an expensive option with disposables and heavy with reusable models.
I’m sure it goes without saying that all the techniques mentioned above can be extremely dangerous if common sense and judgment aren’t used when mixing highly volatile gases with heat.