In part 1 we looked at trying to minimise heat loss through conduction by improving our ground insulation … now it’s time for convection.
Convection happens in response to the movement of a fluid or gas. We can pretty much forget about fluid and concentrate on gas and for our purposes that gas is air. The biggest factor is what can be thought of as ‘forced’ convection by the wind. I’m sure everyone reading this has suffered the effects of wind induced convective heat loss … like those times you’ve ducked down behind a wall for a breather and suddenly found yourself feeling 5 degrees warmer.
If you choose to spend your nights in a ‘proper’ tent with a solid inner, a sewn-in groundsheet and a fly that comes almost to the ground, then your chances of suffering are pretty low. However, if your nights are spent under a tarp or something similar, convective heat loss can start to play a significant part in whether you enjoy the night or just endure it.
The obvious thing to do is protect yourself from the wind as much as you can … The first step is to determine where the wind’s coming from and make use of your surroundings. Anything that blocks the wind will be an advantage, so utilise walls, trees, hedges and hollows when you’re selecting your spot for the night. Now you’ve selected a spot, it’s time to erect your shelter but you might have a decision to make before you do. If the wind’s light consider pitching your shelter side on to it. This should give you greater protection from the wind and reduce the effects of convection … you can maximise the benefit by pitching the windward edge of the tarp at ground level. Now, lets consider that the wind’s blowing with a bit more force … pitching your shelter side on to a high / blustery wind is likely going to result in you not having a shelter for very long. Common sense should tell you to pitch your shelter with the smallest surface area facing into the wind. In the case of a simple A frame set up, this will be the lower, foot end of your tarp. A tarp set up like this should survive most things but it won’t be doing much to prevent convective heat loss as the end of the tarp facing the wind is likely to be open … so, it’ll be channelling cold air at high speed over the top of you, all night!
Like I said, you might have a decision to make before you set up your shelter … in light winds maximise protection from the wind but as the wind increases you’ll need to consider the stability of your shelter too, which could result in an increase in convective heat loss.
All is not lost though. Firstly, if you’re sleeping under a tarp there’s a good chance you’re also sleeping in a bivvy bag and all bivvy bags are wind-proof. However I wouldn’t advise using your bivvy bag as your first line of defence against the wind, instead try to think of it as your last. Beneath your wind-proof cocoon lies your fluffy sleeping bag which relies on trapping air within its insulation to keep you warm … the more air it traps the warmer you’ll be. What happens to that air trapping insulation when there’s a 30mph wind pushing against it through your bivvy bag? That’s right, it compresses, which reduces how much air can be trapped and you end up shivering all night.
Your first defence against the wind should be your surroundings, your second barrier should be your tarp / shelter and your bivvy bag should bring up the rear and block out any sneaky draughts that got through.
The tarp in the picture above will certainly keep the rain off you but if there’s wind it’ll offer little protection.
The same tarp set-up in a slightly different guise. Set with the tail into the wind you’ll be much warmer than you would be under the basic ‘A frame’ set-up. You’re possibly carrying a certain amount of gear that’s redundant through the night, don’t waste it … use bags, waterproofs or even your bike to ‘block up’ any holes or gaps. A frame bag and harness along the bottom edge of the tarp or your waterproof jacket used as a makeshift door can make a massive difference.
Besides forced convection (wind) there’s a second type of convective heat loss that should concern us … passive convection. Passive convection can be thought of as a ‘chimney effect’
and while it can have a bearing on your shelter, the effect tends to be greater within your clothing and sleeping bag. We all know that hot air rises … but when that hot air moves, cold air moves in to take its place. So, you’re wearing long johns and a down jacket … the jacket’s zipped up but there’s still a gap around the neck and the hem of the jacket is slightly baggy, there is a drawcord around the hem but you’ve not really paid much notice of it. The nice warm air trapped between you and your jacket is free to escape (rise) via the neck, it will be replaced by cooler air entering from the loose hem … result, passive convection.
By making sure that any gaps in your clothing are sealed by tucking things in, using hoods and tightening adjusters and by using chest / neck baffles and the hood on your sleeping bag, you’ll be making it much harder for warm air to leave and cooler air to enter, so you won’t continually be trying to warm up fresh, cooler air … which means you’ll stay much warmer!