Keeping warm at night...a scientific analysis for Valerie's class
A good sleeping bag can keep you warm at night, as my niece Valerie learned on her Girl Scout trip a few years ago. But what makes a good sleeping bag? How do you stay warm if it's -20°C outside? To stay warm, you want to minimize the amount of heat that is conducted away from your body and the amount that is advected away.
Solid objects conduct heat. If you touch a cold object with your bare finger, heat moves from your finger to the cold object, making your finger cold and the object warmer. This is conduction. Different objects conduct heat differently. Metal conducts lots of heat whereas feathers conduct very little. Thus, feathers are good insulators, and we want our sleeping bags made out of feathers rather than metal. They are also softer!
Air, water, and other fluids conduct heat, but they also advect heat. When air moves, the warm air next to your body is blown away and replaced by cold air. This is advection and is one of the causes of the wind-chill effect. You can lose heat very quickly when a cold wind is blowing against your bare skin. Thus, to keep warm, you want objects next to your body that stop advection. Solid objects stop advection because they don't flow.
Down sleeping bags are really nice. The feathers stack up on each other making a light mixture of air and solids. The air doesn't advect through the feathers, and neither the feathers nor the air conduct heat. It makes a really light weight, warm sleeping bag. For those of us camping in Antarctica, NSF provides nice down sleeping bags made by the company Feathered Friends. My own favorite sleeping bag is also made by them.
Here is a sketch of me in a sleeping bag in a tent:
The white areas are air, and the main process removing heat is advection. Because the air flows, the temperature in any one air area is the same everywhere. For example, all the air inside the tent in my sketch is -5°C. In contrast, the temperature in the solids varies because heat is being conducted. They are warmer closer to me (the source of heat) and colder farther away. The partial differential equation that describes these processes is:
We won't use it here, but it is a very important equation you should look forward to learning! We'll use graphs.
If I have a good sleeping bag, I'm warm enough as illustrated in the next sketch. Note that 20°C is 68°F, so warmer than that at my body-clothes contact is comfortable.
Note that the areas with air have a constant temperature and the solids have varying temperature.
If my sleeping bag is too thin (or conducts too much heat), I am too cold. The temperature next to my body is less than 68°F.
Wearing clothes inside the sleeping bag also really helps. Here's what happens if we remove the clothes.
If my sleeping bag isn't warm enough for the weather outside, I can put on more clothes to be warmer. The temperature between my clothes and my sleeping bag might be low, but the temperature next to my skin will be comfortable. I could also put one sleeping bag inside another one. That's an excellent way to be toasty warm! (I'll let you think about drawing that graph.)
Questions?
Solid objects conduct heat. If you touch a cold object with your bare finger, heat moves from your finger to the cold object, making your finger cold and the object warmer. This is conduction. Different objects conduct heat differently. Metal conducts lots of heat whereas feathers conduct very little. Thus, feathers are good insulators, and we want our sleeping bags made out of feathers rather than metal. They are also softer!
Air, water, and other fluids conduct heat, but they also advect heat. When air moves, the warm air next to your body is blown away and replaced by cold air. This is advection and is one of the causes of the wind-chill effect. You can lose heat very quickly when a cold wind is blowing against your bare skin. Thus, to keep warm, you want objects next to your body that stop advection. Solid objects stop advection because they don't flow.
Down sleeping bags are really nice. The feathers stack up on each other making a light mixture of air and solids. The air doesn't advect through the feathers, and neither the feathers nor the air conduct heat. It makes a really light weight, warm sleeping bag. For those of us camping in Antarctica, NSF provides nice down sleeping bags made by the company Feathered Friends. My own favorite sleeping bag is also made by them.
Here is a sketch of me in a sleeping bag in a tent:
The white areas are air, and the main process removing heat is advection. Because the air flows, the temperature in any one air area is the same everywhere. For example, all the air inside the tent in my sketch is -5°C. In contrast, the temperature in the solids varies because heat is being conducted. They are warmer closer to me (the source of heat) and colder farther away. The partial differential equation that describes these processes is:
We won't use it here, but it is a very important equation you should look forward to learning! We'll use graphs.
If I have a good sleeping bag, I'm warm enough as illustrated in the next sketch. Note that 20°C is 68°F, so warmer than that at my body-clothes contact is comfortable.
Note that the areas with air have a constant temperature and the solids have varying temperature.
If my sleeping bag is too thin (or conducts too much heat), I am too cold. The temperature next to my body is less than 68°F.
Wearing clothes inside the sleeping bag also really helps. Here's what happens if we remove the clothes.
If my sleeping bag isn't warm enough for the weather outside, I can put on more clothes to be warmer. The temperature between my clothes and my sleeping bag might be low, but the temperature next to my skin will be comfortable. I could also put one sleeping bag inside another one. That's an excellent way to be toasty warm! (I'll let you think about drawing that graph.)
Questions?
