Your body is in a constant balance to regulate the core temperature to around 37 °C (98.6 °F).
That balance is influenced by three main factors: how much heat your body generates, how you're dressed and the temperature of the environment you're in.
To help you manage that balance, we've written two articles - one article about the ways you lose heat and the other on how you generate heat.
In this article, we'll talk about heat loss. And for hunting that's relevant in the following situations:
- Hunting or glassing from a fixed position for a longer time
- Hunting in harsh weather
Harsh weather would include temperatures below freezing. But it could also be above freezing, if hunt in a combination of rain and high winds.
To be able to dress optimally for these types of situations you need to know the four primary ways you lose heat:
- Radiation - heat energy that radiates away from your body
- Wind - heat removed by airflow
- Contact - heat lost when you touch cold objects
- Sweat - Heat you lose through perspiration
Heat energy that radiates from your body
When you put your hand close to a warm mug of coffee, you can feel the heat. That's radiation.
Heat radiates as infrared waves. The same waves that allow us to see marauding hogs in a field with a thermal imager.
It is pretty much impossible to prevent heat loss from radiation with clothing. But, it's typically not a problem when you dress appropriately for the other types of heat loss. And eat and drink enough.
Heat removed by airflow
Back to your mug of coffee.
The warm coffee heats the air above the coffee. When you blow across the surface, the warm air is moved away and replaced with colder air, which the coffee will heat. You speed up the cooling of the coffee by replacing warm air with cold air.
That's called convection.
It's the reason that the wind makes you feel colder.
The principle applies to the flow of water as well, but we'll focus on air for now.
Heat removed by airflow is probably the biggest reason for cold shock (hypothermia). So it's one to pay attention to.
Luckily, the solution is simple.
Stop the wind, trap air and heat it.
In other words, wear a windproof layer to stop the flow of air. Wear a layer of clothing underneath that traps air. This way, you stop the cycle of having to heat new, cold, air. And you minimise your heat loss.
The thickness of the insulating layer depends on how cold the weather is.
Heat lost through contact with cold objects
Remember that mug of coffee?
When you first poured your warm beverage, the warm coffee heated the cold mug. But the coffee was also cooled, a bit.
The same thing happens when you're out hunting. You touch something, and there is a transfer of heat, from warm to cold.
And you are usually the warm object, losing heat.
Heat loss through contact is called conduction.
Conduction is generally speaking, not a big contributor to cold shock (hypothermia). But, things are a little different for hunters.
We might sit up in a tree-stand, rest against a rock to spot for animals or lie down waiting to take a shot. And we use equipment.
Conduction happens with all materials, but there is a big difference in how fast they will remove heat from your body.
Let's look at some examples.
We'll use air as the reference, as it is an excellent insulator.
The table shows how much faster than air; a material transfers heat. That is, how much faster you lose heat by touching something, compared with air.
Speed of heat transfer. Compared with air.
- Air - reference
- Plastic - 1.3 x faster than air
- Wool - 1.7 x
- Cotton - 1.7 x
- Wood - 4 x
- Rubber - 6 x
- Nylon - 11 x
- Water - 24 x
- Ice - 87 x
- Rock - 196 x
- Steel - 1739 x
Wood transfers heat four times faster than air. And steel transfers heat a whopping 1,739 times faster. Yes, that's one thousand, seven hundred and thirty-nine times faster.
So, when you're sitting up or lying prone for a while, or handling equipment in cold weather, think about conduction. It might be that clothing isn't enough to reduce heat loss if you're there for a while.
Heat lost when sweating
The mug of coffee in our example doesn't sweat, but it steams (evaporates). And that removes heat energy.
Water also evaporates from your body. For example, it evaporates out of your mouth and through the pores in your skin as sweat (perspiration).
We sweat all the time. Sweat is insensible (water vapour) at very low activity levels. At higher activity levels sweat is sensible (water). The hotter we get the more we sweat.
Both types can condensate in your clothing.
Because water removes heat 24 times faster than air we want to avoid that condensation.
So we need to prevent or reduce sweating and dress in a way that lets sweat escape from our clothing system.
Knowing how much we'll roughly sweat will help us plan what, how much or how little to wear.
The following list shows how much more an adult sweats, compared with when they're sitting down.
- Sitting - reference
- Gentle walking - 2 x more than sitting
- Active walking, no pack - 3 x
- Active walking, light pack - 4 x
- Active walking, heavy pack - 5 x
- Mountain walking, heavy pack 6 x
These numbers are guidelines as there are variations between individuals.
The key point is that if your body gets hot enough, you will start to sweat. Don't wear too much insulation and make sure sweat isn't trapped in your clothing.
Summing it all up
You lose heat in four main ways, radiation, airflow, contact and sweat. They create different requirements for your clothing.
Make sure that you take weather conditions, your activity level and the tasks you perform into account when you dress for hunting.
Sources and recommended reading
- Hypothermia, Frostbite and other cold injuries - prevention, survival, rescue and treatment (G. Giesbrecht, J. Wilkerson) ISBN 0-89886-892-0
- Waterproof and Water Repellent Textiles and Clothing (editor J. Williams) ISBN 978-0-08-101212-3