After a run of very nice warm winter weather out here on the hilltop, while much of the rest of the country has undergone a particularly cold and brutal winter, we have momentarily reverted to the cold and wet that winter is generally thought to be about. It’s 50 degrees out and pouring rain today, but the good news seems to be that the rain will end some time this morning and the temperature will begin its slow climb back up towards the low 70s by the weekend. We people are funny. We like to be warm, but don’t want to get overheated. We like to stay cool, but don’t want to get chilled. What exactly is it that we prefer in terms of body temperature? What’s up with that?
Your body constantly generates heat through metabolism – just keeping your cells running, your heart beating, your brain thinking, etc. To stay at 98.6°F, you need to dump that excess heat into your environment. This works beautifully when the air around you is cooler than your body temperature. But when the outside temperature equals your body temperature (or gets close), you lose your temperature gradient. Heat naturally flows from warmer to cooler, so when the air is 98.6°F, there’s nowhere for your metabolic heat to go efficiently. You’re still producing heat internally, but you can’t offload it effectively through radiation or convection. That’s why an outside temperature of 98.6°F feels hot to us even though that’s our normal metabolic body temperature. At that temperature your body tries to compensate by sweating more – evaporative cooling becomes your primary mechanism. But if humidity is high, even sweating becomes less effective because the moisture can’t evaporate efficiently from your skin. Plus, at 98.6°F ambient temperature with typical humidity, you’re often dealing with a heat index that feels even hotter. Your body is essentially in distress mode, working overtime to cool itself while producing heat it can’t effectively eliminate. That’s why 72°F feels comfortable – there’s a nice temperature difference allowing your body to passively dump its ~100 watts of metabolic heat production without much effort.
So much for staying cool, but let’s get back to warming up like on a cold wet morning like today. There are several good reasons why people often prefer cooler ambient temperatures and keep the option to add layers for comfort. There’s the issue of generally better temperature control. It’s much easier to warm up by adding a blanket or sweater than to cool down once you’re overheated. You can fine-tune your comfort level precisely by adjusting layers, but you can’t do much about being too hot beyond removing clothes (which has obvious limits). We certainly deploy that strategy when it comes to sleep quality. Our core body temperature naturally drops during sleep as part of our circadian rhythm. A cooler room (typically 65-68°F) supports this process and generally promotes deeper, more restful sleep. Being too warm disrupts this natural cooling and can fragment our sleep. Who among us does not understand why one second we feel snug under our blankets only to feel the urgent need to kick them off our legs?
Cooler temperatures tend to keep us more alert and focused. Warmth makes us drowsy (think of that post-lunch warm room feeling), while cooler air is generally more stimulating. Alertness and productivity seem like good reasons for staying cooler and then using our layers to cozy up. In shared spaces, it’s easier to accommodate different comfort levels and allow for personal preference variation by using this approach. Most people seem to find their sweet spot around 68-72°F with the ability to adjust via clothing or blankets as needed. and that causes us to make sure that our wardrobes have plenty of sweaters and long-sleeved shirts. For this morning, anticipating the cold and wet, I purposefully chose a grey heather long-sleeve t-shirt. It actually looked like it would be cozier on a cold and wet morning. Now that has to be some cultural bias driven by the thought of a morning on the heath in Scotland, right? It’s also why I tend to prefer Polartech vests. They seem to be the perfect blend of looking like they’ll be warm and comfortable and yet given their lack of sleeves, they seem to give me the freedom to stay unencumbered and cooler should I start to heat up.
We are all told to layer up to stay warm. Layering works so well because of how heat transfer actually happens. It’s all about trapping air. Air is an excellent insulator and still air has very low thermal conductivity. The problem is that a single thick garment allows air to circulate inside it through convection currents, which carries heat away from your body. Multiple layers create separate air pockets that can’t easily mix, drastically reducing convective heat loss. You create a sort of layer sandwich. Each fabric layer traps a thin boundary of warm air against it. With multiple layers, you create multiple insulating air gaps. Three thinner layers often outperform one thick layer of the same total thickness because you’ve got more discrete air pockets. It ain’t rocket science, but it works. You also gain adjustability. You can regulate your temperature precisely by removing or adding a layer as your activity level or environment changes. This prevents the sweat/chill cycle where you overheat, sweat, then get cold when the moisture evaporates. If only it was easy to carry around those layers that you need to disrobe.
Also, different layers actually do different jobs. A base layer wicks moisture away from skin (keeping you dry and thereby staying warm). A middle layer(s) provide the bulk of insulation through trapped air. And that outer layer blocks wind and precipitation as needed for the occasion. Keep in mind that wind protection matters a lot. Even great insulation fails if wind can penetrate and flush out those warm air pockets. This is why windbreakers are so effective despite being thin. They preserve the insulation of your other layers. It’s the same principle as double-pane windows or a thermos (who can forget the famous thermos joke…how do it know???) Let’s face it, I rapped air is one of nature’s best insulators, and layering is essentially building that system around your body. It’s really about minimizing conductive, convective, and evaporative heat losses simultaneously…now that’s a handful for a cold and wet February morning on the hilltop. Buddy is just sitting here next to me wondering what all the fuss is about. He has his fur and that’s that…he makes do with his own version of polartech while we carefully select our garments and layering strategy to achieve our cold comfort. Meanwhile, he just goes about his day, indoors or outdoors, using his surroundings and what nature has given him to take care of of his cold comfort.