What Clothes Should I Pack for Under 40°F Weather?

Everything above 40°F is a comfort problem. Below it, the failure mode changes: get it wrong and you're not just uncomfortable, you're wet, and wet is what turns cold into a genuine problem. The single most useful thing to understand about this range is that moisture, not temperature, is what you're managing.

Quick Answer

Why Overdressing Backfires

The intuitive approach to cold is more insulation. Below 40°F that's only half right, because insulation works by trapping still, dry air, and sweat destroys that.

NWS cold weather guidance states it directly: avoid overheating, because when you sweat you lose heat faster, and water, sweat or dampness will pull heat from your body. That's the mechanism. A base layer soaked with your own sweat conducts heat away from your skin far more efficiently than dry fabric, so the person who dressed too warmly for a brisk walk ends up colder at rest than the person who started slightly chilly.

The practical version: dress so you're a little cold for the first five minutes. If you're comfortable standing still before you set off, you're overdressed for moving.

This is also why the layering system exists in the form it does. Three layers with distinct jobs let you shed the middle one when you warm up, rather than committing to one thickness for the whole day.

The Three Layers

Base layer. Against the skin, and its job is moving moisture away rather than providing warmth. Merino wool or synthetic. This is the layer where cotton does the most damage, because it absorbs sweat and holds it against you.

Mid layer. The insulation. Fleece, down, or a wool sweater. This is the piece you remove when you warm up and add back when you stop, so it should be easy to get on and off over the base layer.

Outer shell. Wind and precipitation. Wind resistance matters more than thickness here, because moving air strips heat from the whole system regardless of how much insulation sits underneath.

Bottoms follow the same logic: a thermal base layer under trousers on the coldest days, which most people skip and then regret.

Extremities, and a Correction

Below 40°F your body prioritises your core, reducing blood flow to hands, feet, ears and nose. That's why those are where you feel the cold first and where frostbite starts.

Mittens over gloves. NWS recommends mittens, snug at the wrist, because fingers grouped together share warmth rather than each being separately insulated.

A hat, but not for the reason you've been told. NOAA lists "the body loses most of its heat through the head" among cold weather myths, explaining that the head is about 7% of total body surface area and loses heat at roughly the same rate as any other 7% of exposed skin. An older NWS brochure still in circulation repeats the 40% figure, which is presumably where a lot of the confusion originates. The practical advice doesn't change: cover your head, because it's usually the part you've left bare. The reasoning does.

Warm socks, wool or synthetic, and enough of them to keep feet dry. Damp feet are the most common way an otherwise well-dressed person ends up genuinely cold.

What Wind Chill Actually Tells You

Wind chill is the rate of heat loss from exposed skin caused by wind and cold, and it's a real effect: at 0°F with a 15 mph wind, the wind chill is -19°F and exposed skin can freeze in about 30 minutes.

Two things it doesn't tell you, both from NWS directly.

It can't cause frostbite above freezing. You can only get frostbite if the actual air temperature near your skin is below freezing. Above that, wind makes you feel much colder and can lead to hypothermia, but tissue won't freeze.

The number alone isn't a risk assessment. NOAA gives the example that different temperature and wind combinations can produce the same wind chill value while carrying different frostbite times, because wind's ability to remove heat varies. Use it as guidance rather than a threshold.

Two Things That Make It Worse

NWS also notes some non-obvious factors. Caffeine constricts blood vessels and works against warming your extremities. Alcohol reduces shivering, which is one of your body's warming mechanisms. And hydration matters more than people expect, since it increases blood volume, which helps protect against frostbite.

Dirty clothing also insulates worse than clean, because accumulated grime clogs the air pockets that do the work.

For the band immediately above this one, where the temperature reading becomes trustworthy again, see what to pack for 50-60°F weather. For how the layers fit together when you can't pack a piece for every job, see when layers have to do all the work.