Clothing warmth is tested at three levels. Fabrics are measured on sweating guarded hotplates (ISO 11092) for thermal and vapor resistance. Complete outfits are measured on heated thermal manikins (ASTM F1291, ISO 15831), with results in clo. Heat-reflective surfaces are rated for emittance with instruments such as portable emissometers (ASTM C1371). Each test answers a different question.

The clothing microclimate is the thin layer of air trapped between your skin and your clothing. Its temperature, humidity and thickness largely decide how warm and dry you feel. Comfortable conditions are often described as about 32°C and 50% relative humidity. Still air in gaps of up to about a centimeter insulates well; wind, movement and moisture disrupt it.

A radiant barrier in clothing is a thin, low-emissivity metallic layer that reflects infrared heat radiated by your body back toward you and radiates little heat outward itself. It works only when it faces an air space, and it addresses radiant heat loss, not conduction, wind or evaporation. It matters most in still, cold conditions and in low-bulk layers.

Wind chill is the temperature it feels like on exposed skin when wind and cold combine. Wind strips away the thin layer of warm air around your skin, so skin cools faster than it would in still air. The U.S. National Weather Service index, in use since November 2001, models heat loss from a face at walking pace and estimates how quickly frostbite can occur.

Normal adult body temperature is a range, roughly 97 to 99°F (about 36.1 to 37.2°C), not a single number. It varies by person, time of day, age and where it is measured. The hypothalamus keeps core temperature in that range by adjusting skin blood flow, sweating and shivering. Below 95°F (35°C) is hypothermia, and above 100.4°F (38°C) is generally considered a fever.

Hands and feet get cold first because, when the body senses cold, it narrows blood vessels in the skin of the extremities to keep warm blood near the vital organs. Fingers and toes also have a large surface area for their small volume and sit far from the core, so they lose heat quickly once warm blood flow drops.

Conductive heat loss is the transfer of body heat directly into a colder object you touch, such as a metal tool, cold ground, a wet shirt or water. Its rate depends on the temperature difference, contact area and the material's thermal conductivity. Water conducts heat roughly 25 times better than air, and metals conduct far more, which is why they drain warmth so quickly.

A clo is the standard unit of clothing insulation. One clo equals a thermal resistance of 0.155 square meter kelvin per watt, roughly the insulation of a typical business suit, and a nude body is 0 clo. Higher clo means more resistance to dry heat loss. Values are measured on heated manikins and are used in comfort standards such as ASHRAE 55.

Emissivity is a number from 0 to 1 that describes how efficiently a surface gives off thermal (infrared) radiation compared with a perfect emitter at the same temperature. For opaque materials, emissivity and reflectance add up to 1, so a surface with low emissivity reflects most incoming infrared. Human skin sits near 0.98, while bright aluminum can be as low as 0.02 to 0.03.

The body stays warm by balancing heat production against heat loss. Metabolism generates heat continuously, and the hypothalamus defends a core temperature of about 37°C by narrowing skin blood vessels and, if needed, triggering shivering. Clothing helps by trapping still air and slowing the four routes of heat loss: radiation, conduction, convection and evaporation. Wind, water and cold surfaces speed those losses.