Thermal comfort glossary: warmth, cold and clothing terms defined
Thermal comfort is the condition of mind that expresses satisfaction with the thermal environment, as ASHRAE Standard 55 defines it. This glossary defines 80 terms across body heat, heat transfer, clothing, weather indexes, standards and clinical warming. The two core units: 1 clo of clothing insulation equals 0.155 m²·°C/W, and 1 met of metabolic heat equals 58.2 W/m².
How to use this glossary
Definitions are written for plain reading, and where a term is set by a standard, the number comes from that standard or the agency that publishes it. Units are shown in the form the source uses.
Clinical and injury terms are here to explain vocabulary, not to diagnose. If you have symptoms such as numbness, color changes in the skin or unusual cold sensitivity, talk to a clinician.
Thermal comfort terms A to C
Terms from acclimatization to core temperature, including the core physiology of cold and the main clothing and comfort units.
| Term | Definition |
|---|---|
| Absorptance | The fraction of incident radiation a surface absorbs rather than reflects or transmits. For an opaque surface it equals 1 minus reflectance. |
| Acclimatization | Gradual physiological adjustment to repeated heat or cold exposure over days to weeks, which changes how strongly and how early the body responds. |
| ACGIH TLV | Threshold Limit Values published by the American Conference of Governmental Industrial Hygienists, guideline exposure limits that include cold and heat stress. For cold, ACGIH suggests a work-warming regimen when continuous work occurs at a wind chill of -7°C (19.4°F) or below. |
| Aerogel | An ultralight solid made mostly of air held in a nanoscale porous network, giving very low thermal conductivity. In clothing it is used as thin insulating sheets or coatings where bulk must stay low. |
| Afterdrop | A continued fall in core temperature after a cold person is removed from the cold, as cooled blood from the limbs returns to the core. |
| Air-activated warmer | A disposable heat pack in which iron powder oxidizes when exposed to air, releasing heat for several hours. Salt, activated carbon and water-holding fillers control the reaction rate. |
| Ambient temperature | The temperature of the surrounding air, measured in the shade and away from heat sources. |
| ANSI/ISEA 107 | The American national standard for high-visibility safety apparel. It sets requirements for fluorescent background material, retroreflective tape and garment designs by type and performance class. |
| ASHRAE 55 | ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy, which defines indoor conditions most occupants find acceptable. It combines air temperature, radiant temperature, humidity, air speed, clothing and activity. |
| Base layer | The garment worn directly against the skin, chosen mainly to move moisture away from the skin and add a thin layer of insulation. |
| Brown adipose tissue | A type of fat that burns energy to produce heat directly, the main source of non-shivering thermogenesis. |
| Carbon fiber heating element | A flexible heater made of conductive carbon fibers or carbon-coated fabric that warms when current flows through it. It is one of the most common elements in battery-heated garments. |
| Chilblains (pernio) | Itchy, painful red or purple skin patches from repeated exposure to cold that is not freezing. NIOSH describes exposure between freezing and 60°F damaging small blood vessels in the skin of the cheeks, ears, fingers and toes. |
| Clo | A unit of clothing insulation. ASHRAE 55 defines 1 clo as 0.155 m²·°C/W, close to trousers, a long-sleeve shirt and a suit jacket (0.96 clo). |
| Clothing microclimate | The thin layer of air between skin and clothing, whose temperature and humidity largely decide how warm and dry a person feels. |
| Cold shock | The involuntary gasp, rapid breathing and surge in heart rate in the first moments of sudden immersion in cold water. It can cause inhalation of water before hypothermia has time to develop. |
| Cold stress | Conditions in which the body loses heat faster than it can produce it, raising the risk of hypothermia, frostbite and other cold injury. |
| Cold-induced vasodilation | A periodic widening of blood vessels in cold fingers and toes that briefly rewarms them, also called the hunting response. |
| Conduction | Heat transfer through direct contact, such as skin against a cold seat, a metal tool or wet clothing. |
| Convection | Heat transfer by moving air or water carrying heat away from a surface. Wind and water currents increase it sharply. |
| Core temperature | The temperature of the deep tissues and organs, which the body holds within a narrow range. It is measured at sites such as the esophagus, bladder, rectum or pulmonary artery. |
Thermal comfort terms D to H
Terms from down fill power to hypothermia, including heat loss routes, forced-air warming and cold injuries.
| Term | Definition |
|---|---|
| Down fill power | The volume one ounce of down occupies under a standard test. Higher fill power means more loft, and more warmth, per unit weight. |
| DWR (durable water repellent) | A finish on outer fabrics that makes water bead and roll off. It wears with use and washing and can be renewed. |
| Emissivity | How efficiently a surface emits thermal radiation compared with a perfect emitter, on a scale of 0 to 1. Skin and most fabrics are high; polished metal is low. |
| Evaporation | Heat loss as water, mainly sweat, turns to vapor on the skin. It is the body's main way of shedding heat in hot conditions. |
| Evaporative resistance | How strongly a fabric or garment resists the passage of water vapor. Lower values mean a more breathable garment. |
| Forced-air warming | A patient-warming method that blows heated air through a disposable blanket over or under the patient. NICE CG65 recommends it for adults having anesthesia for longer than 30 minutes. |
| Frostbite | Freezing of body tissue, most often of the nose, ears, cheeks, chin, fingers or toes. Severe cases can permanently damage tissue. |
| Frostnip | The mildest cold injury, in which skin turns pale and numb without deep tissue freezing. It resolves with rewarming but signals frostbite risk. |
| Heat flux | The rate of heat flow through a surface, expressed in watts per square meter. |
| Heat index | The National Weather Service apparent temperature, combining air temperature and relative humidity to describe how hot it feels. It is calculated for shade. |
| Heat stress | The total heat load on the body from environment, clothing and work, which can lead to heat exhaustion or heat stroke. |
| Heated garment | Clothing with built-in electric heating elements, usually powered by a rechargeable lithium-ion battery with selectable heat settings. |
| Hypothermia | A drop in core temperature to an abnormally low level because heat loss outpaces heat production. It impairs thinking and movement, and can become life-threatening. |
| Hypothermia stages | Mild, moderate and severe hypothermia, graded by core temperature or by signs. Shivering is typical early and can stop as it worsens, while confusion and drowsiness increase. |
Thermal comfort terms I to O
Terms from insulation to operative temperature, including the ISO and ASHRAE indexes used to rate cold and comfort.
| Term | Definition |
|---|---|
| Insulation | Material that slows heat flow, mostly by trapping still air in fibers, down or foam. |
| IREQ | Required clothing insulation, the index in ISO 11079. It estimates how much clothing insulation keeps the body in heat balance at a defined level of strain for a given cold environment and activity. |
| ISO 11079 | The international standard for assessing cold stress using required clothing insulation (IREQ) and local cooling effects such as wind chill. |
| Latent heat | Heat absorbed or released when a material changes phase, such as sweat evaporating or a phase change material melting or freezing. |
| Loft | The thickness and springiness of an insulating layer. More loft traps more still air and usually means more warmth. |
| Mean radiant temperature | The uniform surface temperature of an imaginary enclosure that would exchange the same radiant heat with a person as the actual surroundings. |
| Met | A unit of metabolic rate. ASHRAE 55 defines 1 met as 58.2 W/m² of skin surface, the heat produced by an average person seated at rest. |
| Metabolic heat | Heat produced by the body's chemical processes, rising with muscular work and shivering. |
| Non-freezing cold injury | Damage to nerves and blood vessels, usually in the feet or hands, from prolonged cold and wet exposure above freezing. Trench foot is the classic form. |
| Non-shivering thermogenesis | Heat production without muscle contraction, mainly by brown adipose tissue. |
| Operative temperature | A single temperature in ASHRAE 55 that combines air temperature and mean radiant temperature to represent their joint effect on a person. |
Thermal comfort terms P to R
Terms from perioperative hypothermia to rewarming, including comfort indexes, heater types and radiant heat.
| Term | Definition |
|---|---|
| Perioperative hypothermia | Unintended low core temperature around surgery. NICE CG65 defines inadvertent perioperative hypothermia as a core temperature below 36.0°C. |
| Phase change material | A substance that absorbs heat as it melts and releases it as it solidifies, buffering temperature near its transition point. |
| PMV (predicted mean vote) | An ASHRAE 55 index that predicts the average thermal sensation of a group on a scale from -3 (cold) to +3 (hot), with 0 as neutral. |
| Poikilothermia | Inability to hold core temperature independent of the surroundings, so body temperature drifts toward ambient. It occurs after some high spinal cord injuries. |
| PPD (predicted percentage dissatisfied) | An ASHRAE 55 index giving the share of people expected to be dissatisfied, derived from PMV. A PPD below 10% corresponds to a PMV between -0.5 and +0.5. |
| Prewarming | Actively warming a patient's skin before anesthesia to reduce heat loss after induction. NICE CG65 recommends starting active warming at least 30 minutes before induction where possible. |
| PTC heater | A positive temperature coefficient heater, whose electrical resistance rises as it warms, so its power falls and it limits its own temperature. |
| R-value | A measure of thermal resistance used for building insulation and sleeping pads. Higher values resist heat flow more. |
| Radiant barrier | A low-emissivity, high-reflectance layer that reduces radiant heat transfer across an air gap. It works only when facing an air space. |
| Radiation (thermal) | Heat transfer by infrared waves between surfaces at different temperatures, requiring no contact or air. |
| Raynaud's phenomenon | Episodes in which small arteries of the fingers or toes narrow excessively in response to cold or stress, turning skin white or blue, then red on rewarming. It is called primary when no underlying disease is found and secondary when one is. |
| Redistribution hypothermia | The drop in core temperature after anesthesia induction, as widened blood vessels let core heat flow to cooler peripheral tissues. |
| Reflectance | The fraction of incident radiation a surface reflects. For an opaque surface, reflectance plus absorptance equals 1, and absorptance equals emissivity at the same wavelength. |
| Relative humidity | The amount of water vapor in air as a percentage of the maximum it could hold at that temperature. |
| Resultant clothing insulation | Clothing insulation corrected for wind and body movement, always lower than the static value measured on a still manikin. |
| Rewarming | Restoring body temperature after cold exposure. Passive rewarming relies on the body's own heat under insulation; active rewarming adds external or internal heat. |
Thermal comfort terms S to Z
Terms from shell layer to windproof, including the weather indexes used for cold and heat warnings.
| Term | Definition |
|---|---|
| Shell layer | The outermost garment, designed to block wind, rain and snow while letting some water vapor escape. |
| Shivering thermogenesis | Heat produced by involuntary, rhythmic muscle contractions when the body is cold. |
| Sweating guarded hotplate | A laboratory instrument that measures the thermal and evaporative resistance of fabrics under controlled conditions. |
| Thermal comfort | The condition of mind that expresses satisfaction with the thermal environment, the definition used in ASHRAE 55. |
| Thermal conductivity | How readily a material conducts heat, in watts per meter kelvin. Lower values mean better insulators; still air is among the lowest. |
| Thermal manikin | A heated, sensor-covered human-shaped model used to measure the insulation of whole garments and ensembles. |
| Thermoneutral zone | The range of surrounding temperatures in which the body holds core temperature through skin blood flow alone, without shivering or sweating. |
| Thermoregulation | The body's control of core temperature, led by the hypothalamus through blood flow, sweating, shivering and behavior. |
| Tog | A unit of thermal resistance used mainly for duvets and bedding. Higher tog means warmer. |
| Trench foot | A non-freezing cold injury from prolonged wet and cold. NIOSH notes it can occur at temperatures as high as 60°F if feet are constantly wet, since wet feet lose heat 25 times faster than dry feet. |
| Vapor barrier | A layer that blocks water vapor, keeping insulation dry from sweat but trapping moisture against the skin. |
| Vasoconstriction | Narrowing of blood vessels, especially in skin, hands and feet, which reduces heat loss and keeps warmth in the core. |
| Vasodilation | Widening of blood vessels in the skin, which increases heat loss to the surroundings. |
| Watt-hour (Wh) | A unit of energy. For a battery, volts multiplied by amp-hours gives watt-hours, and dividing by a heater's watts gives an approximate runtime in hours. |
| WBGT (wet bulb globe temperature) | A heat stress measure for work or activity in direct sun that accounts for temperature, humidity, wind speed, sun angle and cloud cover. The military, OSHA and athletic programs use it. |
| Wicking | Movement of liquid sweat along and through fibers by capillary action, spreading it so it can evaporate. |
| Wind chill | The air temperature that would feel the same on exposed skin as the actual combination of air temperature and wind speed. It applies to exposed skin, not to objects or clothed body areas. |
| Windproof | Describes fabric or membranes that block air from passing through, preserving the still air in insulation beneath. |
Key takeaways
Clo measures clothing insulation (1 clo = 0.155 m²·°C/W) and met measures activity (1 met = 58.2 W/m²).
Heat leaves the body by conduction, convection, radiation and evaporation; most warmth strategies target one of these routes.
ASHRAE 55 uses a PPD below 10%, a PMV between -0.5 and +0.5, as its comfort target.
Wind chill and the heat index describe how conditions feel; WBGT adds sun and wind for heat stress in direct sun.
NICE CG65 defines inadvertent perioperative hypothermia as a core temperature below 36.0°C.
Frequently asked questions
What is the definition of thermal comfort?
ASHRAE Standard 55 defines thermal comfort as the condition of mind that expresses satisfaction with the thermal environment. It depends on air temperature, radiant temperature, humidity, air speed, clothing and activity, which is why two people in the same room can disagree.
What is the difference between clo and met?
Clo measures how well clothing resists heat loss, and met measures how much heat the body produces. ASHRAE 55 sets 1 clo at 0.155 m²·°C/W and 1 met at 58.2 W/m², the output of a person seated at rest.
What do PMV and PPD mean?
PMV, the predicted mean vote, estimates average thermal sensation on a scale from -3 (cold) to +3 (hot). PPD, the predicted percentage dissatisfied, converts that into the share of people likely to be uncomfortable; ASHRAE 55 targets a PPD below 10%.
What is the difference between wind chill and the heat index?
Wind chill describes how cold exposed skin feels when wind adds to low air temperature. The heat index describes how hot it feels when humidity slows sweat evaporation, and the National Weather Service calculates it for shade.
Is trench foot the same as frostbite?
No. Frostbite is freezing of tissue, while trench foot is a non-freezing cold injury from prolonged cold and wet conditions. NIOSH notes trench foot can occur at temperatures as high as 60°F if the feet stay wet.
Related reading
Sources
ANSI/ASHRAE Standard 55-2013, Thermal Environmental Conditions for Human Occupancy, ASHRAE
Cold-related Illnesses in Workers, CDC, National Institute for Occupational Safety and Health (NIOSH)
Cold Environments: Control Measures (working in the cold, ACGIH guidance and wind chill), Canadian Centre for Occupational Health and Safety
Wet Bulb Globe Temperature (WBGT) and the heat index, National Weather Service, La Crosse, WI
Hypothermia: prevention and management in adults having surgery (NICE clinical guideline CG65), National Institute for Health and Care Excellence, via NCBI Bookshelf
ISO 11079:2007 Ergonomics of the thermal environment, Determination and interpretation of cold stress when using required clothing insulation (IREQ) and local cooling effects, International Organization for Standardization
About HEATJAC. HEATJAC is a thermal architecture company founded by an anesthesiologist. We design patented garment systems that capture, conduct and broaden warmth across the body, and we publish this knowledge hub because the science of staying warm should be public. HEATJAC products are not medical devices and are not intended to diagnose, treat, cure or prevent any disease.