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Apparent temperature
View on Wikipediafrom Wikipedia
 | This article needs additional citations for verification. (April 2020) |
| Apparent temperature | |
|---|---|
 Instrument face indicating the value of the heat index at the intersection of its two hands (indicating temperature and relative humidity), with a chart showing windchill according to the temperature (indicated) and wind speed (acquired by some other method) | |
| Dimension | Index calculated to be similar to a temperature |
Apparent temperature, also known as "feels like",[1][2] is the temperature equivalent perceived by humans, caused by the combined effects of air temperature, relative humidity and wind speed. The measure is most commonly applied to the perceived outdoor temperature. Apparent temperature was invented by Robert G. Steadman[3] who published a paper about it in 1984.[4] It also applies, however, to indoor temperatures, especially saunas, and when houses and workplaces are not sufficiently heated or cooled.
- The heat index and humidex measure the effect of humidity on the perception of temperatures above +27 °C (81 °F). In humid conditions, the air feels much hotter, because less perspiration evaporates from the skin.
- The wind chill factor measures the effect of wind speed on cooling of the human body below 10 °C (50 °F). As airflow increases over the skin, more heat will be removed. Standard models and conditions are used.
- The wet-bulb globe temperature (WBGT) combines the effects of radiation (typically sunlight), humidity, temperature and wind speed on the perception of temperature. It is not often used, since its measurement requires the use of a globe thermometer exposed to the sun, which is not included in standard meteorological equipment used in official weather conditions reporting (nor are, in most cases, any other explicit means of measuring solar radiation; temperature measurement takes place entirely in a shade box to avoid direct solar effects). It also does not have an explicit relationship with the perceived temperature a person feels; when used for practical purposes, the WBGT is linked to a category system to estimate the threat of heat-related illness.[5]
Since there is no direct measurement of solar radiation in U.S. observation systems, and solar radiation can add up to 15 °F (8.3 °C) to the apparent temperature, commercial weather companies have attempted to develop their own proprietary apparent temperature systems, including The Weather Company's "FeelsLike" and AccuWeather's "RealFeel". These systems, while their exact mechanisms are trade secrets, are believed to estimate the effect of solar radiation based on the available meteorological data that is reported (such as UV index and cloud cover).
Australian apparent temperature
[edit]The Australian apparent temperature (AT), invented in the late 1970s, was designed to measure thermal sensation in indoor conditions. It was extended in the early 1980s to include the effect of sun and wind. The AT index used here is based on a mathematical model of an adult, walking outdoors, in the shade (Steadman 1984). The AT is defined as the temperature, at the reference humidity level, producing the same amount of discomfort as that experienced under the current ambient temperature and humidity.[6]
The formula[7] is:
where:
- Ta is dry-bulb temperature (°C)
- e is water vapour pressure (hPa)
- v is wind speed (m/s) at an elevation of 10 m (33 ft)
The vapour pressure can be calculated from the temperature and relative humidity using the equation:
where:
- Ta is dry-bulb temperature (°C)
- RH is relative humidity (%)
- exp represents the exponential function
The Australian formula includes the important factor of humidity and is somewhat more involved than the simpler North American wind chill model. The North American formula was designed to be applied at low temperatures (as low as −46 °C or −50 °F) when humidity levels are also low. The hot-weather version of the AT (1984) is used by the National Weather Service in the United States. In the United States, this simple version of the AT is known as the heat index.
See also
[edit]References
[edit]- ^ Muroi, Millie; Layt, Stuart (14 July 2022). "In weather forecasts, what does 'feels like' temperature mean?". The Sydney Morning Herald. Retrieved 4 December 2022.
- ^ "BBC Weather - Help and FAQs - BBC Weather". BBC Online. 9 August 2022. Retrieved 4 December 2022.
- ^ "Online calculator: Apparent temperature". planetcalc.com. Retrieved 7 April 2020.
- ^ Steadman, Robert G. (December 1984). "A Universal Scale of Apparent Temperature". Journal of Climate and Applied Meteorology. 23 (12): 1674–1687. Bibcode:1984JApMe..23.1674S. doi:10.1175/1520-0450(1984)023<1674:ausoat>2.0.co;2. ISSN 0733-3021.
- ^ "Wet Bulb Globe Temperature is Great for Heat Warnings - Why Don't We Use It?". Forbes.
- ^ "The Apparent Temperature (AT) – Heat Index". Bureau of Meteorology, Australia. 5 February 2010. Retrieved 1 August 2018.
- ^ "The formula for the apparent temperature". Bureau of Meteorology, Australia. 5 February 2010. Retrieved 9 August 2013.
Apparent temperature
View on Grokipediafrom Grokipedia
Apparent temperature, also known as "feels like" temperature, is a meteorological index that estimates the temperature equivalent perceived by the human body, accounting for the combined effects of air temperature, relative humidity, wind speed, and in some models, solar radiation.[1] This perceived temperature reflects how hot or cold conditions feel to a person at rest in the shade, wearing light clothing, rather than the measured dry-bulb air temperature alone.[2]
The concept was developed by Australian meteorologist Robert G. Steadman, who introduced a universal scale in 1984 to quantify thermal discomfort from environmental factors, building on earlier work in human biometeorology.[1] Steadman's model calculates apparent temperature using empirical formulas that adjust for humidity's role in impeding sweat evaporation (increasing perceived heat) and wind's enhancement of convective cooling (lowering perceived cold).[2] In practice, national weather services adapt this framework: the Australian Bureau of Meteorology employs the Steadman apparent temperature for forecasts, while the U.S. National Weather Service uses a similar approach by integrating heat index above 27°C (80°F) and wind chill below 10°C (50°F), defaulting to ambient temperature in moderate conditions.[3][2]
Apparent temperature is critical for public health warnings, as it better predicts risks of heat-related illnesses like hyperthermia or cold-related issues like hypothermia compared to air temperature alone, influencing everything from outdoor activity advisories to urban planning for thermal comfort.[4] High apparent temperatures, often exceeding 35°C in humid tropics, signal elevated heat stress, while low values below -20°C indicate frostbite hazards, with trends showing increases, such as in the United States, due to climate change exacerbating humidity and temperature extremes.[5]
These categories inform NOAA's heat advisories and warnings, emphasizing precautions for vulnerable populations.[10]
These thresholds reflect physiological responses such as increased cardiovascular strain or hypothermia risk, derived from validation against human trials and heat balance simulations.[35] By accounting for radiant heat and wind effects more precisely than earlier indices, UTCI offers enhanced accuracy for global applications, though it assumes a standardized reference person (35-year-old, 1.75 m height, 75 kg mass).[35]
Definition and Basics
Core Concept
Apparent temperature refers to the temperature equivalent as perceived by the human body, which incorporates the effects of air temperature along with non-thermal environmental factors such as humidity and wind speed.[3] This measure aims to reflect the actual sensation of warmth or cold experienced by individuals, rather than the thermometer reading alone, by accounting for how these factors influence the body's heat exchange with its surroundings.[2] The concept of apparent temperature was first developed by Robert G. Steadman in 1979 through his work on assessing sultriness via a temperature-humidity index grounded in human physiology and clothing science.[6] Steadman formalized a universal scale for apparent temperature in 1984, expanding it to include variables like wind and solar radiation for broader applicability in meteorological contexts.[1] In everyday weather reporting, apparent temperature is commonly presented as the "feels-like" temperature to provide a more relatable indicator of comfort or discomfort for the public.[2] As a broad term, it encompasses various specific indices tailored to different conditions, such as the heat index for hot and humid environments or the wind chill index for cold and windy ones, each simplifying the underlying physiological interactions into a single equivalent temperature value.[3]Influencing Factors
The perceived temperature, or apparent temperature, is shaped by a combination of environmental and personal factors that interact with human thermoregulation to influence thermal comfort. Air temperature serves as the baseline, directly determining the potential for heat gain or loss from the body, with higher temperatures increasing the sensation of warmth by reducing the gradient for radiative and convective cooling.[7] Relative humidity affects this perception by altering the efficiency of sweat evaporation, a key cooling mechanism; elevated humidity impairs evaporation, trapping heat near the skin and elevating the felt temperature, particularly in warm conditions.[8] Wind speed enhances cooling through increased convection, which removes the warm air layer adjacent to the skin and accelerates heat dissipation, making environments feel cooler even at moderate temperatures.[7] Solar radiation contributes additional heat load, especially outdoors, by directly warming the body via short-wave absorption, which can raise perceived temperature by several degrees depending on exposure and orientation.[9] Personal factors like clothing insulation and activity levels modulate these effects; thicker clothing reduces heat loss in cold settings, while physical exertion generates internal heat that amplifies discomfort in hot environments.[9] At the physiological level, human thermoregulation maintains core body temperature around 37°C to support metabolic processes, relying on mechanisms such as sweating, vasodilation, and convection to balance heat production and loss.[7] Sweating dissipates heat through evaporation (releasing approximately 0.58 kcal per gram of water evaporated), but high humidity hinders this by saturating the air, leading to less effective cooling and a heightened sense of heat stress.[7] Vasodilation widens skin blood vessels to promote heat transfer to the surface for dissipation via radiation (about 60% of total loss) and convection (15%), while skin temperature—typically lower and more variable than core temperature—serves as a primary sensor for these adjustments through peripheral thermoreceptors.[7] Wind augments convection by forcing air movement over the skin, increasing the rate of heat loss proportional to wind speed, which is particularly pronounced in cooler conditions.[8] Individual variability further influences perception through metabolic rate and acclimatization. Higher metabolic rates, such as during exercise, elevate internal heat production, intensifying the apparent temperature by overwhelming cooling mechanisms.[7] Acclimatization to repeated environmental exposure enhances thermoregulatory efficiency, such as improved sweating onset and reduced physiological strain, thereby altering how factors like humidity and wind are perceived over time.[7]Heat-Related Indices
Heat Index
The Heat Index is a measure developed by the National Oceanic and Atmospheric Administration (NOAA) that combines air temperature and relative humidity to estimate the apparent temperature felt by the human body in hot, humid conditions, specifically for air temperatures above 27°C (81°F).[10] It accounts for how humidity impairs the body's ability to cool itself through sweat evaporation, providing a more accurate indication of heat stress than air temperature alone under shaded, light-wind conditions.[10] Full sunlight can increase the effective Heat Index by up to 15°F (8°C).[11] The index originated from research by Robert G. Steadman, who in 1979 published a physiological model for assessing sultriness based on human thermoregulation, clothing, and environmental factors, which the U.S. National Weather Service adopted and adapted for public use.[6] Steadman's work focused on temperatures and humidity to derive an apparent temperature table, forming the foundation for the NOAA's operational Heat Index.[12] The current regression-based formula was refined in 1990 by Lance P. Rothfusz using multiple regression on Steadman's model data.[13] The Heat Index (HI) is calculated using the following equation in degrees Fahrenheit: where is the air temperature in °F and RH is the relative humidity in percent; adjustments apply for very low or high humidity extremes.[13] To use Celsius values, convert to Fahrenheit via , compute HI in °F, then convert back if needed ().[13] This formula is valid for HI values of 80°F (27°C) or higher and assumes a standard human physiology model.[13] Heat Index values are classified into risk categories to guide public health responses, with escalating dangers from heat-related illnesses:| Classification | Heat Index (°F) | Likely Effects on High-Risk Groups |
|---|---|---|
| Caution | 80–90 | Fatigue possible with prolonged exposure and/or physical activity |
| Extreme Caution | 91–103 | Heat cramps or heat exhaustion possible with prolonged exposure and/or physical activity |
| Danger | 104–124 | Heat cramps or heat exhaustion likely, heat stroke possible with prolonged exposure and/or physical activity |
| Extreme Danger | 125+ | Heat stroke highly likely |
Limitations and Adjustments
The Heat Index assumes shaded conditions and light winds (below 5 mph or 8 km/h), focusing on relative humidity's impact on evaporative cooling for individuals at rest. It does not account for direct sunlight, which can increase values by up to 15°F (8°C), or strong winds, which may exacerbate heat stress in very hot, dry air by enhancing convective heat gain despite potential evaporative benefits.[14][15] These assumptions make it less accurate for direct sun exposure, high-activity scenarios like outdoor labor, or non-acclimatized individuals, where metrics like Wet Bulb Globe Temperature (WBGT) are recommended for occupational safety.[16] A key limitation is its scope: the formula applies only above 80°F (27°C), below which the Heat Index approximates air temperature regardless of humidity; it also includes built-in adjustments for extreme relative humidity (below 13% or above 85%) to prevent unrealistic outputs.[13] The index overlooks factors like clothing insulation, metabolic rate from physical exertion, and personal acclimatization, potentially underestimating risks for vulnerable groups such as the elderly or children.[17] In low-humidity environments, it may not fully capture heat impacts, as dry air facilitates evaporation but still poses dehydration risks.[17] Criticisms include occasional underestimation of extreme heat events, as noted in 2025 studies evaluating its performance during intense humidity spikes, though the core model remains unchanged since 1990 with no major updates as of November 2025.[13] To address these, the National Weather Service advises combining Heat Index with alerts for sun exposure and activity levels, and some research proposes enhanced indices incorporating radiation for better climate change adaptation.[15]Cold-Related Indices
Wind Chill Index
The Wind Chill Index is a standardized measure developed jointly by the United States and Canada to assess the apparent temperature in cold, windy conditions, specifically for air temperatures at or below 10°C (50°F) with wind speeds above 4.8 km/h (3 mph), where wind accelerates convective heat loss from exposed human skin, making the environment feel significantly colder than the actual air temperature.[18][19] The index is calculated using a formula derived from heat transfer models applied to the human face, the most exposed area during cold weather activities. In imperial units, the wind chill temperature (°F) is given by: where is the air temperature in °F and is the wind speed in mph (valid for mph).[19] The equivalent metric formula for (°C) is: where is the air temperature in °C and is the wind speed in km/h (valid for km/h).[20] These equations provide a single equivalent temperature that bare skin would experience under calm conditions, aiding in public safety assessments.[21] The modern Wind Chill Index was developed in 2001 by the Joint Action Group for Temperature Indices (JAG/TI), a collaboration involving the National Weather Service, Environment Canada, and experts from Indiana University-Purdue University Indianapolis and Defence Research and Development Canada, as an update to the original 1945 model created by Paul Siple and Charles Passel during U.S. Army Antarctic expeditions.[19] This revision incorporated biometeorological research, computer modeling of facial heat loss, and validation through human subject trials conducted in 2001, replacing the older empirical formula to improve accuracy and consistency across North American weather services, with implementation starting November 1, 2001.[21][19] The index categorizes risks based on frostbite exposure times for unprotected skin, which decrease as wind chill values drop. For example, at an air temperature of -5°F with 10 mph winds (yielding a wind chill of approximately -22°F), frostbite can develop in about 30 minutes; similarly, at 0°F with 15 mph winds (wind chill of -19°F), the risk materializes in 30 minutes.[22] These thresholds guide weather warnings, such as Cold Weather Advisories issued when wind chill or air temperatures reach or fall below location-specific thresholds (typically -20°F/-29°C or lower, varying by region) for several hours. As of October 2024, the NWS updated its alert system, renaming Wind Chill Advisories to Cold Weather Advisories to better encompass both temperature and wind chill effects.[23] This emphasizes the need for protective clothing to mitigate rapid cooling.[24]Limitations and Adjustments
The wind chill index assumes a standard walking speed of 3 mph (approximately 5 km/h) to represent typical human movement, which incorporates this velocity into the "calm" wind threshold of 5 km/h; however, this assumption leads to inaccuracies for stationary individuals or scenarios with wind speeds below this level, where the index overestimates the cooling effect.[19] In still air conditions, the index's formulation, which relies on convective heat loss models, fails to accurately reflect actual heat transfer from the body, as it does not adequately account for the absence of forced convection.[25] Additionally, the index largely disregards humidity's influence, rendering it less precise in high-humidity cold environments where evaporative cooling from moisture on the skin could exacerbate perceived chill, though such effects are minimal in typical dry winter conditions.[19] A primary criticism of the wind chill index is its focus on facial cooling—particularly the cheeks, nose, and ears as the most exposed areas—rather than whole-body heat loss, leading to an overestimation of cooling for clothed individuals where only limited skin is bare.[25] This localized model, derived from heat transfer principles applied to human subjects, does not incorporate broader physiological responses such as metabolic heat production, vasoconstriction, or clothing insulation, which vary significantly across individuals and activities.[19] To address these limitations, adjustments have been proposed for stationary people, such as the Adjusted Wind Chill Equivalent Temperature (AWCET), which reduces the wind's cooling impact to about 28% of the standard value to better suit urban or low-mobility contexts, validated through mortality risk analyses in subtropical regions.[26] For wet-cold scenarios involving high humidity, while the core wind chill index does not integrate moisture directly, extensions like combined thermal indices incorporate humidity to model enhanced evaporative losses, providing a more comprehensive assessment of cold stress.[25] Validation studies, including human trials conducted in 2001 by the Defense Research and Development Canada (DRDC) with 12 volunteers, confirmed the need for revisions by measuring skin temperatures and heat flux under controlled conditions, revealing variations in individual thermal resistance.[19] These findings led to the 2001 update by the National Weather Service (NWS) and Environment Canada, which redefined wind speed measurements at face height (10 meters adjusted by a two-thirds factor), adopted a 38°C core body temperature, and implemented a new formula that reduced wind chill values by approximately 20% compared to prior models, making the "feels like" temperatures less severe while improving accuracy for frostbite risk thresholds.[19]Comprehensive and Regional Indices
Steadman Apparent Temperature
The Steadman Apparent Temperature index, developed by Robert G. Steadman, provides a comprehensive measure of thermal sensation by equating current environmental conditions to an equivalent air temperature experienced by a clothed human under standard reference conditions of shade, light wind, and moderate humidity. Introduced in 1979 to assess sultriness in warm and hot climates, it was expanded in 1984 into a universal scale applicable across the full temperature range, from cold to hot extremes. This index represents the temperature at reference conditions (typically 50% relative humidity, 2.5 m/s wind speed, and no extra radiation) that would require the same total thermal insulation for human heat balance as the actual conditions.[6][27][1] The index is derived from a physiological model of human heat transfer, focusing on the equilibrium temperature of a clothed body to maintain thermal comfort, incorporating convective, radiative, evaporative, and conductive heat losses. It accounts for air temperature (Ta), humidity via vapor pressure (e), wind speed (v), and solar or extra radiation, with calculations often involving iterative solutions to heat balance equations or precomputed tables for practical use. For instance, in hot conditions, high vapor pressure elevates the apparent temperature by impairing sweat evaporation, while in cold conditions, increased wind speed lowers it by enhancing convective cooling; a representative approximation simplifies to AT ≈ Ta + 0.348e (in appropriate units) adjusted for wind and radiation effects, though the full model uses detailed charts for accuracy across variables. This balanced approach distinguishes it as a foundational metric that has influenced subsequent thermal indices.[6][27][1] By covering both heat stress and cold stress scenarios, the Steadman index offers a versatile tool for evaluating human thermal environments beyond narrow temperature bands, serving as the basis for many derivative models in meteorology and ergonomics.[1]Australian Apparent Temperature
The Australian Apparent Temperature (AT) is an operational index adopted by the Australian Bureau of Meteorology in the 1990s as an adaptation of R.G. Steadman's thermal comfort model, designed to quantify human-perceived temperature across all seasons by accounting for the combined effects of air temperature, relative humidity, and wind speed.[28] This index provides a simplified measure of thermal sensation for weather forecasting and public advisories in Australia's diverse climates, emphasizing practicality over complex biometeorological simulations.[29] The formula used by the Bureau of Meteorology for AT is AT = Ta + 0.33 × e - 0.7 × v - 4.00 (°C), where Ta is the dry-bulb air temperature in °C, e is the water vapor pressure in hPa calculated as e = (RH/100) × 6.105 × exp((17.27 × Ta)/(237.7 + Ta)), RH is relative humidity in percent, and v is wind speed in m/s measured at a standard 10 m height.[30] This equation approximates the heat balance on a human body under moderate metabolic activity and clothing, excluding solar radiation in its base calculation to focus on shaded conditions.[31] Apparent temperature values are categorized to guide comfort and risk assessments: the comfort zone spans 0 to 27.8°C, where minimal thermal stress is experienced by a clothed adult at rest; values between 27.8 and 39°C indicate low to moderate risk of discomfort or fatigue during prolonged exposure; and temperatures exceeding 39°C signal high risk of heat stress, potentially leading to health impacts like dehydration or heat exhaustion.[32] Adjustments account for environmental variations: indoor AT omits wind effects (setting v = 0) to reflect sheltered conditions without air movement, resulting in higher values in humid interiors; outdoor applications include wind cooling; and direct solar exposure adds approximately 5 to 8°C under Australian midday conditions, depending on sun elevation and surface reflectivity, to estimate full-sun thermal load.[2][33]Universal Thermal Climate Index
The Universal Thermal Climate Index (UTCI) is a comprehensive thermal index developed in 2009 through an international collaboration led by the European COST Action 730, involving experts in human thermophysiology, biometeorology, and clothing physiology to evaluate the full spectrum of outdoor thermal stress on humans via advanced physiological simulation.[34] Unlike simpler empirical formulas, UTCI employs a thermo-physiological model based on the advanced Fiala multi-node heat balance model of the human body, which simulates dynamic physiological responses including core temperature, skin wettedness, and thermal sensation under varying environmental conditions.[35] This approach ensures applicability across all climates and seasons, providing a standardized assessment independent of individual acclimatization but adaptable to behavioral factors. Calculation of UTCI requires multiple meteorological inputs: air temperature, mean radiant temperature, relative humidity or vapor pressure, and wind speed at reference height (typically 10 m, adjusted to body level).[35] The index is defined as the air temperature in a reference environment (40% relative humidity, no wind, mean radiant temperature equal to air temperature) that would elicit the same physiological strain as the actual conditions, computed through iterative simulation rather than a direct equation.[35] Operational implementation relies on specialized software, such as the UTCI calculator or BioKlima tool, which incorporates polynomial approximations for efficiency while maintaining fidelity to the full model.[34] The model also integrates an adaptive clothing insulation function that dynamically adjusts based on environmental demands (ranging from 0.6 to 1.7 clo) and allows for metabolic rate variations (e.g., 100 W/m² for walking), enabling customization for different activity levels without altering the core index.[35] UTCI categorizes thermal stress into nine levels based on the equivalent temperature output, as follows:| Category | Temperature Range (°C) |
|---|---|
| Extreme heat stress | > 46 |
| Strong heat stress | 38 to 46 |
| Moderate heat stress | 32 to 38 |
| Slight heat stress | 26 to 32 |
| No thermal stress | 9 to 26 |
| Slight cold stress | 0 to 9 |
| Moderate cold stress | -13 to 0 |
| Strong cold stress | -27 to -13 |
| Extreme cold stress | < -27 |