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General
Several ECMWF thermal parameters have become operational with model cycle 49r1. They are measures of environmental heat and cold as they affect humans. All are based on a mathematical model of human body heat balance for an adult, walking outdoors, in the shade. Each can be considered as an adjustment to the ambient temperature based on the level of humidity, and in some cases, the also the strength of the wind.
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ECMWF mean radiant temperature is an ancillary variable from which the globe temperature is derived.
Apparent Temperature (AT)
Apparent Temperature is defined as the temperature (in °C) giving the same discomfort as under the current ambient temperature and humidity.
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Example plot:
Heat Index (HI)
Heat Index is defined as the temperature (in °F) the human body perceives in shady conditions when perspiration is limited due to increased relative humidity.
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Example plot:
Humidex
Humidex is defined as the temperature (in °C) the human body perceives in hot, humid weather.
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Example plot:
Normal Effective Temperature (NET)
Normal Effective Temperature (in °C) is defined as the temperature felt by a human body and can indicate thermal exchange between the human body and the environment.
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Example plot:
Universal Thermal Climate Index (UTCI)
Universal Thermal Climate Index is defined as the air temperature of a reference outdoor environment that would produce the same physiological response as the actual environment. The complex model combines a physiological model with temperature-adaptive clothing insulation.
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Example plot:
Wet Bulb Globe Temperature (WBGT)
The Wet Bulb Globe Temperature represents the thermal environment to which an individual is exposed and its value gives a first approximation of the heat stress on a person. The Wet Bulb Globe Temperature is an International Organization for Standardization (ISO) screening method to establish the presence or absence of heat stress.
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Wet Bulb Globe Temperature | Risk |
|---|---|
| 20 ≤ WBGT < 25 | Low Risk |
| 25 ≤ WBGT < 31 | Moderate risk |
| WBGT ≥ 31 | High risk |
Example plot:
Wind Chill Factor (WCF)
The Wind Chill Factor is defined as the air temperature of an equivalent environment that, under calm wind conditions, would entail the same skin surface heat loss to the environment as in the actual, windy, environment. It takes into account the assumptions of convective and radiative heat loss described in modern heat transfer theory, and assumes no impact from the sun. The equivalent environment considers a still airspeed of 1.34 ms−1 (average walking speed) and a wind speed at face level (i.e., it assumes that the adult is walking into the wind).
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Wind Chill Factor | Risk | Effect |
|---|---|---|
| 0 ≥ WCF > -10 | Low Risk | – Slight increase in discomfort |
-10 ≥ WCF > -28 | Moderate risk | – Uncomfortable |
| -28 ≥ WCF > -40 | High risk | – High risk of frostnip or frostbite: Check face and extremities for numbness or whiteness. |
| -40 ≥ WCF > -48 | Very high risk | – Very high risk of frostbite: Check face and extremities for numbness or whiteness. |
| -48 ≥ WCF > -55 | Severe risk | – Severe risk of frostbite: Check face and extremities frequently for numbness or whiteness. |
| WCF ≤ -55 | Extreme risk | – Outdoor conditions are hazardous. |
Example plot:
Mean Radiant Temperature (MRT)
Mean Radiant Temperature is defined as the uniform temperature of a fictive black-body radiation enclosure which would result in the same net radiation energy exchange with a human subject as the actual, more complex radiation environment (Figure 1). It is the numerical representation of how human beings experience radiation. It applies to a human subject placed in an outdoor environment and irradiated by solar and thermal radiation both directly and diffusely. The Mean Radiant Temperature is an international standard for thermal environment ergonomics according to the International Organization for Standardization (ISO). It is also a standard for thermal environmental conditions for human occupancy according to the American Society of Heating, Refrigerating and Air-Conditioning Engineers.
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- Sdn, diffusesurf is the isotropic diffuse solar radiation (Jm-2).
- SUPsurf is the solar radiation reflected from the surface (Jm-2).
- Ldnsurf is the surface thermal radiation downwards (Jm-2).
- Lupsurf is the surface thermal radiation upwards (Jm-2).
- I∗ is the radiation intensity of the sun on a surface perpendicular to the incident radiation direction (Jm-2).
- fa is the angle factors set to 0.5
- fp is the surface projection factor determined from the solar elevation angle;
- σ is the Stefan–Boltzmann constant (5.67 × 10−8 W/m2K4),
- εp is the emissivity of the clothed human body (standard value 0.97)
- air is the absorption coefficient of the body surface area irradiated by solar radiation (standard value 0.7).
Example plot:
Globe temperature
The (black) globe temperature is derived via an indirect measurement of the radiant heat load of the environment. Its name comes from the way the parameter is measured - that is, use of a thermometer installed inside a hollow copper sphere painted matte black.
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- MRT is the Mean Radiant Temperature (in °C)
- Ta is the 2m air temperature Ta (in °C)
- v is the wind speed at 1.1m (in m/s)
- ε is the emissivity set to 0.95
- D is the diameter of sphere set to 0.15 m.
Example plot:
Overview
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