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Universal Thermal Climate Index

Heat stress

Effect

Advised treatment

UTCI > 46Extreme heat stress– increase in rectal temperature (Tre) time gradient
– steep decrease in total net heat loss
– averaged sweat rate >650 g/h, steep increase		Temporary body cooling.  No physical activity. Drinking >0.5Lh−1 water necessary.
38 < UTCI ≤ 46Very strong heat stress– core to skin temperature gradient <1K (at 30 min)
– increase in rectal temperature (Tre) at 30 min		Temporary use of air conditioning.  Shaded places necessary.  Reduce physical activity. Drinking >0.5Lh−1 water.
32 < UTCI ≤ 38Strong heat stress– dynamic Thermal Sensation (DTS) at 120 min >+2
– averaged sweat rate >200 g/h
– increase in rectal temperature (Tre) at 120 min
– latent heat loss >40 W at 30 min
– instantaneous change in skin temperature >0 K/min		Shaded places.  Drinking >0.25Lh−1 water.  Temporarily reduce physical activity.
26 < UTCI ≤ 32Moderate heat stress

– change of slopes in sweat rate, rectal temperature (Tre) and skin temperature:      mean (Tskm), face (Tskfc), hand (Tskhn)                                                                      – occurrence of sweating at 30 min                                                                               – steep increase in skin wettednesswetness

Drinking >0.25Lh−1.
9 < UTCI ≤ 26No thermal stress– averaged sweat rate >100 g/h
– dynamic Thermal Sensation (DTS) at 120 min <1
– dynamic Thermal Sensation (DTS) between -0.5 and +0.5 (averaged value)
– latent heat loss >40 W, averaged over time
– plateau in rectal temperature (Tre) time gradient
0 < UTCI ≤ 9Slight cold stress– dynamic Thermal Sensation (DTS) at 120 min <-1
– local minimum of skin temperature hand (Tskhn)(use gloves)		Some warm clothing, e.g. gloves and hat.
-13 < UTCI ≤ 0Moderate cold stress– skin blood flow at 120 min lower than at 30 min (vasoconstriction)
– averaged skin temperature face (Tskfc) <15°C (pain)
– decrease in skin temperature hand (Tskhn)
– rectal temperature (Tre) time gradient <0 K/h
– 30 min skin temperature face (Tskfc) <15°C (pain)
– skin temperature mean (Tskm) time gradient <-1 K/h (for reference)		Intensify activity. Protect face and extremities.
−27 < UTCI ≤ −13Strong cold stress– averaged face (Tskfc) <7°C (numbness)
– rectal temperature (Tre) time gradient <-0.1 K/h
– rectal temperature (Tre) decreases from 30 to 120 min
– increase in core to skin temperature gradient		Intensify activity. Protect face and extremities. Warm clothing.
−40 < UTCI ≤ −27Very strong cold stress– 120 min face (Tskfc) <0°C (frostbite)
– steeper decrease in rectal temperature (Tre)
– 30 min Tskin temperature face (Tskfc) <7°C (numbness)
– occurrence of shivering
– rectal temperature (Tre) time gradient <-0.2 K/h
– averaged skin temperature face (Tskfc) <0°C (frostbite).
– 120 min skin temperature face (Tskfc) <-5°C (high risk of frostbite)		Intensify activity. Protect face and extremities (frostbite risk). Warm clothing. Reduce time outdoors.
UTCI < −40Extreme cold stress– rectal temperature (Tre) time gradient <-0.3 K/h
– 30 min skin temperature face (Tskfc) <0°C (frostbite)		Stay at home.  If necessary to go outdoors, use heavy and wind protected clothing.

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Fig8.1.18-8: Forecast wind chill factor during a strong surge of very cold air southwards across the United States.  Low windchill wind-chill extended to the southern gulf states (as did snow).  Extended areas with high to severe risk of frostbite and hypothermia across the region. A small area of extreme risk (outdoor conditions are hazardous) over Montana/Alberta area.

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