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Heat Force (Liljegren WBGT)

Heat force (Dutch: hittekracht) is a 0–10 scale that communicates environmental heat stress to the public, introduced operationally by KNMI (the Royal Netherlands Meteorological Institute) from June 2026. It is analogous to the familiar wind-force and UV-index scales.

Heat force is derived from the Wet Bulb Globe Temperature (WBGT) computed with the physically based Liljegren method (Liljegren et al., 2008), which is regarded as the "gold standard" for deriving WBGT from standard meteorological variables. The Liljegren method solves the steady-state energy balance of the natural wet-bulb thermometer and the black globe by iteration, and combines them with the air temperature as WBGT = 0.7·Tnw + 0.2·Tg + 0.1·Ta.

More information:

  • Liljegren, J. C., Carhart, R. A., Lawday, P., Tschopp, S., & Sharp, R. Modeling the Wet Bulb Globe Temperature Using Standard Meteorological Measurements. J Occup Environ Hyg 5(10), 645–655 (2008). https://doi.org/10.1080/15459620802310770
  • Kong, Q. & Huber, M. Explicit Calculations of Wet-Bulb Globe Temperature Compared With Approximations and Why It Matters for Labor Productivity. Earth's Future 10 (2022). https://doi.org/10.1029/2021EF002334
  • KNMI Technical Report TR-26-04. From Wet Bulb Globe Temperature (WBGT) to Heat Force (2026).

How to use

Wet Bulb Globe Temperature (Liljegren)

You need 2 m air temperature in Kelvin, relative humidity as a percentage, surface air pressure in hectopascals, 10 m wind speed in metres per second, instantaneous downward shortwave radiation at the surface (SSRD) in W m⁻², the fraction of that radiation that is direct beam (0–1), and the cosine of the solar zenith angle.

It returns the WBGT in Kelvin.

calculate_wbgt_liljegren(
    2m_temperature, relative_humidity_percent, pressure_hPa,
    10m_wind_speed, ssrd, fdir_fraction, cos_solar_zenith_angle,
)

The KNMI operational guards are applied internally: wind below 0.62 m/s at 10 m is raised to that floor (then scaled to the 2 m sensor height); the direct-beam fraction is clamped to [0, 0.9] and set to 0 when the sun is at or below 89.5 degrees zenith. The cosine of the solar zenith angle can be obtained from the earthkit-meteo library. NaN is returned where the iteration does not converge.

By default the 10 m wind is converted to 2 m with the KNMI/Liljegren stability-dependent profile (wind_scaling="liljegren"); pass wind_scaling="brode" to use the generic scale_windspeed log profile instead. The Liljegren conversion is also available on its own:

calculate_wind_speed_2m_liljegren(10m_wind_speed, cos_solar_zenith_angle, ssrd)

Heat Force

You need the WBGT in Kelvin (for example from calculate_wbgt_liljegren).

It returns the heat force as a whole number from 0 to 10.

calculate_heat_force(wbgt)

Interpret the output

Heat force uses fixed 2 °C bands of WBGT. Heat force 0 means no heat load; heat force 10 is extremely rare in the current Dutch climate. Heat force describes only the environmental heat load — the actual health risk also depends on an individual's "heat fitness" (health, activity level, clothing, acclimatisation).

Heat force WBGT (°C)
0 < 14
1 14 – 16
2 16 – 18
3 18 – 20
4 20 – 22
5 22 – 24
6 24 – 26
7 26 – 28
8 28 – 30
9 30 – 32
10 ≥ 32