Function saturation_vapour_pressure_pa 

pub fn saturation_vapour_pressure_pa(temperature_k: f64) -> f64

Saturation vapour pressure of pure water vapour over a plane surface of liquid water, Pa, at temperature temperature_k. WMO-No. 8 (2023), Vol. I, Annex 4.B, eq. 4.B.1, with t in °C:

e_w(t) = 6.112 exp(17.62 t / (243.12 + t))  hPa

WMO states it for −45 °C to 60 °C. Colder air holds so little vapour (1.9 Pa at −60 °C) that continuing the formula below its range changes density negligibly. The formula vanishes as t approaches −243.12 °C, so at and below that it returns zero.