Approximates vapour pressure as the saturated vapour pressure at the daily mean air temperature using relative humidity. See section 9.3 of the STICS book, as well as M. Launay's explanations on the STICS forum (link).
approx_vapour_pressure(mean_temp, rel_humid)numeric vector
approx_vapour_pressure(15, 60)
#> [1] 10.22252