Hi @chris chris and @josephyang josephyang,
thank you for the provided elements!
I’m a colleague of JulienFBC and I’m also interested in Ladybug UTCI calculator.
After some research I’m here to share with you some findings, even if a check on the source code will probably solve all the calculation questions.
According to http://www.utci.org/isb/documents/develop_utci.pdf “wind should be measured at 10 m height, but we need the wind velocity at a height representative of the human being, which has been defined as 1.1 m. So measured or computed wind data must be adjusted to that height. The additional wind speed induced by the movement of the person requires the calculation of a “relative” wind velocity. We assume that the observed “meteorological” wind blows at 90 degrees of the direction of a walking person”.
So the input air velocity should be the vector addition between u = walking speed = 4 km/h = 1,111 m/s and v = wind velocity at 1.1 m .
It is possible to obtain wind velocity at 1.1 m starting from wind velocity at 10m, which according to UTCI scale varies from a minimum of 0,5 m/s to a maximum of 17 m/s.
The formula, derived from http://www.envi-met.info/doku.php?id=apps:biomet_utci, should be:
v = wind_1.1m = wind_10m x [(ln(1,1/z0)) / ln((10/z0))]
0,5 m/s <= wind_10m <= 17 m/s
z0 = roughness length, which depends on terrain category and which could change depending on the country’s standards (default value is generally z0 = 1m for urban areas).
Since the vectors of wind speed at 1.1m and the walking speed are perpendicular, the relative velocity to be used should be the addition of the two vectors, following the Pythagoras theorem:
R = relative velocity = sqrt(u²+v²)
If this procedure is correct, it’s a quite complex one to obtain the input for the Ladybug UTCI calculation. It could be simpler to add two inputs (wind velocity at 10m, roughness length) and the relative velocity calculation process should be added directly in the source code of the Grasshopper component.
Let me know what you think about that and if you find other references about the UTCI calculation process.