Dear Lachlanhug,

without being able to access your data (link does not seem to work), this behavior is typical for input values being out of range for the UTCI regression equation. This mostly happens for Ta, that must be within +50 and -50°C and wind speed in a height of 10m above ground, that needs to range within 0.5 m/s and 17 m/s. If the valid range is exceeded, UTCI can not be estimated for this row of data and a fail value (-9999) is returned.

Please check your data accordingly.

Best regards,
Domink