From the dataset abstract

Weather data acquisition was originally by manual observation and continued with an automated system installed in Nov 2001. Measurements began shifting from manual to automated...

Source: Daily averaged weather timeseries (air temperature, pressure, wind speed, wind direction, precipitation, sky cover) at Palmer Station, Antarctica combining manual observations (1989 - Dec 12, 2003) and PALMOS automatic weather station measurements (Dec 13, 2003 - March 2019).