The station metadata provides information regarding the conditions and practices of temperature observation at the measurement location. (bom.gov.au)
Between 1960 and today, the majority of stations switched from a late afternoon to an early morning observation time, resulting a systemic change (and resulting bias) in temperature observations. (judithcurry.com)
So the issues are the same as surface temperature observation versus naive projections of the near - future forcings. (skepticalscience.com)