"Temperature observations" refers to the act of measuring and recording the level of heat or coldness in a certain environment or object. Full definition
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)