Author: Dr. Michael Chase
UPDATE (18th March 2017): This post is of dubious quality and may be modified or withdrawn, pending a review of the data at ECHUCA AERODROME, which may well not be a good enough representation of the background temperature variations.
Deniliquin in NSW is on-the-map in both the worlds of abattoirs and climatology, and there is a connection between those worlds. The rural nature of the town, and the current weather station location at the airport, rules out current Urban Heating as being a problem, but the weather station was once in the town, and even small rural towns have walls, sheds and paved areas.
Deniliquin has a very long temperature record, extending well back into the 19th century, but unfortunately it suffered a bout of Urban Heating in the 20th century, and this must be dealt with before the background climatology can be obtained. The simplest way to deal with UHI is to ignore it, but this is only possible if it occurs as a “transient” perturbation within the record, otherwise it will distort the end-to-end variation of temperature. This post presents evidence that suggests that most of the UHI did indeed occur as transient heating within the 20th century part of the record.
The following photo provides some ground truth for the UHI effect at Deniliquin:
When the weather station moved to a better location in the town in 1971 the average daily minimum temperature dropped by around 1C, relative to neighbouring stations (source: ACORN-SAT station adjustment summary, verified visually by me). The key question is how the UHI varied in the decades leading up to 1971. That is a difficult question to answer with any degree of certainty, not helped by a general lack of consistency and completeness of the temperature variations of neighbouring stations, but a comparison with a single near neighbour provides compelling evidence that significant UHI at Deniliquin only began in the 1920s. The following figure shows a comparison between annual average Tmin at Deniliquin and Echuca:
The events annotated on the figure above come from extracts (provided at the end of this post) from the PhD thesis of Simon Torok, which provides a summary of station histories. The onset of UHI at Deniliquin appears to have been in the early 1920s, with variations up and down, presumably as new buildings were added, probably with small moves of the thermometers, until dropping in 1971. There is still some uncertainty about how much residual UHI was present when the station moved to the airport in 1984, but that will be discussed in a later post.
The onset and ending of UHI within the record allows it to be ignored, as it will have no effect on the end-to-end variations of temperature, which will be presented in later posts.
I don’t have any photos of Deniliquin Post Office, but here is one of the “nearby” Mildura Post Office from 1912:
Appendix: Station History Summaries, click to enlarge (source: PhD thesis of Simon Torok)