Perth temperature history
Guest post by Chris Gillham
It’s worth digging a bit deeper into claims by the Bureau of Meteorology that Perth endured its equal hottest summer in 2010/11 and its hottest first four months of the year on record.
On May 2, 2011, The West Australian newspaper published the following story, claiming that “Perth has experienced its hottest start to the year on record, with a mean maximum temperature of 31.5C for the first four months, breaking the previous record of 31.3C set in 1978”.
Oh dear, Tim Flannery was right and Perth really is at the forefront of global warming.
But can we trust the BoM calculations? Was the mean maximum in the first four months of 2011 really 31.5C?
Well, we can do it the BoM way and calculate the average of the rounded monthly figures for January (32.5C), February (34.1C), March (31.9C) and April (27.3C). Yep, that’s an average 31.45C, which further rounds out to 31.5C. Too easy.
But what if we don’t average the rounded monthly figures and instead average the actual maximum temperatures recorded on each day during the first four months? It’s pretty easy to do. Just check the monthly figures for Perth Metro (station 9225), which is the official “Perth” site for the BoM, for January, February, March and April 2011.
The actual maximum temperatures in all 120 days of January, February, March and April 2011 can be seen here. Get your spreadsheets busy and figure out the average maximum. You’ll find it’s 31.42, which rounds to 31.4C.
So the actual mean maximum for Perth in the first four months of 2011 was 31.4C, not 31.5C as claimed by the BoM.
On March 3, 2011, the BoM claimed that Perth’s 2010/11 summer was the equal hottest on record with a mean maximum of 32C. But if you use the daily temperatures instead of the rounded monthly figures, the average summer maximum was actually 31.89C, or 31.9C when rounded. Again, their calculations were based on an average of the rounded monthly maxima rather than the days themselves (see Was Perth’s summer of 2010/2011 a record?).
2010/11 was the third hottest summer recorded in Perth, but not the equal record.
Temperature station location
Ah, but this is being a bit pedantic, is it not, since the first four months of 2011 were still the hottest on record?
That depends upon whether you also want to research a bit deeper into the record itself. The BoM references its official “Perth” temperatures from station 9225 in Mt Lawley, which opened and recorded its first annual temperature means in 1994 – picture below in May 2011 courtesy Stephen Harper.
However, the BoM quotes the previous hottest first four months from 1978. So where was this location?
Before 2004, the official Perth temperatures were recorded at Perth Regional Office (station 9034). Check the first four months of 1978 and the mean maximum was 31.3C, the previous record as stated by the BoM.
Stations 9034 and 9225 are about four kilometres apart and the evidence suggests the Mt Lawley site is slightly hotter during the day than the Perth Regional Office site. Site 9034, which opened in 1876 and closed in 1992, has a 19 metre elevation.
Site 9225, which has had a stable mean temperature since opening in 1993, has an elevation of 25 metres. Proximity to broad river waters and the UHI influence of inner city and university development are probably relevant.
Below are the average annual maxima of station 9034 from 1975 until 1992, bleeding into station 9225 from 1994:
24C
24.6C
24.6C
25.2C
23.9C
23.7C
23.5C
23.8C
24.5C
23.5C
24.5C
23.2C
24.3C
24.8C
24.2C
23.7C
24.3C
1992/93 n/a as switch made from 9034 to 9225 below
24.9C
24.2C
24.5C
24.5C
24.5C
24.9C
24.5C
24.2C
24.5C
24.5C
24.5C
23.5C
24.8C
24.4C
24.6C
25C
25.3C
The average annual maximum at Perth Metro over 17 years is about .4C warmer than it was earlier at Perth Regional Office. If you suspect the increase is because of global warming, not the four kilometre change in location, consider the annual minima in “Perth” from 1975 to 2010:
13.9C
14.5C
14.5C
14.7C
14C
14.1C
13.9C
14C
14.8C
13.9C
14.6C
13.7C
14.2C
14.9C
14C
13.7C
14.3C
1992/93 n/a as switch made from 9034 to 9225
12.9C
12.8C
13C
12.5C
12.6C
13.1C
12.9C
11.9C
12.7C
12.8C
12.6C
12.3C
12.3C
12.5C
12.3C
12.7C
12.4C
The average annual minimum at 9225 over the past 17 years was about 1.6C cooler than it had been at 9034, the drop happening immediately the station change was made. Maybe a rapid onset of global cooling, but a more robust explanation is that the four kilometre change in station locations significantly affected temperature recordings and made historic pre-1994 comparisons dubious at best and irrelevant at worst.
The temperature record at Perth Regional Office
Perth Regional Office (9034) started recording from 1897 and finished in 1992. This location was originally known as the Observatory and started recording temperatures in 1897 at the Mt Eliza site now mostly occupied by Dumas House, adjacent to Kings Park. Although the BoM gives no clues that Perth Regional Office has a history of more than one location, station 9034 is the benchmark for historic temperature comparisons in the capital of Perth.
View of Observatory grounds soon after completion with Fraser Avenue and Kings Park in the background. The meteorological instruments including a Stevenson Screen can be seen in the right of the photograph.
Most of the Observatory was knocked down in 1967 to make way for Dumas House, and at that time the Commonwealth transferred its meteorological instruments to Wellington St in East Perth – about two kilometres away. (Source: Perth Observatory History)
The Bureau of Meteorology Perth Regional Office at 127 Wellington St, East Perth, facing north with the instrument enclosure in the foreground
The elevation changed from about 61 metres at the Observatory to 19 metres in Wellington St …
… and look what happened to the temperatures recorded for Perth Regional Office (9034) …
The trend is a bit more obvious just looking at the mean temp for Perth Regional Office …
It should be noted that in 1963, Perth Regional Office was moved about 300 metres to the east of the Old Hale School building, also atop Mt Eliza (Source: Perth’s Observing Sites)
And for Perth’s temperature record from 1897 to 2010 …
So Perth Regional Office changed its physical location in 1963 and again in 1967, when it moved about two kilometres east into the central city, closer to the river both in distance and elevation but separated by blocks of large buildings, and Perth’s temperatures increased thereafter.
To further illustrate the distortion caused by moving the Stevenson screen temperature recording instrument a few kilometres from its original location, the Mt Lawley 9225 temperatures four kilometres inland show higher maxima and significantly lower minima.
Based on the temperatures charted above, the last two decades in Perth have had the hottest days and coldest nights since records began in 1897. Why are records claimed for average maxima but minima and mean are ignored, and why are any of them relevant for comparison with other locations before 1994?
It’s worth looking at Results of Meteorological Observations Made in Western Australia During 1908 (PDF 29mb) published in 1912 by Commonwealth Meteorologist Henry Hunt:
Well, until 1967 anyway.
Perth Gardens temperature overlap
Perth Gardens was Perth’s first temperature recording location as of 1876, situated about 1.5 kilometres from the Observatory site on Mt Eliza in an area since renamed as the Supreme Court Gardens (see map above).
The 1912 Hunt document also contains a table with monthly and annual mean temperature readings at Perth Observatory from 1897 to 1907, and at Perth Gardens from 1876 to 1907, which provides a useful 11 year overlap.
The 11 year mean from 1897 to 1907 at Perth Observatory atop Mt Eliza is 63.9F (17.72C) and the same 11 year mean at Perth Gardens is 64.9F (18.27C). That’s almost 0.6C difference in the same city in the same weather in the same years. The Perth Gardens did not record temperatures from a Stevenson Screen and this may have artificially increased the temperatures, but Commonwealth Meteorologist Henry Hunt suggests the thermometer was well shaded:
The mean temps for Perth Regional Office from 1897 to 1907 (go to BoM Climate Data and look up min/max for station 9034) are exactly the same as the mean temps for the Observatory in the 1912 Hunt document. These identical temperatures confirm that Perth Observatory on Mt Eliza was the original location for Perth Regional Office temps in its early years starting 1897.
Conclusion
When informing the public that record high temperatures have been experienced, the Bureau of Meteorology should ensure the accuracy of its figures by averaging daily temperatures instead of rounded monthly calculations.
Rounded averages of rounded averages are not accurate and a .1C error is significant if claiming record temperatures, particularly when Australia’s economic and political future is likely to be determined by a public understanding of the need or otherwise for a carbon dioxide tax.
The BoM should also acknowledge that temperatures recorded at its Mt Lawley 9225 station are being compared with stations in different locations where local environmental factors have a significant influence on Stevenson screen thermometers.
Perth’s temperature history is too disjointed to make meaningful comparisons and present-day recordings at Mt Lawley should only be compared to recordings from the same site beginning in 1994.
It might mean Perth has a very short history of temperature recordings, but at least the data will be relevant.
============================================================
Chris Gillahm is a 50 year old print/electronic journalist based in Perth who also does graphics, writes html, takes photographs, etc. His interest in the climate change issue stems from the fact that he was university-trained as a journalist to do old-fashioned research and not rely on the accuracy of government media statements. Digging such as this almost always reveals the public only gets half the facts about issues such as climate change. Visit his website here: http://www.waclimate.net/
Discover more from Watts Up With That?
Subscribe to get the latest posts sent to your email.
Oops @ur momisugly Stephen Harper.
It looks like the ‘Rules of Rounding’ have changed since I leaned them.
From my schooldays (40+ years ago), you rounded towards the even number/digit and not always upwards as seems to happen now. The rule was in the case of a final digit 5, to make the final digit an even number after rounding.
Examples..
31.55 rounds to 31.6…upwards as nowadays
31.45 rounds to 31.4 ….downwards in my system but would be 31.5 in the present way of doing things.
Is it an ‘American’ vs ‘British’ thing or what…..
Look up at the skies tonight. There are a zillion things to see. Something that you will not see is an average. Not in the entire universe. An average, even the simplest, is an anthropological concept – an artifact if you like. No one was ever saved from frostbite nor heat stroke by a tempered average for the day. Old Mother Nature does not concern herself with averages. It is her extremities which bite.
The big problem here, as it is with the models, is that the standard deviation is not ‘carried’ through with the further processing. An average figure of just 23.5 for a month could mean every day of that month was 23.5, or half were 24.5 and the other half 22.5, or straight line trend going through 23.5 in the middle etc… without the standard deviation being carried through you have no idea how ‘significant’ or reliable that figure really is.
As for rounding on rounding – it just shows how much they are trying to squeeze significance out of figures when there really isn’t any to be had..
Hector Pascal:
“It’s more to do with the Indian Ocean Dipole than ENSO. I can’t find much in the way of meaningful information, but I believe there is a decadal trend towards increasing dryness. It needs more research: send me money ;-)”
Hehe…yeah….
That’s very interesting – the Indian Dipole. We should perhaps ask Earl Happ to look into this since he is in Western Australia (making fantastic wine) and has some very interesting things to say on such matters.
One of the things that residents (friends and family) were saying is that the minimum temperatures during summer were very high. But I think the main issue was consistently high temperatures day after day with very little change – a blocking we experienced here in Europe during December with the opposite effect. If the Indian ocean was very warm (and swimming off Rottnest I noticed it was particularly warm from just a year ago), there would be less differential between the inland desert and the ocean, meaning the sea breeze should have been weaker, and leaving slightly elevated and more humid (and therefore hotter) air.
Pete in Cumbria UK says:
May 19, 2011 at 4:02 am
That is “bankers’ rounding”. It eliminates the slight bias towards higher numbers you get when rounding fixed decimal place numbers up to the next significant figure when a 5 is found. Half the time it is rounded down instead.
In nature, where numbers are not of a fixed number of significant figures, (often called ‘real’ numbers) then the bias should not exist. In practically every situation, there being an infinite number of numbers between any two numbers, if a 5 exists, there will always be some more ‘less significan figures’, so always rounding up is valid. For example 31.5 could be something like 31.500000065.
In accounting you pretty much always deal with a fixed number of significant figures, so need to adjust to avoid bias.
That is my understanding anyway, but I have only programmed it, not fully investigated it.
http://www.xbeat.net/vbspeed/i_BankersRounding.htm
A remark by René Rhéaume, 21.09.01
@ur momisuglyHector Pascal
“Oops @ur momisugly Stephen Harper.”
????
@Stephen Fisher Harper – you’re right – this summer lacked any of those over 40C days – I have to say that because of that, I found this summer to actually be cooler than the few summers before it. We could use some more rain though..
As mentioned above by Ian, Perth does have a number of other nearby weather sites which could be used to produce a Perth area average (whatever that means). However, as always the devil is in the detail – how many of these are long record sites, unchanged since startup? For example Rottnest Island, some 20 km offshore, would have been ideal to get a long term trend from but its station was shifted in 1995. Perhaps Perth airport? (just kidding).
As regards the Mt Lawley site, I sometimes cycle past there – and yes it is unattractive vegetationless dirt. Even weeds struggle in Perth with lack of rain. It would be real interesting to have a duplicate station a few metres away with grass underneath for comparison. Dirt is noticeably hotter than grass during the day especially, which is why our houses have lawns.
OTT, it’s great to see Greens Leader Bob Brown squirming under the heat of the local press over his party’s carbon tax policy. Brown claims the press is being unfair because it gives both sides of the debate airtime, but then if we are being asked to pay billions in some new tax, he has to convince us of its merits. Which he can’t.
Speaking as another Perth resident, we have a particularly strong sea breeze known as the Freemantle Doctor. It causes a sharp drop in temperature when it arrives typically mid-afternoon on hot days, around the time the daily maximum typically occurs.
The timing of the sea breeze at a particular location is determined by its proximity to the ocean and to a lesser extent the Swan River. I believe the Swan River channels the breeze.
Annual comparison between sites even a few kilometers apart in the Perth Metro area will be meaningless unless the effect of the sea breeze is taken into account. And that is a difficult task.
I’d expect the sea breeze to arrive later at Mount Lawley than East Perth and hence Mount Lawley should have higher maximum temperatures.
There should be little variation in night time land breezes over central Perth and hence comparisons of daily minimum should not have the same locality issues as daily maximums.
Nice work, Chris.
Interestingly, the first four months of this year ALSO set the record for mean maximum temperature at the nearby Perth Airport meteorological station (32.3 C), and this record also beat out 1978 as the next highest mean maximum (31.74). Here the record go back to 1944. And here the difference is quite dramatic. (These are averages of the daily values for highs from Jan 1 through Apr 30 each year)
I have no idea how the siting of the airport’s station may have changed, or how UHI might have affected the readings, but it does give some credence to the claim that 2011 was hottest in Perth. (The next hottest years for Jan-Apr are 2010, 1972, 1988, and 2009)
There are several other official stations in the area, but most only have very short time-frames (a decade or two). Still, it would be interesting to see if they also show 2011 as the hottest.
Pete in Cumbria UK says: May 19, 2011 at 4:02 am
… you rounded towards the even number/digit …
Of course, the slightly better method is to keep a few more digits during intermediate steps and round at the end. The actual results for the months are:
JAN: 32.49
FEB: 34.15
MAR: 31.87
APR: 27.31
AVERAGE: 31.46
This DOES indeed round to 31.5!
Of course, the mistake now is that they should have done a weighted average. The shortest month (FEB) had the highest temperatures. By giving the shortest month the same importance as the longer months, they threw off the average. The true number is indeed 31,42, which should round to 31.4.
@ur momisugly Stephen Harper
Sorry. I made a comment above mistakenly addressed to Richard Verney, agreeing with your comment that Perth doesn’t look like a sandpit. It doesn’t, Perth looks fine, as long as it is reticulated. The Mt Lawley site looks like a sandpit because it is not irrigated.
You will pardon me, but I have a hard time understanding why the mean of the maximum means anything. The time weighted average means something since that is a measure of the total heating/cooling requirements. The maximum or minimum mean something because they are a measure of the worst case stress on the heating and cooling. But the average of the maximum? A nice phrase for people that have no understanding of statistics I suppose, but unless somebody can illuminate me with a real use for this statistical nonsense, I must regard it as simply that; nonsense.
ShrNfr says: May 19, 2011 at 7:43 am
>You will pardon me, but I have a hard time understanding why the mean
> of the maximum means anything. The time weighted average means
> something since that is a measure of the total heating/cooling requirements.
Yes, a time weighted average would be best. But most records only show daily max & min, so that is what we need to work with.
> The maximum or minimum mean something because they are a measure
> of the worst case stress on the heating and cooling.
OK. Sounds good.
>But the average of the maximum? A nice phrase for people that have no
>understanding of statistics I suppose, but unless somebody can
>illuminate me with a real use for this statistical nonsense, I must
>regard it as simply that; nonsense.
Well, if the daily maximum means something (as you said in the previous sentence), then knowing the typical value for daily maxima also means something. It is the simplest summary of a measure that you think is valuable. It would give a typical stress on cooling systems over the course of a few months.
I remember the Perth summer of 1990 well. Just moved to Perth, wife arrived to look at accommodation, air conditioning in rented house at Swanbourne broke down, airconditioning in new car broke down, visited Maylands area where we were thinking of living, temp 45 deg or there abouts for several days, wife says ‘I’ll stay in Sydney’ (luckily she didn’t), and I don’t think its been hotter since – but thats just my memories.
Ciao
John Gorter
Milan
Chris, any way to get the monthly average minimums, maximums and relative humidity for those four month period for 1972, 1978, 1988, 2009, 2010, and 2011? Just curious if the amount of water vapor in the atmosphere had anything to do with this during warmer years. Or have you already followed that same line of thought?
Nick Stokes says: You can get the daily data for Mt Lawley from here. It is station 94608. For some reason it’s in Fahrenheit. But by averaging daily max I get, yes, 31.4C for the first 4 months of 2011. And 30.2C for the first 4 months of 1978.
Thanks for drawing my attention to that ncdc/noaa (WMO) data re Perth Metro. However, digging into their records makes me wonder if we’re talking about the same location. Station 94608, wherever that is, started in 1978 and the mean max for that full year was 24.2C. This compares to an annual mean max of 24.5C since the BoM’s Perth Metro (Mt Lawley 9225) began in 1994, and indeed the last couple of years were 25C and 25.3C.
However, the ncdc/noaa 1978 figures show an annual mean min of 15.1C. This compares to an annual mean min of 12.6C since the BoM’s Perth Metro (Mt Lawley 9225) began in 1994. I doubt it. The Perth Regional Office had an annual mean min of 14.7C in 1978, compared to an average 14.1C from 1968 to 1991 (post Observatory so the same location for a change).
I’ve compared the Perth Metro figures for 1994, although the ncdc/noaa data only show 346 days for that year. However, they seem to be fairly random missing days so:
9225 average annual max = 24.9C;
ncdc/noaa average annual max = 24.7C;
9225 average annual min = 12.9C;
ncdc/noaa average annual min = 11.9C.
It’s good to see the ncdc/noaa average min come down from 15.1C in 1978 to 11.9C in 1994!
How about 1995 from ncdc/noaa? Nah, they’ve only included 318 days of the year and that’s getting a bit too sparse for a reliable comparison.
1996? Eureka! Every day of the year:
ncdc/noaa average annual max = 24.4C;
9225 average annual max = 24.5C;
ncdc/noaa average annual min = 11.8C;
9225 average annual min = 13C.
Hmmmm.
I’ll jump forward to 2010 and give it a final roll of the dice:
ncdc/noaa average annual max = 25.3C;
9225 annual average max = 25.3C;
ncdc/noaa average annual min = 11.2C;
9225 av annual min = 12.4C.
Why don’t we have a look at another station in the Perth metropolitan area – Perth Airport, which is 94610 in ncdc/noaa-land and 9021 in BoM-land. For 2009 (I’d compare 2010 only the BoM hasn’t yet calculated last year’s December temp (?):
ncdc/noaa average annual max = 25.5C;
9021 average annual max = 25.5C;
ncdc/noaa average annual min = 11.1C
9021 average annual min = 12.3C
I’ve got no idea how ncdc/noaa has created a Perth Metro station dating back to 1978 as there’s no record of Perth Metro before 1994 on the BoM website. A cursory study of the ncdc/noaa Perth Metro data suggests it’s as reliable as an ice cube in an oven, and their Perth Airport figures also suggest they’re getting their temperature data from a dodgy operator – particularly their minima.
Agnostic – check http://www.waclimate.net/bureau.html for a variety of explanations re Perth’s temperature patterns and current dry spell. The BoM does attribute some of the past year to La Nina but, like Stephen Harper above (thanks for the pic, Steve!), I’m intrigued by the fact that the southern coastal sea surface temperatures off Western Australia have been well above average compared to the rest of the world for some time now… see http://www.osdpd.noaa.gov/ml/ocean/sst/anomaly.html. Our west-east weather flows have to push southerly cold fronts across that warm ocean water if we’re to get rain, and the BoM weather outlooks usually show they’re on their way four or five days beforehand but give up as they get closer to the coast. The regularity and/or strength of our afternoon sea breezes might also be affected. I’m no expert on SST and I’ve got nothing to back up my belief except for what looks like an obvious explanation. Also note that across the 2.6 million square kilometres of Western Australia, this past summer recorded the highest-ever rainfall and below average maxima.
Tim Folkerts – I’m not sure if you’ve repeated the BoM rounding mistake but the average max of every single day in the four months was 31.4241666666667. That’s 31.4 when rounded.
Re Perth’s summer hitting 40C – we just got there on two occasions and there’s nothing particularly remarkable about that if you go through the BoM records. I also thought it was a comparatively cool summer because our “hot” days were mostly around 37C with none of those 42-43C scorchers, although a long run of warm minima above 20C made it a bit sticky at night. It was (almost) Perth’s record hottest summer even though it didn’t get particularly hot.
Thanks to all who’ve made complimentary comments.
BoM sources:
Perth Metro max annuals – http://www.bom.gov.au/jsp/ncc/cdio/weatherData/av?p_nccObsCode=36&p_display_type=dataFile&p_startYear=&p_c=&p_stn_num=009225
Perth Metro min annuals – http://www.bom.gov.au/jsp/ncc/cdio/weatherData/av?p_nccObsCode=38&p_display_type=dataFile&p_startYear=&p_c=&p_stn_num=009225
Perth Airport max annuals –
http://www.bom.gov.au/jsp/ncc/cdio/weatherData/av?p_nccObsCode=36&p_display_type=dataFile&p_startYear=&p_c=&p_stn_num=009021
Perth Airport min annuals –
http://www.bom.gov.au/jsp/ncc/cdio/weatherData/av?p_nccObsCode=38&p_display_type=dataFile&p_startYear=&p_c=&p_stn_num=009021
wayne says: Chris, any way to get the monthly average minimums, maximums and relative humidity for those four month period for 1972, 1978, 1988, 2009, 2010, and 2011? Just curious if the amount of water vapor in the atmosphere had anything to do with this during warmer years. Or have you already followed that same line of thought?
Nope, I haven’t looked into relative humidity. There are some longer term averages for Perth Metro at http://www.bom.gov.au/climate/averages/tables/cw_009225_All.shtml and you can track down temps, rainfall and solar exposure for each year back to 1992 at http://www.bom.gov.au/climate/data/ (search for Perth Metro at 9225 and Perth Regional Office at 9034) – but I don’t know where to find yearly or monthly rh stats.
Chris Gillham comments: May 19, 2011 at 10:26 am
Tim Folkerts – I’m not sure if you’ve repeated the BoM rounding mistake but the average max of every single day in the four months was 31.4241666666667. That’s 31.4 when rounded.
Actually, I was agreeing with you.
* If the average is done for the four months separately and those four numbers are averaged, then the answer would be just over 31.46, so it would very legitimately round to 31.5. But as I said, they should actually have done a weighted average, not a simple average.
* When the average is done the correct way, then I posted the same numbers above as you — that the correct average is 31.42, which could be rounded to 31.4 if you only want to keep one decimal place of precision.
The more significant point ( in a comment at May 19, 2011 at 6:42 am) was that the data from the Perth airport the first four months of this year ALSO set the record for mean maximum temperature. And the last three years are all in the top 6 hottest years.
Wayne asks: May 19, 2011 at 10:08 am
Chris, any way to get the monthly average minimums, maximums and relative humidity for those four month period for 1972, 1978, 1988, 2009, 2010, and 2011?
Weather Underground has daily data going back a while for various locations. I saw RH & Dew Point back about 10 years for Perth. But the data is presented for each day — I don’t think there is a way to download data for a month or year at a time, so extracting the data could be painful! And I don;t think you will find ’72, ’78, or ’88.
Wooot! Perth on the map … yes it was a fairly toasty summer, but then I have only lived here since 1999. We get a fair few hot summers. As I live in the centre of town (near that red dot on the right actually) I shall be happy to take a few photos of those red dots as they are currently if you like.
Quick question. If they rounded their previous average in the same way as the new one, then shouldn’t the difference be all that matters? I think your point about the rounding only matters if they changed method doesn’t it?
Allan
But no-one in the rest of Australia cares about Perth.
Perth – 2011 Record Summer Temperatures
When examining historic temperatures, one needs to consider: (1) whether the weather station has remained in the same location for the duration of the historic records; (2) whether the weather station site has been impacted by surrounding development (i.e., urban heat island effect or UHI), and (3) the impact of nocturnal cloud cover. Chris Gillham (WUWT, May 19) has given an excellent account of the consequences of a change in weather station location in Perth, Western Australia.
In 1947 Perth supported a population of 303,000. Today, the population of Perth has increased to nearly 1.7 million.
Now, Perth Airport is 10 kilometres east of the Perth CBD. Land was purchased to the south of Guildford and east of Ascot for the construction of the airport (YPPH) in 1938. The airport opened in 1940; initially as an RAAF base. A few years later (1944) the airport supported a mix of military and civil aviation. After WW2 the airport attained international status in 1952 and now comprises two intersecting runways and separate domestic and international terminals. Development of the airport, therefore, coincided with the post WW2 “Baby Boom” (1946-1963) and an influx of migrants from Great Britain and Europe.
In parallel, the Perth CBD shot up off the back of the Western Australian mining boom of the 1960s and 70s. At the same time there was an influx of new residents from the eastern states of Australia. Consequently, the airport that used to be on the outskirts of metropolitan Perth is now surrounded on three sides by residential and industrial development and to the east by remnant buffer of vegetation and more residential development.
A direct consequence of these changes is that temperatures in both the Perth CBD and at Perth Airport are now very much infected by UHI.
Weather station data at Perth Airport extends back to 1944, so we have a continuous record on how temperatures have changed from the end of WW2 right up to the present day.
The whole AGW argument is now largely being driven by a bias towards “warm” weather stations and continued increase in UHI in CBDs and areas of urban sprawl. This is also true of the Perth CBD and its nearby airport.
More recently, and in response to weakening sunspot activity, there has been a corresponding increase in low level cloud. This has exacerbated the nocturnal component of UHI. This is evident in the fact that, whilst the average daytime temperature at Perth Airport was 1º C above the long-term monthly mean maximum for January, the night time mean temperature for January minimum temperature was 3º C hotter than the corresponding long-term mean; 20º C cf. 17º C!