Are We Getting More Heatwaves?

As it is now two weeks (nine business days) since I questioned Dr Vertessy on his claims in his ABC Radio interview, it appears an answer is still to be given, so I shall post what I have found so far.

Dr Vertessy claimed that we are seeing “of the order of five times the number of very serious heatwaves” as in the middle of last century.  Not knowing Dr Vertessy’s definition of a “very serious heatwave”, I have used the following metric:

“Three days or more in a row in summer (December- February) where the maximum temperature is in the top 5% of temperatures for that day at that location, with daily benchmarks calculated using daily maxima for each month from 1961 to 1990.”

I have also used as an absolute metric of very hot days the Bureau’s own definition, days above 40 degrees Celsius.  I have used ACORN-SAT maxima to 31 December 2014 downloaded directly from the Bureau’s Acorn site, and daily maxima from 1 January to 28 February this year for each site, downloaded from Climate Data Online.

Note that this does not consider other serious factors such as humidity or night time minima.

I have initially looked at all state capitals, and will later look at other locations.

I have calculated decadal running counts of the number of days meeting the criterion to show how hot weather has changed over time.

So what did I find to be the answer to “Are we getting five times more heatwaves than we did 60 years ago”?  Mostly no, but it depends where you look.

Fig. 1: Decadal count of heatwave days in Adelaide

Decadal cnt 95 3d heatwaves summer Adelaide

Yes, but the peak may be past.

Fig. 2: Decadal count of >40C days in Adelaide

Decadal cnt 40 Adelaide

This shows a distinct recent rise.

Fig. 3: Decadal count of heatwave days in Brisbane

Decadal cnt 95 3d heatwaves summer Brisbane

A peak 10 years ago, dropping to zero heatwaves in the decade to 2015.

Fig. 4: Decadal count of >40C days in Brisbane

Decadal cnt 40 Brisbane

One day, 22 February 2004.

Fig. 5: Decadal count of heatwave days in Darwin

Decadal cnt 95 3d heatwaves Darwin

As Darwin doesn’t have “summers”, the count was of all days.  Note the 1930s and 1970s.  Darwin is not seeing more heatwaves.  Darwin has never had a day over 40C.

Fig. 6: Decadal count of heatwave days in Hobart

Decadal cnt 95 3d heatwaves summer Hobart

Hobart has had no heatwave days in the past decade, compared with five in the 1950s.

Fig. 7: Decadal count of >40C days in Hobart

Decadal cnt 40 Hobart

Hobart has fewer extremely hot days than in the past.

Fig. 8: Decadal count of heatwave days in Melbourne

Decadal cnt 95 3d heatwaves summer Melbourne

Melbourne has fewer heatwave days than the middle of last century.

Fig. 9: Decadal count of >40C days in Melbourne

Decadal cnt 40 Melbourne

Melbourne has more very hot days than it did in the 1950s, but less than the 1940s.

Fig. 10: Decadal count of heatwave days in Perth

Decadal cnt 95 3d heatwaves summer Perth

Perth has had 10 heatwave days in the past decade.  In the decade to 1955 it had 6- but in the 1960s it had three times the current number.

Fig. 11: Decadal count of >40C days in Perth

Decadal cnt 40 Perth

The recent peak was one more than in the 1960s.  The warming since the 1970s is clearly visible.

Fig. 12: Decadal count of heatwave days in Sydney

Decadal cnt 95 3d heatwaves summer Sydney

Four days in the last 10 years, compared with zero in the 1940s and 1950s- but less than the 1960s.

Fig. 13: Decadal count of >40C days in Sydney

Decadal cnt 40 Sydney

The current peak of seven days in the past 10 years of very hot days is about the same as the 1940s and 1960s, but much less than the 1980s.

Technically, Dr Vertessy is correct in his claim of “of the order of five (four to six?) times the number of very serious heatwaves” as in the middle of last century, at Adelaide, Darwin, and Sydney, but not at Brisbane, Melbourne, Hobart, or Perth.  However, Sydney had far more in the 1960s, and Darwin had as many in the 1970s and far more in the 1930s.  Adelaide alone shows a clear picture of many more heatwave days in the past 10 years.

In several of the records it is possible to see cycles of 15 – 20 years duration.  While there is an argument that heatwaves and extremely hot days are weather events, not climatic, resulting from blocking highs or the lateness of sea breezes, these apparent cycles indicate a climatic influence.  What would cause blocking highs to be more persistent, or sea breezes to be consistently later, for 10 years or more?  Atmospheric circulation patterns, including the location of the sub-tropical ridge, would appear to be the major influence.

The longer term analysis from 1910 shows a more complete picture than since the 1950s.  Wouldn’t it be good to use “carefully curated” Acorn maxima from before 1910.

In a future post I will look at other locations, as a continent’s climate extremes can’t be usefully analysed with only seven sites.  As well, this analysis has used ACORN-SAT data only.   What will the raw data show?  Therefore I will also compare results for Acorn and raw.   Bourke might prove interesting.

Meanwhile, I am waiting patiently for Dr Vertessy’s response.  Apart from Adelaide, the state capitals certainly don’t support his claim.

Tags: , , ,

13 Responses to “Are We Getting More Heatwaves?”

  1. reichforthesky Says:

    Hi Ken, I have done as you suggested above and analysed for Melbourne the number of days where maximum temperature is greater than or equal to 40 degrees. The results are as follows.

    For the 1910s the number of days are Raw 10 Acorn 12, 1920s- Raw 9 Acorn 15, 1930s- Raw 8 Acorn 13 , 1940s- Raw 17 Acorn 24, 1950s- Raw 8 acorn 10, 1960s- Raw 10 Acorn 15, 1970s- Raw 4 Acorn 7, 1980s – Raw 14 Acorn 14, 1990s – Raw 15 Acorn 15, 2000s -Raw 21 Acorn 21, 2010 until end 2014 – Raw 13 (26) Acorn 13 (26).

    The bracketed values for 2010 until the end 2014 are the values equivalent to a full decade.

    The trend lines for the Acorn data show an increase of the number of extreme days (greater than or equal to 40 degrees) for the Acorn data (from 1910 till end of 2014) of 0.7 days per decade.
    Similarly the Raw data shows a trend line increase of 1.3 days per decade.

    The alarming increase in such extremely hot days for Melbourne from 1970 until end of 2104 are illustrated by a trend for this period of 4.5 days per decade for the Acorn data while for the Raw data the trend is 5.1 days per decade.

    You can see why I strongly support Jennifer Marohasy’s request for an inquiry into the BOM data (maybe even a Royal Commission). This data clearly supports the contention that the BOM, by its nefarious massaging of its temperature data, is understating the degree of warming, at least for the number of extremely hot days in Melbourne.

    By the way the data for the above can be obtained from for the Acorn data while the Raw data can be obtained as a file IDCJAC0010_086071_1800_Data.csv from the BOM site through .

    The use of the numeric filter function in Excel makes it a fairly straight forward exercise to check this data.

    • kenskingdom Says:

      See Fig. 9 above. Sometimes Acorn reduces trends, sometimes increases them. Nearly all capitals are subject to UHI and Acorn corrects somewhat for this. No argument there. This post is about incidence of heatwaves. Melbourne’s heatwaves are NOT 5 times more frequent now.

  2. reichforthesky Says:

    Hi Ken, you are quite right in pointing out that the BOM manipulation of data sometimes cause trends in temperatures to increase and sometimes decrease. There is no consistent picture due to the differences between sites.

    As someone who suffered through last years heat wave in Melbourne, this gave me the distinct impression that the number of extremely hot days has increased. From the raw data the largest number of extreme days (greater than or equal to 40 degrees) was the highest on record last year (7 days) which beat 1991 (5 days). Accordingly I have decided to look at the data in more detail.

    Analyzing the same set of data used for figure 9 above (decade summed Acorn data) we get an average increase in number of hots days per year of only 0.001 days per year , or over 105 years, an increase of 0.1 days (compared to the average of 1.5 days per year over the entire 105 years) which is a 6.5% increase since 1910. If we uses the un-smoothed Acorn data (i.e not summed over a decade) we get an average increase in number of extremely hot days of 0.0042 days per year and for the 105 years this amounts to an increase of 0.44 days i.e. a 28% increase in extremely hot days from 1910 until end of 2014.

    For the raw data we get a much larger increase of 0.0071 hot days per year which corresponds to an increase over 105 years of 0.7 days of extreme heat per year (compared with the average for this period of 1.16 days) . This corresponds to a 64% increase in number of extreme days per year since 1910.

    For un-smoothed raw data we get an increase of 0.0098 days per year (for the decade summed raw data) which gives an increase, over 105 years, of just over 1 day of extreme heat per year (compared with the average for this period of 1.25 days) . This corresponds to about an 83% increase in number of extreme days per year since 1910.

    The BOM’s Acorn process allows comparisons to be made that remove ‘extraneous factors’. However what people on the ground in Melbourne experience and suffer from, is more of a reflection of the increase in raw temperatures, rather than the adjusted Acorn temperatures. Comparing the trends for the raw and Acorn data, it is apparent that the large increase in the number of extreme days in Melbourne is mainly from UHI and minimally from, at least for the case of Melbourne, climate change.

    In summary, despite claims to the contrary, we are getting more extremely hot days for Melbourne. If we use the raw data we get about 60-80% increase in extreme days since 1910. Using the Acorn data which presumably compensates for the urban heat island effect (and possibly other factors) instead, we get a much smaller increase in extreme days by about 6-28%.

    Note that I have, rather than restricting myself to January and February,for the Acorn data, I have included extreme days that have occurred in November (on 3 occasions), December (on 27 occasions) and March (on 7 occasions) . I suspect this will not have changed the outcome to any large extent.

    • kenskingdom Says:

      The 40C graphs above are for the whole year, not just summers. But whatever makes you happy I guess.

      • reichforthesky Says:

        Sorry I misread paragraph four above in your original post. My humble apologies for my final paragraph.

        Care to comment on the remainder?

        • kenskingdom Says:

          Who am I to discourage you from your fervent belief that ‘very serious’ heatwaves are increasing, despite evidence to the contrary- especially in Melbourne? Is Dr Vertessy correct in saying we are getting about 5 times more heatwaves- yes or no?

  3. reichforthesky Says:

    To quote from Dr Vertessy at the Senate Estimates Committee for the full transcript see –;db=COMMITTEES;id=committees%2Festimate%2F7c7082a0-a5a5-40aa-b2e0-29c66070f91d%2F0003;query=Id%3A%22committees%2Festimate%2F7c7082a0-a5a5-40aa-b2e0-29c66070f91d%2F0000%22

    ‘We can see, when we look at a breakdown of the distribution of temperatures, there is certainly a shift towards much hotter events. If we look at some of these very low frequency heatwaves, we are seeing something in the order of a fourfold or fivefold increase in the frequency of those in the modern record as compared to the mid-century’s last years.’

    The definition of extreme events is vague but he may have been referring to even more extreme events and consequently even more infrequent events than the greater than 40 degrees events that we have been discussing until now.

    If we chose 42 degrees as our reference point we find for the raw data for Melbourne the following-

    Form 1910 until 1999 – the number of days great than 42 are 14 days. From 2000 until end of 2014 there were 12 days. So prior to 200 these events occurred at a frequency of 14/90 = 0.16 while for years great than 2000 the frequency is 12/15 = 0.8.

    This corresponds to an increase by a factor of 5.

    It is interesting that as heat wave events become even more extreme and infrequent overall then the increase over time of these events become more obvious compared to the less extreme events.

    Now Melbourne is probably a bad example if we want to claim this is due solely to climate change. It would be interesting if this holds for other locations. Maybe if you have time Ken you could have a look at this.

  4. kenskingdom Says:

    “The definition of extreme events is vague”. You are SO right. Vague enough to create alarm and media attention by loose references. He has still not replied to me after 12 business days, but Dr Vertessy was clearly referring to heatwaves, not single days. There was one 3 day 42C heatwave in 2009, and one in 1959. Unless the definition has recently changed to a heatwave being a single hot day, it still does not hold up.

  5. reichforthesky Says:

    Sorry for the minor quibble but 1959 did not have 3 days in a row above or equal to 42. January 18th, 1959 was 41.8 degrees. Close but no quite there.

    So the only years where we have had more than 1 day in a row of 42 degrees and above have been 2009 with 3 days in a row and 2014 with 2 days in a row.

    I have also looked at the number of days in a row above a particular temperature for the period 2000-2015 compared to the period 1910-1999.

    For temperatures greater or equal to 38 degrees we have had 144 days of consecutive temperatures prior to 2000 and 27 days after. This correspond to 1.6 days per year prior to 2000 and 1.8 days per year after, a 12.5% increase,

    The figures for 39 degrees and above are 127 consecutive days before 2000 and 24 after 2000. The numbers per year before 2000 are 1.41 and 1.6 after, a 13.4 % increase.

    For 40 degrees and above the numbers are 28 days before 2000 and 11 after 2000. The numbers per year are 0.31 and 0.73 respectively an increase of about 236%.

    For 41 degrees and above the numbers are 9 days before and 7 after 2000 giving 0.1 and 0.47 per year respectively, a 470% increase.

    As above, for temperatures greater or equal to 42 degrees we have no sequences prior to 2000 and 7 occurences after 2000.

    So it seems if you are looking at extreme heatwaves (greater than 41 degrees), at least in the case of Melbourne, the 5 fold increase in frequency of these events is evident.

    It would be interesting for someone with time on their hand to repeat this exercise for other locations to see if Dr Vertessy’s statement is appropriate for Australia in general.

  6. kenskingdom Says:

    Sorry for the minor quibble but according to Acorn, 17, 18, 19 Jan 1959 were 43.1,42.3, and 43.3.
    Your faith in manmade global warming is touching but not contagious. One site out of 112 is never going to be worth so much effort. I have better things to do so will leave you to your musings.

    • reichforthesky Says:

      Ok Ken, it is clear now you were referring to the Acorn data and my calculations are using the Raw data. If we use the Acorn data which does give 3 days in a row over 42 degrees in 1959 , this essentially changes nothing,

      i.e we have 3 days prior to 2000 and 5 days after 2000. this gives an average of 3/90 = 0.033 days per year while for years after 2000 the rate is 5/15 =0.33 days per year. This corresponds to a 10 fold increase in the rate of extreme days for this century as compared to last century!

      So what we are seeing is that the frequency of heat waves in Melbourne is increasing and interestingly the rate of increase is related to how extreme the heat wave is.

      I have said all along that I have been using raw data and not the Acorn massaged data. Consequently this raw data still contains the influence of urban heat island effects which may dominate and as a result, mean that climate change by itself is not likely to be the primary cause of the increase in heat waves for Melbourne.

      Finally I would like to sign off by saying that I appreciate your integrity in that you have accepted my contributions despite our diferences in opinions and interpretation of the data. I wish the discussions concerning this topic woud be conducted in the same vein at other venues on the web.

  7. harrytwinotter Says:

    I don’t understand some of your charts. Can you clear up these points for me?

    – Sydney (station 066062) 26/01/1915 the maximum was 40.4C yet there is nothing on your chart.

    – What definition of “decadal” are you using?

    – I plotted out the decadal counts of Very Hot Days for Sydney (station 066062) and get a slight upwards trend.

    • kenskingdom Says:

      Gday Harry
      Yes 26/1/1915 was 40.4, 28/1/1915 was 39.7, both above 95% threshold, but were isolated days, not part of a 3 day heatwave (27th was only 27.8 according to Acorn).
      Decadal means the running count of days meeting the criteria in the previous 3,652 days. Therefore the decadal count to 28 February 2015 is the total number of days meeting the criteria since 1 March 2005. 26/1/1915 is counted as part of the decade to 25/1/1925.
      I haven’t bothered with linear trends with such obviously non-linear data. I am interested in the actual count of very hot days as it changes over time, thus the decadal count which is a useful way of showing periods when hot days are more or less frequent.
      I hope this answers your questions.

Comments are closed.

%d bloggers like this: