Coldest Month in 10 Years

Update- more detailed explanation below.

I have been monitoring minimum temperatures and rainfall since 2003 for 10 locations in South and Central Queensland.

These are: Rockhampton, Clermont, Longreach, Charleville, Cunnamulla, Surat, Oakey, Taroom, Maryborough, and Amberley.

If you thought August was cold, you’re right.

I have been calculating temperature anomalies using my own method, which is:

The difference between daily raw Tmin and the running 30 day mean of long term monthly means, both converted to Kelvin (to reduce the relative size of winter variation), expressed as a percentage of the monthly mean.  This smooths the transition from month to month of the daily anomalies.  I then calculate a 30 day running mean of these anomalies for the 10 locations.  (See below.)

On 21 August this value reached -1.273 -1.282.   The next lowest was on 25 August 2008 when it reached -1.252  -1.261.  (Made a small correction!)

Here’s the plot of the minimum percent anomalies from 2003 to the present:

The 4th order polynomial trend line shows the past, not the future, but there is no doubt that this is the coldest month and year of this decade.

It is not, however, the coldest 3 month period.  Here’s the 90 day running mean:

And 365 day running mean:

The trends (for this decade) are similar.

Update: My method is rather idiosyncratic and needs further explanation.

Download from Climate Data Online raw daily Tmin.

Convert to Kelvin (Celsius + 273.2) (TminK)

Enter BOM’s long term monthly mean for each month. Convert to Kelvin.

For each day of each month calculate the centred 31 day running mean of the long term monthly mean. (MeanK).

Calculate daily % anomalies ((TminK – MeanK)/ MeanK %).

Calculate 30 day, 90 day, and 365 day means.

Here’s a plot comparing the normal anomaly calculation with mine.  Notice how mine is smoother, but also is more conservative.  The normal (blue) trend is greater.

Vertical gridlines are at 30 day intervals.

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7 Responses to “Coldest Month in 10 Years”

  1. Ray Tomes Says:

    It seems that the title is misleading. To the extent that I understand your description of your calculation (it is not clear) it seems that you are saying that temperatures are lower than recent average trend by more than any other time, which is not coldest.

  2. Ken Stewart Says:

    I’m sorry for the confusion- no, the anomalies are calculated from the long term averages (as published by BOM), not the recent years. The difference is that I smooth monthly transitions, use Kelvin, and calculate the percentage difference. The 30 day mean of daily anomalies is lower than at any other time since 2003.
    Does that make sense? I will update shortly with a clearer explanation.
    Ken

  3. Geoff Sherrington Says:

    Ken, some images are not showing, just a border with the usual no-show symbol top left. Could be my end.

    In general, do all 10 stations show this trend or do you have some mavericks? Would be nice to show cloudiness data if that was collected.

  4. Ian George Says:

    Statement from the BOM’s recently released August summary.

    ‘The Australian diurnal temperature range (the difference between maximum and minimum) anomaly was the highest on record for August and the third-highest on record for any month; the anomaly was 2.32 °C.’
    Yet the max temp anomaly was +1.49C and the min temp anomaly was -0.83C. Surely the above statement is a mistake? Seems if they added the two together and deleted the minus sign.
    Shouldn’t the range be around +0.33C?

    Summary here:
    http://www.bom.gov.au/climate/current/month/aus/summary.shtml

    Seems even if it is cold, they can make it the hottest August on record. I wonder if the ABC et al will be quoting this.

    • Ken Stewart Says:

      Hi Ian
      No, they’re right. The range is always positive (unless Tmax is less than Tmin!)- from -0.83 to +1.49 is 2.32 C. The problem for AGW is that DTR should be decreasing as nights and winters get warmer due to greenhouse warming. High DTR and decreasing Tmin, if they continue, would be an embarassment for BOM and no doubt we would see some more adjustments! By the way, the temperatures I have downloaded are real raw- haven’t yet been “quality controlled”-so BOM’s final figures are very likely to chnage and may prevent this problem.

  5. Ian George Says:

    Whoops. I assumed it was the average mean temp. Pays to read it more carefully.
    Thanks, Ken.

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