Tuesday, 2 January 2018

Weather Report December 2017

It had wet bits and hot bits: must be Summer!

Rainfall

For the third month in a row we actually got some decent rainfall.  It totaled 96.4mm, which was 30.8mm above the 34 year average (and 15.7mm above the average for the last 10 years) and 18.8mm more than December 2016.
It rained on 12 days, with falls > 5mm on 7 days.  The rain was concentrated in the early part of the month with the dust being laid a couple of times later.
Overall we ended at 68.95% of annual average rainfall.  Not bad considering that at the end of September we were at 52.6 %.

Readers may be interested in this post comparing, at an annual level, rainfall and flows in Whiskers Creek.

Temperatures

After a cool start to the month we entered a period of cycles with a few hot days followed by a cooler change.

 Maximum temperatures

The average maximum temperature through the month was 27.4oC a little below the average since 1993 and below 2016.
During the month I had a closer look at the long term temperature series, with a particular focus on the maximum temperatures.  This caused me to wonder about some of the readings, where the temperatures seemed very high, in the period prior to 2010.  I have concluded that I should only comment on the period since 2010 as the series seems more stable since then.  In contrast to the chart above, the average daily maximum for 2017 was almost exactly on the average for the period  2010 -2016
A further aspect of maximum temperatures is that of heat waves.  I define a heat wave as 3 or more consecutive days with a maximum of 30+oC and minima above 10oC.  I then count the number of days in the month which form part of  heat waves.  

In December 2017 there were 6 such days (from the 11th to the 16th).  We got very close to another 'wave from 22nd to the 24th, but the 22nd topped out at 29.9oC!   The count of 6 was a little below average (but a count of 9 would have been a little above average).

Minimum temperatures

The average minimum for December 2017 was very much above the 24 year average.  (Minimum temperatures seem to be not so badly affected by the issues described in the post linked above under maximums.)
An interesting issue with minimum temperatures relates to timing.  The BoM operates on a day starting at 9:00am (I suspect for tradition and/or industrial relations issues) whereas my weather station records for a day starting at midnight.  (I could fix this by adjusting the time setting in my weather station to run 9 hours late or by fiddling with EXCEL or ACCESS but am not sure that is worth it.)  What this means is that a very unpleasant night with a high minimum under BoM rules is often followed by a day which cools down so that my weather station minimum occurs late in the evening.  This is illustrated in the following chart 
The BoM-defined minimum of 22.3oC on 20 December was the highest minimum ever recorded in the area. (I have 2 more "BoM definition" minima above 20oC and my predecessors have a further 3.)  It was followed by a cool change so that the default minimum on the weather station, recorded in the evening was 14.6oC. 

Average temperatures

I only have actual average temperatures for a few years.  For most months I have 4 or 5 years of readings so is probably worth reporting. Obviously December was above average for average, due to the very warm minima.

Humidity

A relatively humid month with a 3pm average of 46.5% in contrast to an average of 37%.
This was not surprising in view of the number of rainy days.  
I was surprised that the rH was no higher on the 29th -30th but presumably the humidity built up after 3pm on the 29th (the rain started about 10pm) and had dissipated by the 30th.

Wind

The data indicates this was a rather windy month with the average gust at 9.2kph compared to an average for the month of 7.3.
I have attempted to validate this using BoM data for Canberra Airport which proved rather difficult as there are several missing data values in that dataset.  I would have thought that wind speed was a fairly basic piece of information for an airport!  Given the deficiencies of my data (the site isn't really ideal) when looking at the average of daily maximum gusts (the variable shown by BoM) rather than the average of hourly gusts I have shown above:
  • the BoM data shows December 2017 as similar to December 2016;
  • my data is about half that of BoM, but broadly similar between the two Decembers.
This suggests that the winds were more sustained in 2017 than 2016.  The caveats noted above argue against making too much of this comparison!

The dominant wind here is from the W-NW.  However on hot days in summer a cool Easterly comes up from the Coast in the evening.  Just recently I have noticed that as it approaches the BoM Meteye site shows the westerly blowing over Carwoola with the Easterly somewhere on the Hoskinstown Plain.  We are at the mauve tear-drop and the wind is pretty negligible as we are more or less at the seam!

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