Weather Report June 2018

The month featured cool days and (relatively) mild nights (for most of the month.  It was still dry, continuing an event that started at least 18 months ago, and possibly 7 years back. month.

In my analysis, based on biology rather than astronomy (or astrology) June is the last month of Autumn and a seasonal analysis will be done in a later post.

Rain

We recorded non-trivial amounts of precipitation on 6 days across the month, with the majority (12.8mm) occurring on 28 June.  The monthly total was 33.2 which, while still below average, was a marked improvement on the desperate 1.6mm recorded in 2017!

By the end of June we are 160mm down on the average year to date. 😭 However we are now only 10mm down on the same period last year. 😕!

Looking back at past records the current very dry spell began in January 2017 - a period of 18 months.  I calculated the total rainfall over that period as 666 mm - the number of the Beast!!!  This is the lowest 18 month total recorded since Carwoola records began in  May 1984.  The previous period of very low 18 month aggregates was in 2003-04.

On looking at an initial version of the chart I was struck by the apparent downwards slope since a peak in about March 2011.  That coincides with comments by a birding friend, with connections to farming in Victoria that there have been 7 dry years in a row.  He thought this explains some unusual bird sightings on the Coast.  Here is a chart of 18 month rainfall aggregates since March 2011.

Even applying a very basic linear trend function gives a significant downward trend.

The final comment about rainfall is that pro-rata projections continue to forecast the lowest annual total OF ALL TIME for this area.  (Not for everywhere - I suspect the more arid places in the Simpson Desert are drier some years.)  It is currently ~370mm.

Temperatures

It is clear from this chart that maximum temperatures stayed largely in a band between 10^oC and 15^oC while the minima clearly show a cold snap from 21st to 26th.

Maximum Temperatures

The mean daily maximum for June was 11.1^oC slightly below the average since 2010 and rather more above the mean for last year.

It was also well below the long term average.

Minimum Temperatures

The mean daily minimum temperature was +2.1^oC well above last year and the long term average, both of which are very close to 0^oC (they are one the chart but the columns are too short to be seen).  This continues the run of high minimum temperatures going back to January 2016.

The long term average minimum for June is quite variable. This is understatement!

Although the value of R² is not fantastic I suspect it is strong enough to confirm the warming trend.  

Further evidence of the warmer minima is the low number of frosts in June.  This chart shows the proportion this year of the average number of frost days.

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Mean temperature

There are two ways available to measure mean temperature.  The first, and preferred method is the simple arithmetic mean of all (in my case hourly) readings during the day.  Where that is not available it can be approximated by the average of the daily maximum and minimum. (I'll explore this a little more in an ad-hoc post later: in the meantime I'll note that the comparison of the two approaches is quite good for June 2018 apart from the cold snap, where the true reading is significantly lower.)

Our mean temperature for June was just about on average, and somewhat above last year (which was probably depressed somewhat by the very dry weather meaning the cloud blanket was not present.

Mean temperature appears to be the basic tool for indicating warming climate.  This is demonstrated as an anomaly, calculated as (current period mean - average mean). As I only have exact means available for the period since 2014 I have calculated the anomaly using those averages and the ones calculated by differencing going back to 2010 (the maxima prior to 2010 have some issues - due I suspect to site shading problems).

The difference between the two sets of anomalies is not great but indicates that January and April were the two particular problem months.

Humidity

My standard time for comparison is 15:00 as this is usually the time cited by the media.  Possibly this reflects it being more variable than the 09:00 values also reported by BoM in their summaries.  This chart shows June as a little below average, but well above the very dry value from 2017.

 I have plotted below the 0900 and 1500 values for the last month.  As expected the 0900 values are typically higher than the 1500 values and pretty much confined to a band between 75% and 95%.   The 1500 values show a particular drop for the cold snap 21st-26th with clear days.

Wind

As usual I am dubious about the quality of the wind data as it continues a long series of recent months being windier than average.

I am beginning to suspect that this is a result of the dry weather.  The steps involved are something like:

1. Spiders need moisture to make webs:
2. Less moisture, less webs;
3. Specifically less webs around the spindle of my anemometer;
4. No webs supplying friction, so anemometer spins faster;
5. Thus appears windier!