August weather report

At least we got some rain this month!  Overall a wintery month with low temperatures on occasion and a couple of light snowfalls.  Here is the Gazette report: it is identical to that in the hard copy.
As explained in this report last month, and the hard copy report, for temperatures I will report here for the period since 2010, while for rainfall I'll continue to over the whole period since 1985.

Rainfall

The fall totaled 37.2mm with non-trivial amounts on 8 days. It is a bit of a worry when getting 70% of average rainfall seems like a Good Thing!  The first chart compares this year with last plus an 11 year average (all at our house).
 The second chart is a pro-rata projection of the rainfall to date.  Unfortunately it seems to have settled down to about 350mm for the year, compared to a long term (since 1985) average of 670mm.
 My records suggest this drought actually started in January 2018, meaning that it has now lasted for 20 months.  The next chart shows 20 month and 19 month totals since 1985.  About the only good thing in this is the fact that the 20 month total is relatively higher than the 19 month total.  It is still the lowest on record.
The last two months are the lowest 20 month values in the series.  They are followed by several months in the 2003-04 drought (aka the Millennial Drought).  In summary, these data confirm the view of a friend that the country is in as bad a shape as it ever has been.

Looking more closely at the chart above I wondered if this current drought actually started in 2012 as that was the "high point".
Applying a linear trend shows a quite strong downward trend.  Applying a 4th order polynomial trend gives a slightly different picture.  That suggests a retreat from the (unusually?) high value of March 2012 until early 2013 followed by a fairly stable level to mid-2016 at which pont the downwardslide begins.

Temperatures

The daily extremes were a little strange, with the highest temperature occurring early in the month and the lowest late in the month.  The average temperature shifts around a little on a daily basis but overall has no trend across the month.

Maximum temperature

The value is very similar to last year and down a little on the average since 2010.

Minimum temperature

As has been the case recently (even after deleting the unduly cold readings prior to 2010) the minimum is a little above average.  I shall return to this in talking about mean temperatures below.

The coldest minimum of the month was -6.3oC.  This is the lowest temperature so far in 2018, and the third lowest August temperature since 2010.

A key aspect of the cold weather is the number of frosts.  A light, or ground, frost occurs when temperature at the ground is 0oC whereas a hard, or air, frost occurs when the temperature at the height of a standard meteorological screen is 0oC. There are many ways of presenting recent experience of frosts and the chart below shows the number of frosts of each type in 2018 as a percentage of the average.
We recorded above average light frosts, but below average hard frosts.  

Average temperatures and temperature range.

There are two ways of calculating average temperatures.  The most accurate is to take the average of hourly (or more frequent) temperatures across a time period.  The alternate approach, when the detail isn't available, is to calculate the average of the daily maximum and the daily minimum.  The alternate approach appears to understate the mean temperature in the Winter months (possibly by 0.3oC) so is probably best suited to "emergency" applications.

A particular use of mean temperatures is to calculate a temperature anomaly defined as the difference between the mean temperature for a period and the long term average mean.  I have calculated  two versions of the anomaly:
  1. using the period since 2013 as the base average (all data from my weather station, using themore precise method); and
  2. using the period since 2010 (data for 2010-13 using the alternate approach).

Allowing for the bias introduced by the alternate method in the colder months this analysis shows that mean temperatures are close to the long term average in recent months.

I was also interested to see how the range of temperatures varied through the year, and how this year compared with average.  I took the range as the average maximum for a month less the average minimum.  This chart shows the average range for 2013 -2017 and the range for 2018.
I haven't calculated a confidence interval for the average, but would be astonished if the values for 2018 fell outside it.

Humidity

The traditional 1500 hrs readings for relative humidity show the month to have been drier than average.  This is not surprising since, while rainfall in August was better than in several recent months, it wass till only 70% of the August average.
 Looking at daily readings I have plotted both the 9am and 3pm values.
For the 9am series I was struck by the string of low values from the 16th to the 21st but can offer no particular explanation for this.  WRT the 3pm series the value for the 30th surprised me greatly - that would be a low reading for mid-Summer!  However it appear that the value fell gradually through the day and no observational error seems to have occurred.

Wind

My subjective view was that it was a windier month than expected.  My data shows it to be above average as measured by average speed of hourly gusts.


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