Wednesday, 1 November 2017

Weather Report October 2017

There have been a couple of trivial changes to the report published in the latest Gazette, due to that report being compiled before the exact end of the month.  Here is the final table.
Overall the month had good rain and mild maximum temperatures.  Humidity and wind were almost exactly on average levels.  The most unusual aspect of the weather was the relatively warm overnight minimums.  

Rain

The month ended with a couple of very useful falls, at my station 27mm and 10.6mm.  This meant that for the second time this year we ended with more than 50mm for the month.  We also exceeded the fall in October 2016 and the 10 and 34 year average falls.
During the rain around 6 pm on 26 November Frances opined that it was about the heaviest she had seen here.  (I was elsewhere at the time.)  She was not greatly mistaken: my weather records contain 1751 records of a rain rate >0 mm/hr and on this occasion the rate was 135.6 mm/hr, the 7th highest rate recorded.

As will be obvious from the previous chart this has been a very dry year.  Exactly how dry is a matter that can be assessed in a number of ways.  My usual approach is perform a pro-rata approach:
  • so far this we have received x mm and on average we'd have received y mm;
  • on average we get z mm in a full year;
  • thus we could expect x*(z/y) mm in the whole of this year.
Following that approach gives the estimate of 420.6 mm shown in the Gazette report above. There has only been one year (2006) since 1983 with lower rainfall than this.  Another approach is say the next two months will be average, and add those averages to the cumulative fall to date, which gives a total of 489 mm (as November and December are typically wet months).  A further approach is to assess the trend in cumulative rainfall to date and project that forward 2 months as illustrated below.  That gives a total of just under 400 mm for the year.
Whichever approach is followed we end up with a dry year: perhaps the driest since 1983 (if below 409 mm) or the 4th driest if we get average falls in the last two months.  

Temperatures

In essence a fairly benign set of temperatures: like Baby Bear's porridge, not too hot and not too cold.
Indeed the average maximum temperature was a smidgin (sorry about the technical term) above the average since 1993.
 However the average minimum temperature was noticeably above the long term average minimum
The warmth of the overnight lows is well demonstrated by the number of high minimum values.  Before this month I had 744 observations of a minimum temperature, of which 42 (~5.55%) were  > 11oC.  Of the 31 days in October 6 (~19.3%) were 11oC.

As any Monaro tomato grower will attest October is really the last month in which frosts could be expected.  This year we had:
  • no hard frosts (below 0oC) while the average is 2 such minima
  • 2 light frosts (minimum in the range 0 - 2oC)in contrast to the average of 10 such days.
October is also the first month in which temperatures above 30oC could be expected (on average there are 0.9 such maxima per year.  This year no day was that warm.

In summary fairly normal maxima, but warm minima.

Humidity

As usual the value for 3pm humidity varies greatly over the month.
 I was initially surprised at the number of values below my "normal" level of 40% rh.  However looking at the longer term (in this case 4 years observations) the value for this month was only a little below average - which is in itself raised by the very moist October 2016.

Wind

The month was about normal for average hourly gust, but much less draughty than August and November.
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