Tuesday, 1 August 2017

July weather report

In summary a cool dry month with a pleasant drop of rain on the last day.

There have been a few minor changes, which are highlighted in yellow in the graphic below, since I submitted the weather report to the editor of the Gazette.  The most significant is that we received 13.2mm of rain on the 31st  rather than the 10mm which I had guessed.

Rain

Essentially, it was nice to get some!  The fall on the 31st was the first significant rain for 10 weeks.  July was still well below average (and last year) but at least it was a good steady drop.
Despite the late falls the total for the year to date is barely over half the average fall to the end of July.
I tried applying a projection to the 2017 series but it proved to be very sensitive to the type of projection used which I took to be an argument against doing anything fancier than the simple pro-rata estimate quoted in the Gazette report.  It is notable that that estimate comes out at 38mm below the lowest annual total recorded in the area going back to 1984.

Against that background I have been astonished to see Whiskers Creek continuing to flow through the month.  The rain on the 31st increased the flow a little, but it seemed that the gentle and steady nature of the falls meant the moisture went into the ground rather than running off.

Temperature

In addition to comments about the lack of rain I have seen various comments about this July being the coldest month since whenever.  As usual with comments about the weather the truth of that statement depends upon how you measure it!

The maximum temperatures were "reasonable" for the month hovering around 10oC for most of the month and the minimums were cool but not dramatic.
Looking at the longer term averages the maximum temperature for July 2017 was insignificantly above both the average (since 1993) and 2016.
 The minimum temperature was almost on the average but markedly below the value for 2016 (since last year was very cloudy).  So it really wasn't that cold.
 To really get a handle on cold I look at frosts.  The first measure is the number of days the minimum temperature is below 2oC.  We had 25 such days in 2017, slightly above average.
 The number of air frosts (below 0oC at screen-height) was well above average.
Bringing these frost measures together shows that it was a bit colder than average.
My conclusion is that it was a bit colder overnight than it has been recently.  However looking back to the early days of recording in the area for several Julys in the period 1993 to 1998 there were 26 or 27 air frosts and the average minimum was below -4oC.  So in summary a cold July but not one that threatened any records.  The glaciers are still retreating!

Humidity

Not surprisingly in view of the comments about rain, the month (like June 2017) was low on humidity when compared to the average (since 2013) and last year..

Wind

Judging by what I encountered on my (too infrequent) jogs in this area it was quite a windy month.  The data (with all its deficiencies due to site issues) suggests it was about average for July.



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