Friday, 1 June 2018

Weather Report May 2018

I have updated the Gazette format weather report with the final data making slight changes to average temperatures (the last two days of the month were cool) and rainfall (there was enough fog on the 30th to condense as 0.2mm of rain).
In summary a(nother) warm dry month, although less of a departure from longer term averages than April this year.  The most outstanding feature, as has been the case for several months, has been the relatively high minimum temperatures.

Rainfall

The total precipitation for the month was 23mm, of which 15.8mm fell on 4th of May.  Two other days had non-trivial falls.  As expected for Autumn there were a few fogs which condensed to give records of 0.2 or 0.4mm of precipitation.
 May is, on average, the lowest rainfall month in this area.  However it is also very variable.
The average fall for the month is 36.5mm and removing the two 'outlier months' of 1995 and 2010 still gives 31mm which is well above this year's miserable effort.

Looking at the year to date we have recorded 137mm which is the second lowest since 1985 (2004 was lower at 112mm).  It amounts to almost exactly 50% of the average rainfall to date and doing a pro-rata projection gives us 371mm for the year which is ungood.    This next chart shows how the projected annual rainfall has varied, on a daily basis through the year,
Looking at a website which offers 12 month forecasts of rainfall compared to average the next few months are rated as close to average with the first forecast of above average falls not appearing until next February!  The water carters are going to be busy!

In terms of run off, Whiskers Creek ran heavily following the heavy rainfall on the 4th, but had dried up within a week and stayed dry for the rest of the month.

Temperatures

The daily extremes and average are shown below. 
The averages shown here are the mean of hourly readings through the day.  As a quick estimate it is possible to approximate this by calculating (max+min)/2 but, for this month at least, that gives an unduly high estimate.

Maximum temperatures

The average maximum temperature was slightly above last year and the average since 1993.  However the anomaly (ie the difference between current and average values) is much less than for April.  

The long term average maximum temperature for May varies considerably ...
.. even for the period since 2013 where reading are from the same device at the same location.

Minimum Temperatures

The average minimum temperature for the month (2.48oC) was almost the same as last year, but well above the long term average (1.32oC).  This continues a pattern of above average minima evident for many months.

A chart of average minima for May shows considerable year-on-year fluctuations for most of the series, but has several elements to an increasing trend in recent years.
My other commentary about cold weather reflects the number of days with a frost.  An average May features 18 frosts overall (screen temperature less than  2oC) of which 10 would be hard frosts (screen temperature less than  0oC).  The equivalent numbers this year were 13 and 6: further emphasising the relatively mild month.

Humidity

 The average 1500hrs humidity was a little below the average value and rather more below the value for last year.  This is quite consistent with rainfall amounts.
 The daily readings for humidity at both 9 am and 3 pm are reasonably stable across the month with the exception of very high 3 pm readings for the damp period of 10th to 12th when some rain fell.

Wind

The average hourly maximum gust was somewhat above average for May and for May 2017.  However it was calmer that the first 3 months of this year.

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