Thursday, 3 May 2018

April 2018 Weather report

A few numbers in the draft report shown in the hard copy Gazette have changed since that was produced.  They are shown in red below.
From watching the records as they occurred my impression was that the month was dry and relatively warm.  Going though the data has confirmed this with:
  • very little rain (effectively one rainy day);
  • warm temperatures (both maxima and minima averaging out at ~3oC above the long term average);
  • low 3pm humidity; and 
  • above average gustiness.
I think people have been very responsible in not lighting up their piles, and we have been very lucky not to have had a nasty bushfire.

Rain

Basically 1 day of rain in the month giving a total (18.6mm) well below average and below last year.
However, looking at records since 1985, this was not extraordinary.  Of the 34 years, 22 have recorded less than 40mm.
The more worrying issue is that this year in total is very low.  Year to the end of April we are at 42% of the average.  It is the third lowest fall to the end of April.
The YTD value can be projected forward in many ways giving estimates for the year in the range 300 to 350mm.  Here is one approach.
The take away message from this is that we usually get an average of 53mm per month.  This year (to date) we have scored an average of 23mm per month.

Temperatures

My starting point is the daily extremes.
As would be expected both series tend downwards over the month.

Maximum temperatures 

The average maximum (23.3oC) for April 2018 is well above both the long term average (+2.9oC) and the value for last year.
 Looking at the time series of average maxima shows this months value to be above every year since 1993, except 2005 (when I suspect there were shading issues with the site at that time).
Looking at the average maximum since 2010 - more consistent sites - gives a slightly higher difference (+3.2oC).

Minimum temperatures

As with the maximum temperature, the average minimum (8.4oC) is well above average (by +3.8oC)  and last year.
The long term picture shows a continued warming trend - even the linear trend is a significant rise.
My final comment on low temperatures relates to frosts.  On average April would feature ~8 minima with light frosts, above 0oC but below 2oC: this year we have not had one.  Obviously we have not had any hard frosts (below 0oC).

Average temperatures

It should come as no surprise that the mean temperature is also above average.

Humidity

The standard for humidity is 3pm (1500 hrs).  At that time the average humidity or April 2018 was well below average.  This is quite consistent with the pathetic rainfall.
The alternate time cited by BoM is 9am, which I believes relates to the time they normally take the first readings of the day.  For this month I find 6am to be more interesting as that is the time of day when fog (or at least mist) can form.  That is reflected in the number of values >90% rH at that time.  The relative dryness of the month is indicated by number of readings below 40% rH (the value of 15% on 7/4 is very notable).

Wind

My usual cautions regarding the poor siting of my anemometer apply.  That being said the month was above average in windiness, but less so than some recent months.


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