May 2019 Weather Report

In summary the month was quite close to average.  Rain was only a little below median and most temperatures were close to average.  Humidity was close to average.  The month wasn't too windy.

Possibly the most 'different' thing for May was how much better (ie higher) our rainfall has been so far this year compared to last year.  The vegetation is growing again: pity about the mosquitoes!

Rain

I have looked at the variability of monthly rainfalls and for most months the pattern is a small number of high values and a much greater number of lower values.  Another way of expressing this is to say that occasional very high values inflate the mean so below mean could be expected in most years.  Here is a comparison of mean and median (the value that half observation are below) rainfalls.(since 1975) for each month.

The median rainfall  is probably a more useful measure and I will use that in the future.

We ended up with 62.8 mm of rain with falls >1 mm on 10 days.  This total was a little less than May 2018 and 92% of the median value. 

We are currently at 77% of the mean for year to date.  Doing a pro-rata expansion of the year to date value gives an estimated fall for the year of 721 mm.  If that comes to pass it would be the 5th lowest annual fall since the series began in 1975.  This next chart shows how the pro-rata estimate has changed as this year has progressed.

I heard an interesting comment about the weather recently, looking at the current number of rain-free days which was very high in most Capital cities.  I have found it easier to calculate the number of rain-days.  (I define a rain day as >0.2 mm since a decent fog or dew gives 0.2 mm.)

We had more than an average number of rainy days but most of them had small amounts.

Temperatures

The usual overall view of the temperature is to look at the maximum and minimum daily temperatures.  These data come from my weather station.

The  chart clearly shows the very warm spell at the start of the month.  The impact of this can be summarised by noting that the average temperature for the month was 0.24^oC above the long term average.  Excluding the first 3 days makes the average for the rest of the month 0.22^oC below the long term average.

For temperatures the mean and median series are very similar for both minimum and maximum temperatures.  Since "Mean" is the usually accepted non-technical meaning of "Average" I will use means in this section.

Maximum Temperatures

The average maximum temperature at the BoM station on Betka Rd was 18.3^oC.  This is a little higher than both2018 and the average.

 The time series of average maxima for May is somewhat unstable but shows a slight (but with an r² of  0.13 not significant) upwards trend.

 Minimum temperatures

The average minimum temperature at the BoM station on Betka Rd was 9.24^oC.  This is of a little higher than 2018 and almost the average of 9.30^oC.

The time series for minimum temperatures resembles a profile of the Himalayas,  Not surprisingly there is no trend in these data, but is it interesting that the ups and downs are within a very tight band from 8.03 to 10.30^oC.

Humidity

The first chart of daily data is taken from my weather station.  Not surprisingly tis tends to have slightly lower readings for rH than the BoM station which is much closer to the sea, and more exposed to wind.  Nothing surprises me in this chart.

 For the comparison with longer term values I have used data from the BoM site.  The usual "Climate Data Online" which I use for temperatures and rainfall only has a restricted amount of data available.  I have been able to extract from that humidity data as:

• a series of averages for all years 1993 to 2010; and
• individual month data from February 2018 onwards.

 These are used to generate the following graphs.  Note that the vertical axis has been truncated which exaggerates the month to month variation,

My conclusion from this is that thus far 2019 is more humid than 2018 but overall about similar to the average.

Wind

I posted recently about the interpretation of the wind observations from my weather station and how they compared with the official BoM readings from Mallacoota.  My concluiosns were:

• Wind run is an appropriate measure to use as an indicator of longer term windiness,as it includes a lot of information; and
• The information from my Weather Station is a reasonable measure of relative windiness for Mallacoota.
• Wind run has a high correlation with the 1500 speed (which occurs after the day has developed a reasonable weather pattern) and the 1500 speed is available on a daily basis so I'll choose that as my standard indicator of current windiness from now on. 
• In 44 (73%) of cases the the wind direction recorded by my weather station  was within one compass segment (22.5^o) of that reported by the BoM site.  To my mind that is an astonishing level of consistency and indicates that the wind direction data from my weather station is also fit for the purpose.

Having got all that off my chest, what about some data?    This first chart shows the 1500 Hrs speed, indicating a couple of very draughty periods with relative calm through the middle of the month.

This next chart is pretty pathetic in showing the windrun, comparing a 15 year average from BoM for Mallacoota with the three months for which my weather station has data.  As I build up data the chart will get better!

 A rather interesting wind rose in which the slices of the pie reflect the proportion of observations of wind.  There were no observations for the directions ENE, E or SSE.I have marked the position of South outside the pie to reinforce the view that 3/4 of our wind comes from the Western half of the rose, but spread fairly evenly between those 8 segments.  (An eyeball of the BoM climate summary for May comes to a very similar conclusion: looking at the directions of max gust; 0900 and 1500 reading there are very few readings between NNW and S!)

I have just checked the maximum gusts for the month:

• Angophora WS: 54.7 kph at 2000 on 27 May
• BoM Mallacoota: 76 kph at 1933 on 27 May
• BoM Gabo Island: 98 kph at 2000 om 27 May

The relativities are as expected (possibly my WS has done better than expected) and the timings all line up very nicely!