August 2020 Weather Report

 Overall a rather mild, damp month.

Rain

Non-trivial rain (ie >0.2 mm) fell on 14 days, and the total recorded at my Weather Station  (WS) was 128.6 mm.  This is just over twice the median fall for August and puts the total fall for the year to date up to 135% of the YTD fall.   By the end of the month we were 303 mm in front of last year to that date (and indeed 154 mm above last year's miserable total).  Here is the basic comparison.

Estimating the annual rainfall by a prorata expansion of the fall year to date gives an estimate of 1,044 mm at the end of the month.  The following chart shows how that estimate has varied over the year.

If we end up close to that level it will be the 11th highest annual rainfall since BoM records began at this site in 1975. 

For August the BoM total was 155 mm.  It is difficult to do a day by day comparison due to the BoM habit of starting the day at 0900 hours, reflecting the history of manual collection of data at that time.  However the main differences are attributable to the event of 15 - 16 August where the centre of the rainfall was to our West: Gabo Island had a similar fall to my WS).

Temperatures

My overall assessment of the temperatures is the anomaly, the difference between current average temperature and long term average.  I use BoM temperatures to calculate this.  For August the anomaly ended up at +0.45 C so a little warmer than average.  The daily average was greater than the average for that day on 18 days and the most common situation was the minimum temperature above average and the maximum below average.  (This is to be expected in a soggy month, with cloud insulating at night and blocking the warming sun in the day.)

Here is the movement in the anomaly over the month.

A second approach to overall temperatures is the daily extremes.

The most notable extreme is that of 30 August which reached 26.8 at my WS.  This exceeds any BoM August maximum temperature .  However the Bom station is often somewhat cooler than my WS and on that date only reached 25.5 - which is still the 5th warmest August maximum out of 814 records.

The comparison of WS and BoM temperatures on the 30th is interesting.  The BoM temperature plummeted about 1300 hrs while the WS temperature dropped slowly as an overcast came in. (Both of these phenomena were confirmed by our car thermometer.)

The explanation was simple.  A sea breeze from the SE arrived at the BoM site and dropped the temperature 8 degrees in short order as shown below.  The WS was still under a NWly airflow and thus maintain its warmth.

Minimum Temperatures

The coldest temperature was 2.4C on the 5th. The average minimum temperature for the month was 7.19C at my WS and 7.07C at the BoM site.  The difference is insignificant, and at a daily level shows a mixture of positive and negative differences.  The value is slightly higher than 2019 and the average.

As indicated in the commentary about the anomaly for most days the minimum was above the average value for the day (all measured at the BoM site).

Maximum temperatures

A rather more difficult kettle of fish, as there seems to be a significant difference between the BoM site and my WS and previous investigations have shown a different pattern of differences between 2020 and 2019.  The two major factors appear to be:

1. The impact of the sea breeze (demonstrated above for 30 August); and
2. The difference between days beginning at midnight (my WS) and days beginning at 0900 (BoM).

The latter case is exemplified by 31 August where the WS maximum on a calendar day was 17.9C (recorded at 0100) and for a day starting at 0900 was 12.8C (BoM) or 13.2C (WS).  I have yet to work out an efficient means of rebasing the WS data to 0900.  Thus the comparison with the long term average is rather dubious (as the values for 2019 and 2020 are both from WS they are comparable).  

However the warmest day was 30th reaching 26.8C (WS) and 25.5C (BoM).  The latter temperature was the 5th highest record (out of 814).  The average maximum was 16.2C (WS) and 15.1C (BoM).  For what little it is worth here is the comparative chart.

Humidity

The daily humidity values were all over the place, especially for the 1500 hrs reading.

The comparison charts for the two times show similar pictures.  As expected the rH this wet year was notably than higher than the drought of 2019.

Wind

As measured by the daily average run the month was fairly calm.

The daily run was extremely variable.  I have also shown the rainfall to look at the extent to which the wind was the passage of a front.  I conclude that fronts were not the key factor: the longest run reflected high pressure to the South interacting with, and blocking, an East Coast Low.  Lots of wind, but limited rain.

The direction of the wind was very much from the Western half of the compass.  The notable exception is the ESE segment reflecting a low pressure system which sat in Bass Strait for 2 days pumping in rain (my WS was just outside the main impact of this.