Thursday, 2 February 2017

Weather Report for January 2017

Here is the final report in Gazette format.

 Compared to what appeared in the Gazette the average temperature is a bit higher as the last two days were hotter and the rainfall is a little lower since the BoM forecast of 1mm didn't happen until early in the morning of the 1st of February.  Note that the projected annual total is simply looking at the 10 year history of January compared to the rest of the year: I really don't expect it to be that low!

Clearly it has been a hot, dry month.  The question is how hot and how dry compared to our (relatively) recent history?   Details are provided below but to avoid subjecting you to excessive chart looking - unless you like looking at charts - here are some points:

  • The month showed the 3rd lowest rainfall since we started recording in 1985;
  • We recorded the most 'heat wave days' since temperature records started in 1993;
  • The number of days with temperatures >30oC was quite high, but not the most recorded and we didn't get to 40oC once;
  • Average maximum was above average; and
  • Average minimum was also above average and the reading on 24 January was our highest minimum ever.

Taking all of that together I am happy to say it was one of the hotter and drier Januaries we have recorded primarily because it set records or was in the upper reaches of many of the indicators.  Whether it was the 'worst ever' is a tricky question involving balancing warm overnight temperatures against not achieving absolute peaks for day time maximum.

Rainfall

We actually had some!  It totaled 9.4 mm, thanks mainly to 5.4 mm on the 20th of the month.  Clearly very far below both the 10 year average and the fall in 2016. 
Looking at the longer series - back to 1985 - there have been two Januaries with lower rainfall (1985 - 3.5 mm; and 2014 - 7.8 mm).  In both cases the rest of the year tried harder and the annual total was only slightly below average.  

Temperatures

The starting point is a chart showing the extremes for each day.
After a mild first few days the maxima climb gradually until the 19th when there is a short lived drop followed by another warming cycle. The minima float between ~12oC and  ~17oC until cloudy nights before a rain period prevents cooling.  From looking at records back to 1993 the minimum of 21.1oC  on 24 January was the warmest (by 0.1oC ) ever recorded.

On the subject of minimums, doing this analysis has led me to realise that I should refine the way I measure minima.  Explaining that would be too big a break from this narrative so I will do an ad-hoc post on the nature of the problem, a solution and the impact on results in the near future.

Heat waves

The tabloid press made much of a couple of hot days in Sydney which didn't even satisfy the BoM criteria for a heat wave (3 or more consecutive days with significantly above average maxima and minima).  For ease of computation I simplified that to 3 or more consecutive days with maximum >30oC  and minimum >10oC .  I then count the number of days in each month meeting those criteria.
The chart shows how much the number of heat wave days (HWD) in January exceeds the average (since 1993) and last year.  The number of days (22) was the highest recorded, although we have had several Januaries with 21 HWD - mainly in more recent years.  It is tempting - on ideological grounds - to say this is evidence of global warming but a time span of 24 years isn't really long enough for that (there is plenty of other, more robust evidence for such a conclusion).

How high the temperatures?

The foregoing shows that it was definitely a hot January.  However in terms of simple maxima, we didn't get over 40 once.  The next chart plots the number of days each January with maximum >30oC and of those how many were >40oC.
While 2017 was hot it was, on these criteria, merely a contender not the champeen!

Intuitively, what has happened this year is that we didn't get the (relatively) cool nights to break up the heat waves.  I can't however, at this stage, work quite how to demonstrate this.

Longer term stuff

The next two charts show that both maximum and minimum temperatures were well above average from 1993.

Humidity

The next two charts demonstrate that the 1500 Hrs humidity was quite low.

First the daily measures through the month.
As a rule of thumb I take 40%rH as a "reasonable value.  (By way of extreme contrast  I have been told that mid-afternoon on 18 January 2003 the humidity was 4%, but all I can find online is that it was "very low"!).  This January there were 21 days below that level.

For longer term data I can only use my own data back to 2013.
Clearly this year's value is below average, although of the 5 years available data 2 are below the 2017 value while 2 are well above it.

Wind

For a number of days this month the weather station has not recorded gusts.  I have no idea why this is so since:
  • temperatures, humidity and rainfall have been recorded so the station is communicating satisfactorily; and
  • the anemometer is spinning, so it isn't the evil spider webs jamming things.
To give an indication of the months draughtiness I have taken the BoM Canberra records of average 1500hrs windspeed and show below the 73 years average and the value for 2017.  (It is relatively hard to get monthly values for past months so I will start this series off now.
I will say no more than 2017 appears to have been quite windy.

No comments: