Sunday, 4 September 2016

Discontent with seasons (part 2)

In my last post I discussed some issues relating to defining seasons in the Carwoola area.  I think I came up with a fair set of definitions in that post, and the idea of this post is to look at the most recent seasons to see what the data can tell us (or at least suggest to us).

In my usual Weather reports I cover Rain, Temperature, Humidity and Wind.  For the last two I only have a short series of data (and the wind data is very dodgy due to the site not being perfect - OK, hopeless - for exposure to wind).  So this will only cover the Rainfall and Temperature attributes.

Another change to usual is that I will try to summarise everything simply in an initial Summary section and then put the methodological material and charts in a Background Section.

Summary

The thought to do some work on seasons came about through a comment on the recent wet weather that "They said this was the wettest Winter for xx years."  So I decided to look at rainfall and temperature for the seasons to see what is occurring.

Looking at rainfall for the last four seasons (as I define them) I note that last Spring (ie September -October 2015) was well below average.  Summer and Winter were about average while Autumn  (ie March - June 2016) was well above average. However Autumn was only the 7th wettest since the dataset began in 1984 due mainly to a very dry April 2016.

In the background section I cover four ways of looking at the temperature (at a time of year when coldness is on our mind - it might change when looking at Summer!).  
  • Maximum temperatures are very similar to the seasonal averages throughout.  
  • Spring 2015 and the following Summer were close to average for minimum temperatures but Autumn had the warmest average minima I have recorded for that Season.  Looking at the minima for individual months, all were 2 -3oC above average.  I am unsure why this was so for April which was - as noted above - very dry.  Winter was also less cold than average but less markedly so than Autumn.
  • The final two measures are for frosts, which primarily affect Winter (not surprising, as that is how I have defined Winter!). 
    • Airfrosts, with minimum below 0oC, were pretty close to normal for the last 4 seasons.
    • Groundfrosts, with minimum below 2oC  were average in Spring 2015, but well below average - the lowest number of frost days for the season - in Autumn 2016.  April and June in particular were well below historical levels.  Winter was below average.
Taking the two frost measures together suggests that Autumn was about normal for very cold days, but the relatively warm average minimum is explained by the lack of moderately cold days (minimum between 0oC and +2oC).

In summary the weather for the last 4 seasons has had a little bit of everything, but the last two seasons have been damper and less cold overnight than average.

Background Section

A first point to note is that when I first started to look at seasons I concluded that looking at whole months was possibly taking too coarse a view.  Perhaps (eg) the first week of June is still Autumnal on average but the last week is rather Wintery?  However I decided that this was too complicated to look at, especially given the limited data I had (not to mention limited skillset)!

I still hold to those positions.

There are many ways to present the data for seasons including a simple time series such as I used in the previous post and reproduce here.

It is however hard to make much of that.  In the original I assessed the trends and none of them were statistically significant.  There are a few other approaches that I might play with before the end of Spring but the basic presentation I am using this time is to present values for the last four seasons contrasted with the maximum, average and minimum values of the statistic for the life of the series.

I decided to put Summer first in the charts, as it covers the first two months of the calendar year, and although that looks a bit daft when described in text, will stick with it pro tem:  I decided it was daft, so have now changed to have Spring first and the horizontal axis of the charts is now in this order.  (I'm not sure if I will change the order each season as fiddling with column orders has much scope for stuffing things up!)
  • Spring (Sep, Oct 2015)
  • Summer (Nov, Dec 2015; Jan Feb 2016);
  • Autumn (March- June 2016);
  • Winter (Jul, Aug 2016)
I am basically putting the charts here, without commentary, to support the conclusions summarised above.

Rainfall


 Temperatures
Maximum


Minimum

Airfrost days

 Groundfrost days

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