Sunday, 4 September 2016

Discontent with seasons (part 1)

Some different patterns of the seasons have been recognised by various folk (but none make Winter into Glorious Summer):

I have occasionally in the past burbled about how the traditional 4 x 3 month seasonal pattern doesn't work too well in this area.  This has been well discussed by Timothy Entwhistle in his book "Sprinter and Sprummer".  Much of what he says is based on the behaviour of plants, on which he is an expert.
He concludes that 5 seasons are needed:
  1. Winter: June and July
  2. Sprinter: August and September
  3. Sprummer: October and November
  4. Summer: December, January, February and March
  5. Autumn: April and May
In some earlier musing on this pattern I concluded that while this might be good for Entwhistle's haunts (mainly Sydney and Melbourne) a better pattern for the Monaro was sticking with 4 seasons but with a slightly different mix of months to the traditional.
  • Summer is defined as the months with maximum temperatures frequently over 30oC.  Looking at some other data I have, this would include November and December as well as January and February.
  • Winter is defined as the period when >50% of days have an air frost.  Thus July and August.
  • Spring is between Winter and Summer (thus September and October). 
  • Autumn is between Summer and Winter.  This is the remainder: March, April, May and June.
Having now got a much longer weather data set to look at I have thought about this a bit more, including some reflections on what I have observed of plants flowering and (particularly) birds breeding or migrating.

To deal with plants first.  In this area Wattles (Acacia sp.) start flowering in profusion in late August.  The first native orchids (eg Cyanicula caerulens) appear in very late August but the family begin to hit their straps in September.  The Fabaceae (beans) do have some blue species around in August but probably don't really get going until October.  So IMHO plants argue for the peak flowering season (ie Spring) to start in September and everything starts to calm down after October, as the warm weather fries everything at our elevation.  I don't think flowers help much with seasons after then.

Breeding birds begin to be reported in July and peak in September/October.  
As many of these records - particularly those later in the year - are of dependent young, I suspect those months are really the peak breeding time, and things are calming down by November.  The tail-off in April-June suggests a similarity between those months.

WRT to migrants, very few species return in July, some in August, several in September and October and some - mainly ones that seem to be extending their range from the Coast (global warming anyone?) - turn up in November.  
Again a vote for September and October being linked, with November being cut off, with the migrants being different in nature.

At the other end of Summer the peak month for migrants-departing is February.
That effectively includes February in Summer (migrants present still) and March in Autumn (many migrants have moved on).

I haven't got any definitive records of when I have seen reptiles or invertebrates.  However reviewing my blogposts suggests that reptiles (at least the larger ones that pause/pose for photos) start to emerge in September and have pretty much retreated by the end of March.  Invertebrates are mostly going to be around when they have food so are linked to flowering (except for coprophagic species and even they tend to get honoured by spotting "the first blowfly of Spring" in September).

These bits and pieces of real life confirm my view stated above (and pictured in the opening graphic).  
The detail which caused me the most thought was whether to treat June as the last month of Autumn or the first month of Winter.  Using my criterion for Winter of incidence of air frosts, applied to the longer time series gives an interesting result:
For air-frosts (minimum below 0oC), June is very similar to August (but well below July).  However when ground frosts (minimum below +2oC) are considered June is well below both the following months).  So it is lineball.

A further approach is to compare the average annual extrema for June with the adjacent months.  The following messy charts try to do that.
None of the relationships are strong, with even 4th order polynomial trendlines failing significance tests.  The relationship, if it can be called that, for maxima is slightly stronger between May and June than between June and July, while for minima the opposite applies.

My end point is that I prefer to regard June as the tag end of Autumn because:
  1. it is supported by being a true break in the breeding season for birds;
  2. the maximum temperatures are slightly less toxic in June; and
  3. sheer contrariness.
Applying my preferred definition of the seasons for average maximum temperatures gives the following graph ...

... which looks reasonably sensible.  Part 2 of this post will look at other aspects of seasonal weather patterns for this area

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