Tuesday, 30 August 2016

Does it always rain on Wednesdays?

The weather for Wednesday 31 August 2016 is again looking rather damp.  In the last few weeks it has appeared that it always rains on Wednesdays.  Or at least it is always forecast to rain on Wednesdays!  Is this true?

Fortunately I have a database that can address this in terms of what actually happened (especially since I have discovered an ACCESS function that delivers the day of the week from a date!)  I can't do anything about the forecasts x day of the week since BoM doesn't release to the public their historic series of forecasts.

Cutting to the chase, the answer is that as far as I can tell rain doesn't occur more frequently on Wednesdays than other days of the week. Nor is the rain on Wednesdays heavier than for other days (on average).  Another conspiracy theory bites the dust.  However: check the Update towards the end of this post.

I began by looking at records for our house from 12 November 2013 up to July 31 2016.  That is a period of 995 days.  Determining when it rained is a tad tricky, since condensation from a decent fog registers as 0.2mm.  Thus I have said it rained if my weather station has recorded >0.2mm of precipitation.  On that basis I have recorded rain on 244 days (24.5% of days).  Here is the distribution by day of the week.

day
# Rainy days
Sun
31
Mon
30
Tues
30
Weds
42
Thur
36
Fri
35
Sat
40
At first glance it looks as though Wednesday is an unusually wet day.  However the difference is not statistically significant.  

Another way of looking at this could be the average amount of rain on a Wednesday:
day
Average mm
Sun
2.66
Mon
1.51
Tues
1.70
Weds
1.89
Thur
1.44
Fri
1.56
Sat
2.25

While lagging behind the days of the weekend Wednesday is a little higher than other weekdays.  However the average of averages (if that makes sense) across the 7 days is 1.89 - so Wednesday is right there!

But wait, there's more.  Not a set of steak knives, but another set of data, collected by another Carwoola resident from 1 January 1993 until 29 February 2016.  That is  8460 records of which 19.3% had rain: obviously some drought(s) in there!  Here is the information on number of rainy days per day-of-the-week from the longer term series.

Day
# Rainy days
Sun
241
Mon
233
Tues
223
Weds
233
Thur
229
Fri
231
Sat
240

The average number of rainy days per day-of-the-week is 233 so Wednesday is spot on the money.  Again the weekends get a little more than average.

Update
After the initial posting of this topic I was asked about a shorter term view "An interesting table would be rainy days this winter."   As I define Winter to be June -August (incl) answering this had to wait until the end of August!  But here is the requested table:

Day # Rainy days
Sun 3
Mon 5
Tues 7
Weds 8
Thur 4
Fri 6
Sat 3
.. and here is the requested data as a chart, with a summarising trendline added:
The trendline has an R2 value close to 705 so shows a pretty good fit.  In summary, there does seem to have been a bit of bias towards Wednesdays being the rainy day.  This is probably emphasised by several Tuesdays also being wet, getting one into a soggy frame of mind before Wednesday.

Another way of looking at this issue is the percentage of days in a period on which it rained, and the percentage of Wednesdays on which it rained.
Time period   % of  days with rain   % of Weds with rain
Since 1993 19.30 19.28
Since 2013 24.50 29.55
Winter 2016 39.13 53.26

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