Australian Football is an outdoors game played in winter, and as such, weather plays a major role in how the game is played. Generally, adverse weather conditions like rain and wind are assumed to reduce scoring by making it more difficult to gather and use the ball effectively.
This week, HPN analysed Bureau of Meteorology data to see if the effect of rain on footy games were backed up by the data, or if the conjecture was more myth than fact.
To do this, HPN looked at daily rainfall totals for a reliable weather station near each stadium from the Bureau of Meteorology’s Climate Data Online service. This shows how much rain fell during the 24 hours of the each game since 2000, measured at 9am (thus the Sunday measurement covers a Saturday gameday’s timeframe). In order to break down rainfall into useful “buckets”, some definition decisions had to be made.
“Heavy” rain has been classified as above the 95th percentile of all matches in the time frame, meaning days with 10.4mm or more rain. “Medium” rain is above the 85th percentile (more than 3.4mm), with “light” rain covering all remaining rainfall. We excluded Shanghai and Wellington games from analysis.
Daily rainfall being measured the following morning probably slightly biases the data, because it is counting days where it rained after the match as a wet game. This means there is likely to be slight overestimation of the frequency of rainy games, and an underestimation of the impact of rain due to those false positives. Dewy night time conditions also escape this analysis, or games with heavy wind, but work in Footballistics demonstrated that scores are a little lower at night.
Rain and scoring
The first question that has to be answer is whether rain affects scoring rates, and if so how much.
The answer is unsurprising – yes, scores are lower when rain hits play.
Essentially, when there’s any sort of rain, teams should be expected to score at least one goal less than dry conditions, rising to nearly three goals less in the heaviest rain 5% of matchdays. This decline comes through a combination of fewer scoring shots and lower accuracy.
Margins would seem to generally be reduced as well, but there’s a little quirk here. In the “medium rain” bracket, containing about 260 matches where daily rain was between 3.4mm and 10.4mm, there’s been higher average margins than any other group of games. There’s been a number of substantial blowouts at the MCG and in Adelaide and Geelong under these conditions, pushing the average up. Assuming this isn’t just random happenstance, it suggests that there’s a level of heavy but not torrential rain which can leave the stronger team able to score relatively easy against a sodden and demoralised opponent.
Rain makes bonus ladder points a bad and unfair idea
Every now and then (for instance, Rohan Connolly in 2014, Josh Jenkins in 2016, Leigh Matthews in 2019), somebody floats the idea of giving bonus points on the ladder to teams which score over 100 points. This is not a good idea.
As evidenced above, rain makes scores lower. Now, here’s how a 100 point threshold interacts with the bonus point brainwave:
About 40% of dry game team scores reach 100. That falls to 33% for light rain and just 27% for medium or heavy rain.
Rain isn’t distributed evenly and some clubs play fourteen or more games at Docklands with a roof as well. A club which only rarely sees rain (some Docklands-centric sides have managed completely dry seasons) should fairly systematically be picking up nearly an entire full extra bonus point a season than a side playing mostly outdoors in the wetter parts of the country.
That’s enough to be giving the dry game teams an inbuilt advantage in tiebreakers on the ladder:
|Scores above 100 since 2000||Team A||Expected bonus points||Team B||Expected bonus points|
|Medium or heavy rain||27%||0||0||4||1.08|
Both of these hypothetical profiles are entirely possible based on previous seasons. For instance, these hypothetical teams could have been the Bulldogs and Gold Coast in 2013.
A single bonus point is about a quarter of a win. 0.25 wins might not seem like a huge amount of impact, but it’s as big as the forecast impact of fixture difficulty on half the sides in the competition this year. Baking that new imbalance into the league every year is not a trivial proposition.
Bonus points are also likely to sometimes give paradoxical incentives to stop scoring from occurring, as well as reduce the value of racking up the score by devaluing percentage as a tiebreaker, but that’s an argument for another day.
When it rains…
So, how much does it rain at the footy? Since 2000, about 40% of games in the five major Australian capital cities have been on days with some level of rainfall. Interestingly this proportion held pretty steady during the drought years in the 2000s (marked in shading):
Even the shares of light, medium and heavy rainfall days stayed reasonably flat. Instead what seems to have shifted is the volume of rain that fell during wet days, while the frequency of wet days stayed fairly steady. That is to say, only about 140mm fell on outdoor gamedays in these five cities in the record dry year of 2006, versus 301mm falling in flood-filled 2011, with roughly the same number of gamedays.
Where it rains
Rainfall of course varies substantially by city. Below is rainfall on gamedays in each city since 2000:
Melbourne stands out as a great place to have built an indoor stadium, because only its seaside neighbour Geelong has been more prone to rainy matchdays since 2000. Barely more than half of the outdoor gamedays in Victoria and Tasmania have been free of rain since 2000, but these games do see relatively low levels of heavy rain, tending instead towards drizzle which should be less expected to impact on play.
Among the other major cities, Adelaide is also quite wet, its relatively low annual rainfall being strongly concentrated into the middle of footy season.
The rest of the country tends to see less rain than these cities, but more intense rainfall when it does come. Two thirds of Sydney matchdays are unaffected by rain, but a full 10% of matchdays sit above the AFL’s 95th percentile of 10.4mm daily rainfall, double the national frequency. Brisbane, Perth and Gold Coast also show variations on this pattern, with Gold Coast’s infrequent winter rain coming almost exclusively in big heavy downpours. The two heaviest downpours in our sample were this game (103mm) and this one (83mm) both at Carrara.
Glorious Canberra, home of HPN, is the only semi-frequent inland venue and has a dry mountain climate. It’s also a place where the AFL tends to avoid scheduling in the freezing winter months, tends to be more reliably dry than anywhere else except Cairns and the Northern Territory.
Who plays in the rain?
This national variation in winter climate obviously means teams are exposed to rain with different frequencies. Below we’ve narrowed the focus to the last decade to get a feel for more recent history and current stadium arrangements, and it turns out it’s an MCG tenant who plays in Tasmania who have seen the most rain over the years.
Barely half of Hawthorn home games have been in dry conditions thanks to being a mostly outdoor Melbourne side who also play in Launceston. At the other end of the scale, the only time Docklands tenants St Kilda, North Melbourne and the Bulldogs see wet home conditions is at secondary venues or the occasional MCG home games.
The Adelaide teams see more rain than most, while the Swans see the most heavy downpours (GWS play three home games in dryer Canberra).
Fremantle and West Coast have experienced strangely different amounts of rain given they share a ground, with West Coast having 77 rain free gamedays and Fremantle 66. This illustrates that there’s some luck involved in all this, but the overall trends and probabilities are still readily apparent.
Can we adapt?
When talking about rain at the football, someone will quickly pipe up with “well, it’s a good thing at INSERT TEAM X is a good wet weather team”.
Unfortunately for that bloke (it is almost always blokes stuck in the past saying stuff like that) HPN couldn’t derive evidence that certain teams “love playing in the wet” or are more successful in the wet.
To get to this conclusion, HPN had to slice it two different ways. First, here’s win/loss record against number of games played in the rain.
There’s plenty of teams who did better in their rainy games, plenty who did worse, and plenty of teams who played nearly half a season in the rain didn’t have a better win/loss record to show for all this experience.
As a second look, we found that for individual team seasons percentage in wet games, and percentage in dry games were reasonably correlated.
The R-squared of 36% suggests dry weather percentage explains about a third of wet weather percentage. It means that to a certain extent, the same teams that have been good in dry conditions tend to have also been good in the wet, bearing in mind the quite small sample sizes involved here which are producing a number of very high rainy weather percentages.
What did we learn today?
Overall, it seems that rain is a thing that does effect football games, but in a somewhat predictable way.
Rain reduces scoring rates. Rain happens a different amount to different teams in different cities, and it’s probably not something teams can get a significant advantage from, just by playing in it a lot.
Finally, rain’s non-adaptability and uneven distribution is a pretty strong argument against bonus ladder points for high scores.