where’s the rain?

We love talking about the weather in Ireland. Absolutely love it. And there’s an unlimited amount of ways to describe it – fierce weather; good drying weather; scorcher; belter; cracker – can all be used to say it’s rather sunny outside. And my favourite – there’s a grand stretch in the evening!

I’ve often wondered why we love to talk about the weather and the conclusion I always come to is that it’s just an easy conversation opener and instantly puts (Irish!) people at ease. Right now, we’re beginning to hit leaving cert weather. Leaving cert weather is how we refer to the first couple of weeks in June – you’re almost always guaranteed to have sunshine in Ireland. However we’ve been a bit spoilt for sunshine in the last 2 months and it’s got me thinking, where’s the rain? So, using Python, I decided to take a look into the data to see what it’s telling us.

Sourcing data from Met Éireann (who fortunately have an open data policy and publish data collected from their 25 weather stations around Ireland on a daily basis) I created a dataset comprising of monthly rainfall and air temperature stats from 2017-2020 for each weather station. The dataset also includes the mean figures for each station over the last 30 years.

rain, rain, go away

First, I decided to take a look into total rainfall across Ireland. Living in Dublin, I’ve always felt that it rarely rains here – and the data backs me up. The boxplot to the left shows the 3 stations with the least amount of rain in the last 3 years are located in Dublin (Casement, Dublin Airport & Phoenix Park) with the wettest stations located in Newport in Mayo & Valentia in Kerry.

A boxplot is a useful tool that shows the range of values for each variable, displaying a condensed view of data that we would otherwise find difficult to compare. The line in the colored box indicates the median value for each variable (the median is the most common value). The larger the box, the larger the range of values.

Turning our attention to Athenry in the above graph, it has a median annual rainfall value of 1200 millimetres but we’ve also had years when the rainfall was 1410 mm and 1100 mm. This proves useful in 2 ways. Firstly, it allows us to compare the median of all stations at once. Secondly, it allows us to compare stations range of values to all other stations – for example, when we compare Athenry to a station like Mace Head, we can see that there is a larger amount of variation in rainfall in Athenry than Mace Head – very useful to know if you’re planning on travelling to either of those places.

too wet, too dry?

When we want to compare something over time, simple line charts are very effective. The graph to the left shows the average rainfall per month over the last 3 years in Newport (the wettest station in Ireland) and Phoenix Park (the driest station in Ireland). By comparing the last 3 years to the mean over the last 30 years, we can see that Newport has generally been wetter during the winter/autumn months and drier during the summer months excluding August which has seen nearly 33% more rain in recent times! While in the Phoenix Park, it’s generally been drier over the last 3 years than it’s been over the last 30 years – excluding the spikes in March and November.

This got me thinking – can we prove with the limited amount of data we’re working with that the weather in Ireland is getting more extreme?

To try and answer this the histogram on the right represents each stations annual rainfall grouped by year. Let’s take the mean as an example. Each one of the yellow bars represent 1 or more station and their relative annual rainfall. Taking the 800 mark into consideration, it shows that 3 stations have had 800mm of rain on average per year while at the 1200 mark it shows that 6 stations have had around 1200mm of rain on average per year. This becomes useful to know when we plot the data for 2017, 2018 and 2019.

What this shows us is that over the last 3 years we’ve seen stations with more rain than before – we have several stations past the 1600 mark while there’s only been 2 stations reaching the 1600 mark over the last 30 years. Of course, there is a possibility of these being outliers (once off events) however when we look at where the spikes are in 2017 and 2019, there are more stations getting less rain than when we compare it to the mean. Could this point to wetter winters and drier summers?

February was a wet month, right?

Come to think of it, it rained almost every day in February. Coming back to our friend the boxplot and by isolating data for February, we can see that there was nearly 3 times more rain across the nation in February 2020 when compared to February of previous years.

but May has been really dry?

By following the same method as above, we can see that May 2020 was drier than previous years, with the median almost half of what’s expected for this time of year. We can also see that there’s been a consistent drop in rainfall in May over the last 4 years and perhaps it’s figures like this that raises concerns over the possibility of a drought in Ireland in the months to come (apologies for the doomsday rhetoric!).

average rainfall per month – an alternative view

Heatmaps are another useful tool for showing the variety in data over time through the use of size representation or colour coding (as is the case in the above heatmap). Each block represents the average rainfall for each station and month colour coded relative to all other stations and months. The darker blue represents a high volume of rain while the light yellow represents a low volume of rain as represented on the color scale on the right of the heatmap. Taking the bottom left corner block as an example- it shows that Valentia in January has a high volume of rain when compared to Athenry in January (the top left corner block). Taking a look at our wettest and driest stations again – we can see it’s pretty wet all year round in Newport apart from June while it’s fairly dry all year round in Phoenix Park.

what’s the relationship between rain and air temperature?

Exploring the relationship between air temperature and rain on an aggregated level is tricky because the air temperature figures used in this dataset are averages per month while the rain is the total rainfall measurement. This is an example of how analysis can be skewed and it’s something readers should be aware of when reading any type of analysis, but we’ll plough on here anyway.

The graph to the left is a scatterplot. Each point on the graph represents a station’s average total rainfall and average air temperature grouped by month over the last 30 years. There are 25 stations and 12 months, giving us 300 individual points in total.

‘Grouped by month’ in this case means that all points that are from the same month have the same colour. For instance, all points for January are coloured pink while those for September are blue. By grouping them like this we can easily see that the relationship between total rainfall and average temperature varies depending on what month it is. Unsurprisingly, winter and spring months bring lower temperatures and a mix of high and low volumes of rainfall while summer and early autumn months bring higher temperatures with lower rainfall.

different season, different relationship

By dividing the months into winter/spring and summer/autumn, the difference between the conditions are clearer. The top graph to the right shows the winter/spring months while the bottom graph shows the summer/autumn months. By dividing the months out like this we also see a change in the correlation figures. The correlation between the total rainfall and average air temperature for the full dataset was -0.21, but when we calculate the correlation for the divided months it jumps to 0.35 and 0.32 respectively. This indicates weak positive correlation but I imagine that if we were to exclude nighttime temperatures from the averages we’d have a different picture.

one more for luck

The final graph I’ll leave you with is a heatmap representing actual heat! Similar to the heatmap used for the total rainfall per station per month, the graph to the left shows the range in average air temperatures for each station per month over the last 30 years. Unsurprisingly, it’s warmer in the summer and colder in the winter so if you’re hoping for some sun in Ireland, July and August is your best bet.

that’s all for now

Thanks for reading and I hope you enjoyed the post. The code used to create this analysis can be downloaded below along with the associated dataset. The data has been sourced from www.meteireann.ie.

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