Thursday 17 October 2019

Sarah Carey: Come rain or shine, our weather forecasts often feel a bit like chaos theory

Dubliners enjoying the sunshine at the 40 Ft in Sandycove, Dublin. Photo: Arthur Carron/Collins
Dubliners enjoying the sunshine at the 40 Ft in Sandycove, Dublin. Photo: Arthur Carron/Collins

I live in the Midlands, which has little to distinguish itself except some fine archaeology and an interesting statistic: we get double the rainfall of Blackrock in south county Dublin.

Fortunately I don’t let a little rain put me off and so never listen to the weather forecast when planning activities. Otherwise I’d never do anything.

The forecast always predicts it will rain when chances are, it won’t. Or even if it does, so what? This is what jackets are for. My motto is; “there’s no such thing as bad weather; only the wrong clothes”.

My other motto in life is: don’t expect too much. I’m looking out the window now and it’s dull and cloudy. But hey! It’s Ireland! It’s not cold and it’s not actually raining. In fact, it’s quite mild.


This characterisation of the weather drives my husband completely mad. “Mild? It’s summer! We should be able to expect something more than MILD!” No, we shouldn’t. Expecting more only leads to disappointment.

Once it’s not actually raining you can do pretty much anything. Don’t watch the forecast - just get on with it.

The problem is that avoiding the forecast isn’t always enough. Lately I’ve been unable to avoid dramatically foreboding headlines on websites.

Last Monday declared; “Get the brollies - it’s going to rain for the rest of the week”. It rained at night but there was good drying during the day. Likewise, we’d been warned that last Saturday’s good weather would be a “one-off” but the sun shone all afternoon.

The relish with which not-great weather is presented as totally-miserable-weather seems unnecessarily intense.

In fairness, this reflects what’s known as the “wet bias” in professional weather forecasting.

This is a tendency to exaggerate the chances of rain because forecasters know that people get more annoyed by a good day being ruined than a potentially poor day turning out well.

In the US, the commercial Weather Channel has a policy of never predicting a 50/50 chance of rain. That would annoy their viewers, who simply wanted to know if it’s going to rain or not. When they predict a 20pc chance of rain, it only rains 5pc of the time.

That’s why when you look at those maps with the little pictures of clouds and suns, most of the time we get that apparently useless icon of a sun with a raincloud stuck over it.

It probably won’t rain, but getting caught without an umbrella is an unpleasant experience your forecaster would like you to avoid.

But it also means that postponing an outdoor activity because “occasional showers” have been forecast will lead to a semi-permanent life indoors. The problem is that forecasters have to live in the land of uncertainty, a place most people don’t like to be. They can never really say it will definitely be sunny, or definitely rain.


They have to deal with percentages, chances, mights and maybes. It’s not their fault. It’s just that conveying uncertainty isn’t easy.

A political or sports pundit can predict an election or game result and get it hopelessly wrong, (which they do with great regularity), and no one cares.

Unfortunately for meteorologists, no one notices if they get it right, but never forget when it’s wrong. Which is harsh really, because predicting the weather is unbelievably difficult.

The sensitivity of forecasting models to tiny changes is what led to the discovery of chaos theory.

Despite the difficulty, forecasts can be brilliantly accurate. So yes, it might rain somewhere tomorrow.

In fact, it probably will. That’s the forecast all summer. Now, who’s coming for a walk?

Online Editors

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