Rain or shine: The tricky art of weather forecasting

How accurate is the weather forecast? Explaining a few of its off-target predictions this summer, Met Éireann says people need to think in terms of probabilities, not certainties

THE BALCONY ON the third floor of Met Éireann’s pyramidal headquarters, on Glasnevin Hill in Dublin, was built to give panoramic views over the city. One morning this week, everywhere you look the sky is grey. We are living through one of those horribly wet Irish summers where the weather is the first topic of conversation.

The nerve centre of Met Éireann’s forecasting services contains banks of computers modelling the weather for the next 10 days – and the news is not good. Jean Byrne, Ireland’s best-known meteorologist, is looking at predictions computed by the European Centre for Medium-Range Weather Forecasts (ECMWF), to which Met Éireann and dozens of other national weather services subscribe. The computers have come up with 50 scenarios for what might happen in both the short term, which is to say the next three days, and the medium term, which is to say the next seven. Known as ensemble forecasting, it follows the principle that the more scenarios are modelled, the better the chance of coming up with the right one. None of the 50 is promising.

“There’s just low pressure everywhere,” says Byrne. Dublin could get between 1mm and 12mm over 12 hours on Tuesday, according to the predictions. In fact, no rain falls in that period. “If the atmosphere was completely stable, [the predictions] all should be the same in theory, but that never happens in practice, and this shows how uncertain the forecast is,” she explains.

Met Éireann’s head of forecasting, Gerald Fleming, shows the 10-day model for Athens. All the predictions converge: there’s not a cloud in the sky. Weather forecasters there must have nothing to do for four months of the year.

Closer to home, farmers, fishermen, hoteliers, holidaymakers and concertgoers are among those who depend on accurate predictions. When the forecasters say it is going to be sunny, they expect it to be sunny; when they say it is going to rain, they expect it to rain. There is no small amount of irritation when forecasters get it wrong. Heavy rain was forecast for the Dublin area last Thursday week, for example, which might have altered people’s plans for going to the Stone Roses concert in the Phoenix Park. It was the right forecast but the wrong day. The heavy rain fell on Friday. Last Saturday’s weather in Dublin was due to be largely cloudy with some showers, and a maximum temperature of 17-18 degrees. Instead the weather station in the Phoenix Park recorded no rain and temperatures of almost 20 degrees in more than 12 hours of sunshine. “The weather system moved very slowly and did not develop how we expected it to happen. The general character of the weather was correct, but there was an error in timing,” says Fleming.

Met Éireann’s computers, which are operated by the Irish Centre for High-End Computing, a State-supported technology agency, process many billions of bits of information each day. These include atmospheric pressure at different altitudes, wind speed and direction, moisture content, hours of sunshine and soil temperatures, among a plethora of other variables.

Fleming acknowledges that Met Éireann might do more to stress that forecasts are predictions, not certainties, even with all the computer power behind them. “We can describe the weather mathematically; we know the movement of the air obeys the laws of physics; but the equations are horrendously complicated,” he says. “We cannot know everything there is to know about the weather even now. We can’t establish the base line as to what today’s weather is with exactitude.”