Meteorologists are able to predict what the weather will do tomorrow only by knowing what it is doing today. Dick Ahlstrom explains the forecaster's art
When it comes to predicting the future, Dr Peter Lynch of Met Éireann would rather forecast the weather than the economy. There are no laws dictating how the economy will perform, but the laws of physics, Newton and thermodynamics govern the atmosphere.
Dr Lynch, the assistant director of the weather service responsible for scientific and technical support, sums up the atmosphere in a nutshell. "It is a compressible fluid on a rapidly rotating sphere. Fluid mechanics describes its behaviour."
This does not make the prediction game any less fraught, of course. It is very easy to get things wrong, particularly because most of our weather blows in from over the Atlantic, where there are too few sources of solid weather information to generate really reliable forecasts.
The key to the forecaster's efforts, he says, is to have as much information about what the weather is doing right now, as a way to understand what it might do one, two or three days hence. "In order to get tomorrow's weather you have to know today's weather," he says. The more accurate the starting point the better, which requires the best possible observational data.
"The buzz word is chaos now. What the essence of it is, is that small changes can have very large consequences," says Dr Lynch. "Minor errors can result in major forecast errors. What is small today may be large four, five or six days later."
A good forecaster is really applying two distinct skills. He or she must run extensive data through "numerical weather prediction" models and apply meteorological expertise to make their predictions. They must also be able to get this information across to the public.
"The forecaster's job is to give the user the best information he or she can. The skill of the forecaster is to use their knowledge of atmospheric dynamics and interpret the forecast to deliver information to the user. They are using their skills as communicators."
The service opened in 1936, "mainly in response to the emergence of transatlantic aviation", says Dr Lynch. The aviators needed the best advice they could get over the Atlantic. "It was a critical matter."
It opened in Foynes, Co Limerick, where the flying boats first landed, but moved to Dublin in the 1950s. "We started more or less the same time that RTÉ started, and there has been a forecast every evening since then."
No Irish university offers meteorology as a subject, so Met Éireann hires graduates and provides education. "We take in people with mathematics or physics degrees and give them extra training in meteorology. They are all trained scientists."
Modern weather forecasting is based on the use of computer models, which crunch up the widest possible range and volume of data the meteorological service can get. Satellite data is a key source of information, but this has a relatively low resolution, giving lots of information but averaged over a wide area.
Very accurate, high-resolution data come in from the service's collection of monitoring stations, from ships at sea and from aircraft fitted with weather-measuring devices. These sources deliver a wealth of information on temperature, air pressure, humidity, wind speed and wind direction.
Met Éireann also launches radio sondes four times a day from Valentia Observatory, in Kerry. These are large helium balloons that carry monitoring equipment thousands of feet up. "It gives you a vertical profile," says Dr Lynch.
Met Éireann has joined with the Marine Institute and the Met Office in the UK to install a series of anchored buoys off our coasts, giving yet another source of quality data, including water temperature and wave height.
The forecasters use a model called Hirlam, or High Resolution Limited Area Model. Eight countries, including the Republic, jointly developed Hirlam to help spread the cost of maintaining such a complex model.
Also run four times a day, it delivers a short-range, 48-hour view of future weather, giving a very high predictive resolution of 15 kilometres square. The forecasters also use a model at the European Centre for Medium-Range Weather Forecasts, in Reading, England. It is run once a day and gives a three- to 10-day forecast at a 40-kilometre resolution.
The meteorologists do not carry out a single prediction before they deliver the nightly forecasts. They use the best view of today's weather as a starting point and then "perturb" it slightly, producing an ensemble of 50 forecasts, which they average to deliver a final forecast.
This is how they iron out the chaos inherent in the weather. "It is an honest facing-up to the reality that the atmosphere is chaotic," says Dr Lynch. Forecasts are probabilistic, not absolute.