Latest statistics help Met Éireann put weather in context

Thu, May 31, 2012, 01:00

AVERAGE TEMPERATURES and rainfall are up, according to the latest Met Éireann statistics. Yet there is no evidence in these figures to prove that global warming is at play.

Met Éireann yesterday released its latest long-term averages for rainfall, hours of sunshine, wind strength and temperature highs and lows.

People may seek evidence that climate change is under way but they won’t find it here.

These figures represent the latest climate averages encompassing weather statistics collected over 30 years from 1981 to 2010.

These long-term averages will replace those covering the period 1961 to 1990, which until now were used by Met Éireann forecasters every day as they presented their weather reports.

The statistics provide a 30-year average against which daily and monthly figures can be compared. They allow the forecasters to describe, for example, temperatures as being slightly above the norm or conditions as being dryer than usual in the east.

The 30-year averages allow Met Éireann to put current weather in context, according to the organisation’s senior climatologist Séamus Walsh. Using a block of 30 years allows highs and lows to be ironed out so that “typical” weather values can be derived.

But while a forecaster can say a given day, week or month is warmer or cooler than average, 30 years is far too short a period to be able to say whether a temperature trend is under way.

Having an accurate 30-year average is essential for several reasons. It provides a baseline against which current weather can be compared. It is also an important tool for researchers, the agricultural sector and policymakers, says Walsh.

So no conclusions can be drawn from the 0.75 degree rise in temperature averages between the new average data and the previous figures, nor from the 5 per cent increase in rainfall.

“This is the increase in rainfall but there is no evidence as yet of an increased number of days with a higher-than-average rainfall,” says Walsh.

It is not in the averages that a climatologist would search for an indication of climate change but in the weather peaks. These might include the exceptionally cold winters of 2007, 2008 and 2009, or a rise in the number of days with more than 10mm of rainfall.

An increase in the number of particularly severe storms would also be a matter of interest to a climatologist, but once again this depends on being able to compare the current situation against a long-term average. This will indicate whether something unusual is happening.

For this reason, it is valuable to be able to look well back into the past. This presents challenges, however, including the conversion of handwritten and collated weather data into a digitised form.

Met Éireann has highly dependable weather statistics dating to the mid-1800s from the Phoenix Park and Valentia, Co Kerry, Walsh says. But rainfall data is digitised only as far back as 1941 and temperatures to 1961.

The currently digitised statistics represent about 10 million individual data points. While Met Éireann would like all statistics digitised as far back as possible in what is known as “data rescue”, this would be a huge cost in time and labour when resources are scarce.

The latest weather stations automatically capture data, with minute-by-minute temperature sampling.

This is a far cry from the laboriously collected weather data of the 1800s from Birr Castle, Co Offaly, or Markree Castle, Co Sligo.

Irish weather: Some key records

Met Éireann compiles its weather records from 750 or so stations, but the ones that have the most accurate records are synoptic stations, of which there were 14 for most of the periods of the current records.

The driest place in Ireland: Dublin airport, 754mm of rain annually.

The wettest place in Ireland: Valentia Observatory, 1,557mm annually.

The average annual rainfall at Shannon airport between 1981 and 2010 was 977.6mm – an average increase of 7.6per cent on the 1961-1990 period

The average annual rainfall at Valentia Observatory in the same period was 1557.4mm – an increase of 8.9 per cent.

The mean number of hours of sun a day at Rosslare was 4.3 (1,570 hours a year). At Malin Head it was 3.5 (1,278 hours a year).

The headline on this article was amended at 12.10pm on Thursday, May 31st for reasons of clarity