Is this weather really normal?

Thu, Aug 23, 2012, 01:00

THIS MIGHT seem like a dire summer without compare, delivering floods of rain and short rations of sun. Yet the weather statistics for summer 2012 are likely to show it was not out of the ordinary and was within the realms of “normal”.

The jet stream, high altitude winds that help dictate weather on the ground, has turned its face against us. Like a conveyor belt, it has dragged low pressure air over Ireland for months, with plenty of rain and wind.

Yet the reason the summer seems so bad is not only related to the weather and may have more to do with our great expectations of what an Irish summer should be.

“The expectations that people have are a little bit unrealistic,” says Séamus Walsh, senior climatologist at Met Éireann. “One or two times a decade you might get a dry, warm, sunny summer,” he says.

“People have an expectation of a Mediterranean summer every year but we don’t get them. We feel we should expect such weather but it doesn’t work like that. We never learn.”

There is plenty of evidence that summer is drawing to a close, not least the “back to school” advertising. Actual summer persists for another eight days however and the final seasonal weather statistics won’t be ready before then, says Walsh.

The current five-day forecast suggests more of the same with several more soakings of rain before the end of the month, but so far the season “is within the range of what we can get”, he says.

The latest figures with a week before autumn still to go show the midlands, south and east have seen rain at about 200 per cent of the 30-year average. Western coastal areas are running at between 100 and 150 per cent of the long-term average.

Average sunshine is also down, between 70 and 90 per cent of typical, he said. Average temperatures have also been depressed with summer season averages running at a half to one degree below average.

If the rain predicted to hit later in the week materialises then some weather stations may see seasonal records set, Walsh suggests. But 2012 was not extrordinary and may not be as wet as the summer of 2008.

In that year June delivered 150 to 200 per cent of average rainfall and between 150 and 300 per cent in July. August 2008 was also wet, with 200 per cent of average rainfall.

He acknowledges however that the statistics can be misleading given that a single storm or heavy thunder shower might deliver a month’s rain in a matter of hours. “You can quite easily get 200 per cent of average rainfall,” he says.

The average summer gives between 200 and 300mm of rain, maybe 60 to 80mm per month, so not a huge amount.

“If you get a really dry summer you might see 20 or 30mm, or a really wet summer that would give you twice that amount,” he says. “You can quite easily get 200 per cent of average rainfall.”

Meteorologists are in no doubt about the cause: the placement of the jet stream relative to Ireland. It has remained south of us all summer, as a result blocking off high-pressure stable air drifting up from the Azores and instead feeding in unstable low pressure air, says Prof Peter Lynch, formerly assistant director of Met Éireann and now professor of meteorology in the School of Mathematical Sciences at University College Dublin.

The jet stream does not follow a line; it drifts and undulates. “When it curves to the south that is known as a cyclonic circulation and when it curves to the north it is an anti-cyclonic circulation,” says Lynch.

This accounts for jet-stream-related weather conditions affecting the continental US, where it has been heatwaves instead of rain. “We have an anti-cyclonic circulation around the US and a cyclonic circulation around Europe,” he says.

Gerald Fleming, head of forecasting at Met Éireann, described the complexity of the planet’s atmospheric circulation systems, something that made it a challenge to keep weather predictions accurate. “The jet stream has been very far south, but the whole atmosphere is linked,” he said.

Uncertainty in the atmosphere is what causes uncertainty in predictions. “The atmosphere is chaotic in nature. The weather that will be going on in six, eight, 10 days is changed by very small things,” he says.

Modern computers running meteorological models are hugely powerful but the weather systems they are trying to predict cover thousands of square kilometres. The amount of data generated by weather systems even when analysed on a square kilometre resolution is enormous and as much as the latest computers can handle.

Yet better resolution could deliver more accurate, longer-term predictions, says Fleming. “If you could calculate on the basis of a square millimetre of air you could apply the laws of physics and decide how weather will evolve over time,” he said.

We have yet to see computers powerful enough to tackle such a task. In the meantime the full picture of the 2012 “summer that wasn’t” will be detailed in little more than a week.

Why the jet stream roasts and drenches in equal measure

YOU CAN’T see it, you can’t hear it but you sure can feel its effects. The jet stream can make a transatlantic flight that bit bumpier, but most of us will know its presence for the way it affects our weather. It has given us a cool wet summer, but on the other hand has left the continental US roasting under the worst drought in almost 60 years.

“The jet stream is like a river of air flowing rapidly along in the upper atmosphere,” explains Prof Peter Lynch, professor of meteorology in the School of Mathematical Sciences at University College Dublin and author of the “That’s Maths” series in The Irish Times.

It occurs at the border between cold arctic air and warm tropical air, with the flow produced by excess heat in the tropics coming up against a deficit of heat at the poles, he says.

The energy differential creates a stream of moving air, sometimes moving at hundreds of kilometres an hour. It does not flow along a straight line however. “There are large meanders and parts bend to the south and some to the north,” Prof Lynch says.

It flows along at between five and 10 kilometres above the Earth, with the strongest wind speeds along its central core. The reason why it is so important to our weather patterns is because it “has a dominating effect on the weather at the surface”, he says.

“The jet stream can really affect things at the surface. It has a strong steering effect that drags the weather systems along.”

Normally the flow meanders, sometimes moving north and other times south. Unfortunately it became locked, with the flow across Europe getting fixed just south of Ireland.

Cool wet weather is the result, Prof Lynch said. “When the jet stream curves down to the south you get lots of low-pressure systems and the air is rising so you get condensation and precipitation.”

The reverse is happening over the US where the jet stream has moved north. This delivers sinking air that heats rapidly and loses its moisture, leading to clearer air and more radiant energy reaching the Earth in a feedback loop, making things hotter and hotter.

“The hot weather in the US and very wet weather over Ireland are connected. They are different aspects of the overall flow of the jet stream,” he says. Meteorologists have no way to predict what the jet stream will do next. “The system is highly non-linear,” he says.

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