Is this weather really normal?

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.