Is this weather really normal?

“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.