The North Atlantic Oscillation (NAO) can have a large impact on the strength of the jet stream. When the jet stream is weakened its direction can change more often (which is why UK weather can be pretty variable!). The best way to describe the NAO is as a particular state of the atmosphere which can change between so called ‘positive’ and ‘negative’ phases, like a seesaw. These phases are identified by calculating pressure differences between the Azores and Iceland. The Azores has a typically high pressure and Iceland has a low pressure, however this difference can vary in magnitude. The diagram below shows that in a positive phase there is a large difference between high and low pressures, and in a negative phase there is a small difference. If you look closely on the diagram you can see the outline of Europe and Africa to the right of the NAO.