The Expert Bean: The Weather Man

We are very excited on the blog today because we have a real meteorologist to speak to us about the weather.

With over fifteen years experience of tracking our weather, here’s James Dodgson with an insight into why, here in the UK, our weather is so variable. Thank you James, over to you

Guest Post by James Dodgson

Why does it sometimes feel like Spring in Winter and Winter in Spring? It is all down to the vagaries of the jet stream, a corridor of very strong winds (100mph plus) that weave west to east across the mid-latitudes of both the northern and southern hemispheres. This jet stream occurs at high altitude, around the height that commercial jet aircraft fly. However, despite occurring several miles above the surface of the earth, this jet stream has direct impacts on the weather below, driving warm and cold fronts across the UK with areas of settled high pressure followed by areas of unsettled low pressure.

The ‘problem’ with the jet stream is that it is constantly changing as it runs eastwards as a series of waves. These waves consist of troughs and ridges, much like oceans waves. Most of the time the jet stream is very active in its movement east, with a trough affecting the UK one day (which brings low pressure and wet and windy conditions) and a ridge affecting it another day (bringing mainly dry and settled weather). This pattern is quite typical in winter time, with spells of unsettled weather interspersed by spells of fairer weather. This brings the UK its usual mixed bag of weather, with very little in the way of temperature extremes. However, sometimes, the jet gets stuck in its trough/ridge pattern for a period of days or even weeks, and this sets up the likelihood of continued unsettled weather if the trough dominates (low pressure at the surface). If the ridge dominates, mainly fair conditions with light winds set up (generally high pressure at the surface).

A further complication is where the wind comes from and the time of year as this can have a big impact on temperature and type of precipitation. For instance, last winter was very much dominated by a trough in the jet stream, bringing very windy and wet conditions. However, winds were often blowing in from a mild southwesterly direction. If the trough is in a slightly different position, low pressure at the surface can draw cold air down from the arctic region. This can correspond to much of the precipitation falling as snow rather than rain in winter, especially over higher ground.

Equally, if a ridge sets up, this generally leads to high pressure at the surface. Depending on where the ridge is located can have a profound effect on the local weather. In winter, the long nights, clear skies and light winds promote surface cooling leading to frosts and fog. However, if the surface high is located slightly differently, this can draw mild southerly flow across the UK. If the season is approaching Spring, the sun can have quite a strong impact on the warming of the near Continent. If the winds draw this air into the south of the UK, it is very possible to get 20 Celsius in some areas away from the influence of the coast.

So next time the weather seems to get stuck in a rut, good or bad, you can put it down to the vagaries of a corridor of fast moving air miles above the surface.


We hope you’ve enjoyed this post. If you’re an expert bean and would like to write a guest post for one of our Green Bean Challenges then you can contact us via info(at)dsnelson(dot)co(dot)uk. We’d love to hear from you.

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