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Guide To ... Uk Summer Setups


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  • Location: Lincoln, Lincolnshire
  • Weather Preferences: Sunshine, convective precipitation, snow, thunderstorms, "episodic" months.
  • Location: Lincoln, Lincolnshire

This is another updated version of my previous articles- I don't know what to do with those!

As in winter, the biggest determining factors in what weather the British Isles gets are the strength and positioning of the jet stream. When the jet stream is strong, we get low pressure systems tracking from west to east on a regular basis, bringing fronts which in turn bring bands of persistent rain, with brighter showery weather in between the rain belts- a "zonal" pattern. When the jet stream slows down we get blocked patterns with high pressure persisting in certain areas- and the weather we get over the UK depends on where those blocking highs form.

Zonal, southerly tracking jet

When low pressure systems track over southern and central parts of Britain the result is often a good deal of cloudy wet weather- especially in the south. It is often drier over Scotland and Northern Ireland, especially the far north. Temperatures can vary either side of the average depending on how many incursions of warmer southerly and south-easterly winds we get from the near continent. This pattern contributed to the very wet early to mid summer of 2007.

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Zonal, lows passing over and to the north of Scotland

This is the most common "unsettled" weather pattern. The Azores High sits out to the south-west not doing much, apart from throwing up the odd transitionary ridge to southern England, and Britain stays in a broad westerly airflow, maintaining rather cool and cloudy weather with bands of rain moving across at intervals. This setup can produce very wet months, especially in Scotland. The chart below is taken from the very infamous case of July 1988, when this setup persisted for almost the entire month. Summer 1998 was also dominated by this pattern.

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Zonal, lows passing well to the north

When the lows pass well to the north of Scotland this allows the Azores High to ridge into southern areas, bringing longer spells of dry sunny weather to those areas. However, it often stays cloudy and damp in the westerly airstream on the northern flank of the high. However, if the jet stream slackens then this setup can be a building block towards a generally anticyclonic spell over Britain as the high ridges further north and east- or perhaps the "three fine days and a thunderstorm" progression when the high transfers east (see eastern blocking and Spanish plume, below). If the jet stays strong, as happened for example in August 1998, then the north-south split persists.

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Blocked, high pressure to the east

This setup tends to be hot and sunny over the UK, sometimes with thundery outbreaks due to generally southerly winds, bringing hot air in from the near continent. If the high is generally close to Britain the emphasis will tend to be on hot dry sunny weather, as happened in July 2006.

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If low pressure is relatively close by to the west, it will often be warm but changeable, as happened in June 2003.

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The notorious "Spanish plume" arises when a high pressure interlude breaks down with the high transferring away eastwards, sucking up hot southerlies from Spain. At the same time, cooler fresher weather pushes east from the Atlantic, and the two airmasses collide, often giving rise to widespread thunderstorms across Britain, with south-eastern areas often the most heavily affected. If the Spanish plume is associated with a strengthening of the jet, it may herald a change to cool cloudy wet weather, but if not, then high pressure will often ridge in from the Azores to replace the high that moved away eastwards.

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Blocked, high pressure over Britain

A large high over Britain is the most reliable source of prolonged spells of warm, dry sunny weather. One common way this can happen is when a ridge from the Azores spreads north-eastwards and covers the whole of Britain- this is the setup most likely to guarantee sustained dry weather as the Azores High keeps reinforcing the ridge, and only weak fronts are allowed into northern areas. This setup was largely responsible for the hot summer of 1976.

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Alternatively, a high can break off from the Azores High and persist over and to the east of Britain, the Azores High is displaced to the west of its usual position, and low pressure forms to the west. This setup was largely responsible for the hot summer of 1995. It is generally more reliable for heat and sunshine than the Azores ridge (as it is more likely to see imports of hot continental air and less likely to have weak fronts moving around the northern periphery of the high) but less reliably dry, because if the high pulls away to the east we end up with the hot thundery "eastern blocking" scenario above.

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However, high pressure does not always guarantee heat or sunshine. Here are some other places that the high can end up, and the resulting weather:

Blocked, high pressure to the north

High pressure just to the north of Britain still often promises dry sunny weather, especially for western parts of Britain. However, the winds off the North Sea often bring a good deal of low cloud into North Sea areas and sometimes this low cloud transfers a long way inland. The formation of low cloud is dependent on how high the hummidity gets out in the North Sea (which is reflected by a very low differential between sea surface temperatures and dewpoints). In an easterly regime, sometimes thunderstorms can be imported into southern and western areas from the continent.

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Blocked, high pressure to the west

If the high is just to the west of Britain we pick up a northerly airstream (this often happens when the jet slows down, the last of our Atlantic lows moves into the North Sea and there are no follow-up lows, so we get a spell of northerlies). Generally speaking it tends to be warmer, drier and sunnier the further south and west you go in this setup. The weather will usually be bright and showery if the air originates in the North Atlantic or the pole.

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However, it will tend to be cloudy and drizzly if the air originates over Scandinavia, as this results in a rather dry stable regime (moistened as it passes over the North Sea) with limited convection.

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Blocked, high pressure to the south

This setup often amounts to much the same thing as the "northerly tracking jet" scenario with southern areas dry and sunny under the high pressure, and cooler cloudier weather further north. However, a stable anticyclone to the south tends to result in a persistent "warm sector" covering Britain, so it often ends up persistently cloudy and drizzly and humid on the high's northern flank, while areas within the anticyclone usually see any cloud break up in the strong summer sunshine, leading to hot sunny weather. Late August 2008 was an extreme case of the cloud persisting across most of Britain, because the high stayed far south enough to keep Britain in a moist westerly flow.

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Blocked, high pressure a long way away, lows stuck over Britain

A blocked setup can result in slow moving depressions stuck over Britain while high pressure resides elsewhere- in contrast to the other "blocked" setups this setup can give rise to very wet months. Rainfall tends to be heavy and thundery rather than frontal, because the slow moving lows tend to result in Britain staying in showery polar maritime air for long periods. This setup was largely responsible for the very wet August of 2004.

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