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Weather Boy

Sunny Highs And Dirty Highs

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Can anyone help me on this.

Short question: how does one know whether a HP cell affecting the UK is going to be clear and sunny or cloudy/misty i.e a 'dirty' high?

Although this question applies summer and winter, I suspect that the factors that apply are different in different seasons.

Is it possible to predict purely from looking at models which type of high you will get and if so, at what range?

If not, then what information do you need in order to make this prediction?

Thanks in advance.

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It can be extremely hard to forecast- I remember being caught out by this in mid-March, predicting a "sunny" high only for banks of cloud to turn up on one of the evenings.

One general rule of thumb is that the more humid the airmass is (as measured by relative humidity), the more likely it is that low-level cloud will form, particularly when the wind is coming off the North Atlantic or the North Sea. This low cloud then gets trapped underneath a layer of stable, sinking air associated with the high pressure, and spreads out into a thick stratus and/or stratocumulus sheet.

Hence high pressure to the south often means a lot of cloud for northern and western Britain in particular. High pressure to the west or north tends to bring cloudy easterlies into eastern areas, although in the summer half-year, winds from south of east sometimes fail to produce much cloud over the North Sea if associated with dry continental air. High pressure to the east tends not to bring much cloud in off the English Channel, so anticyclonic/southerly types are almost invariably sunny in the summer half-year, though in the winter half-year we may import stubborn banks of cloud from the continent due to the weak sun failing to burn them off.

In the summer half-year, the stronger sunshine tends to burn off such cloud sheets more effectively than in the winter half-year, and thus high pressure is a more reliable source of sunshine in late spring and summer than it is in winter. However, that isn't always the end of the story, because when we are under moist airmasses, solar heating generates convection which produces low-level cumulus, which in turn spreads out into stratocumulus- this can give us sunny mornings and evenings but cloudy afternoons.

I generally think that in the winter half-year, the most reliable route to a "sunny" high is an incursion of dry polar or arctic air followed by a build-up of high pressure. Imports of dry continental air with high pressure centred to the east are comparably reliable in the summer half-year, but in the winter half-year it all depends on how cloudy it is over the continent, and thus "southerly" anticyclonic spells in winter range from the remarkably sunny (e.g. 8th-20th February 2008) to the overcast (e.g. much of February 1993).

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It depends on the positioning of the high, and where the air source is from. I'm sure someone will give a better answer than myself, but generally a high which is positioned out in the Atlantic will draw in west or south westerly winds, and because the winds are coming from over a large body of water the air will contain a lot of moisture which will lead to a lot of cloud formation, the east and south east usually fares much better that the north & west in this sort of situation.

A "dry, clean" high is generally positioned over the UK, or on/over near to Europe, so the "Euro High". These bring winds in from the easterly quadrant, and with the continent being much drier and generally hotter than the UK in summer, the winds are coming from a fairly dry source, it also helps that the positioning of the high doesn't take the air source's track across large bodies of water. The air has to cross the North Sea, but this is nothing in comparison to the Atlantic, although low cloud can get trapped in the high from the north sea plaguing eastern areas, although the sun is usually able to burn this off during the day.

There are many different positions a high can take. The most frequently read about are the Azores High (one which comes from the Azores in the south) and the Euro High (as explained above).

The Azores high is more prone to producing warm, but cloudy weather, where as the Euro high has a reputation of providing clear, hotter weather. That isn't to say that the Azores High cannot produce a great blast of summer, because it can.

The forum's favourite is a Spanish Plume. Basically drawing warm/hot air up from Spain, producing clear afternoons with widespread thunderstorm development.

Hope this helps.

Edit: TWS beat me to it, his answer is much better. smile.png

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The high of 13th-17th Jan 2012 was a 5 day completely sunny high, 4 frosts in a row, frost never lifted during day out of sun, and was very thick on morning of 17th

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Thanks guys. Although interested generally, part of my thinking on this is that I am going walking in the Yorks Dales Thurs-Sun and am looking ahead to what (fingers crossed) looks like an advancing high. I'm worried, though, that even if it verfies, cloudy highs could still bring drizzle which can be quite penetrating and miserable on high ground, whilst nearby lower ground can be dry or even bright. Hoping for a sunny one, or at least a dry one, assuming that prediction is correct.

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