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Osbourne One-Nil

Why Do The Pennines Make Such A Difference?

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I know in general terms why high ground affects the weather either side but how can something as piddly as the Pennines make quite so much difference? During the cold spell, while we had snow, the difference between the north east and us in Cumbria was stark, and more recently, I've had 160mm of rain this month while Airedalejoe just 40 miles away in Chester le Street has had 19.2mm!

How can hills, which are only 3000ft at their very highest point, and generally no higher than 2000ft, make for two such differing climates? Are there any other minor hills in the world which make so much difference?

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I know in general terms why high ground affects the weather either side but how can something as piddly as the Pennines make quite so much difference? During the cold spell, while we had snow, the difference between the north east and us in Cumbria was stark, and more recently, I've had 160mm of rain this month while Airedalejoe just 40 miles away in Chester le Street has had 19.2mm!

How can hills, which are only 3000ft at their very highest point, and generally no higher than 2000ft, make for two such differing climates? Are there any other minor hills in the world which make so much difference?

good question OON,i'm no more than a 10 min walk from the eastern edge of the pennines.

yet iv'e had 126mm of rain here.

over to the experts i think.

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Across a pronounced mountain range, rainfall undergoes orographic enhancement as the air rises over the high ground and so the windward side of the mountains sees heavily inflated rainfall totals. On the leeward side of the mountains the air descends and dries out, resulting in an abrupt drop in rainfall.

This process is also very commonly noted in the South Island of New Zealand and in western Norway, just to quote two examples.

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A similar thing occurs in Scotland. I don't have the statistics to hand, admittedly, but the rainfall east of the A9 is generally less than that to the west. As TWS has stated, orographic rain falls when air is forced up, condenses and then releases as precipitation. It therefore stands to reason that land on the leeward side of a range of mountains exposed to the prevailing wind is likely to get less rain.

I've walked on Ben Nevis and Glen Coe in good weather, only to find it raining when I get past the 3000 ft mark. Very annoying!

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Yeah, but I'm just surprised that something as piddly as The Pennines can make such a pronounced effect!

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Yeah, for such a low mountain range they can have a marked effect on weather.

The current very wet west and much drier east, summer weather with a wind off the North sea and cloudy cool and damp at home, becomes sunny and hot at the bottom of Hartside pass. On such day's temps could be no more than 15 at home but once west of the pennines 25-30!!

Haven't had much time to view radar re this months rain but on many occasions in the past daily radar images show a stark contrast between west and east of Pennines.

I always presumed Consett was wetter than my hometown but I'm very surprised at the actual difference this month between myself and peterf, 126mm compared to my now 20.1mm!! Consett is some 16 miles from Chester-le-street.

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The Mendips have the same effect too. What always amazes me is how much difference a bit of height can make to snowfall, drop down off the hills and there's always much less or they've had rain whilst I've got snow.

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I find it interesting that despite it being just to the east of the Pennines, Sheffield actually has higher average annual rainfall than Manchester Airport. Could anyone provide a possible explanation for this? There isn't much in it, however.

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Air which is close to saturation, as is the case with moist south westerlies at this time of year, requires only a very modest increase in elevation to cool it sufficiently to cause precipitation. Even air at 10c with 80% relative humidity would require only to be lifted to 1200'( 365m ) before it reached saturation point so the 3000' elevation of the Pennines, although tiny in comparison to the Alps, Andes or Rockies, is more than sufficient to cause copious precipitation in a moist air mass.

The precipitation is enhanced if the air flow is rapid as the air is then forced over the hills and a greater amount of precipitable water is aviailable. As condensation and precipitation occurs latent heat is released into the air mass and the greatest rate of precipitation is reached somewhat below the summits. On the lea side of the hills the air is drier ( having released most of its moisture ) and the air is warmed as it descends, effectively turning off any source of precipitation.

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The same does happen in parts of Scotland. Annual rainfall at Killin at the Western end of Loch Tay is double that of Aberfeldy or Pitlochry a few miles to the East of the loch and only about 20 miles North East as the crow flies. Conversely Aberfeldy is colder in Winter with many more snow lying days than Killin but warmer and drier in Summer.

The mountains can make a significant difference over a short distance even if by other parts of the world standards they are not very high.

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the north downs can have a similar effect, shielding the london basin from the worst of rainfall / snowfall from events from the south. tends only to have an effect with light rainfall events though it is noticeable.

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Two reasons simply - orographic uplift as thundery wintry showers explained and the fact the prevailing wind is predominantly a westerly or southwesterly airstream.

The Pennines being a north-south mountain range means the western upslopes will always recieve much copious rainfall totals than the eastern upslopes due as fronts and showers move in from the west. If our prevailing wind was an easterly then the reverse would happen.

An exceptionally extreme example of similiar geography is in south america where the west slopes of the Andes are very wet, compared to the east which quickly become the Atacama desert.

In the Lake District we have mountain ranges on all sorts of an axis, many in the west being on a SW-NE axis or NW-SE axis and those in the east on a north-south axis. It is fairly normal to be on one side of a mountain and it be pouring down with low cloud only to descend on the opposite side into no rain and cloud - this happens alot in showery westerly and north westerly airstreams especially in the summer.

Northeasterlies and easterlies bring cloud and rain/snow to the northern and eastern fells, whereas the southwest fells can languish in brilliant sunshine. During December most of the snow fell on the eastern upslopes of the eastern fells only, by the time precipitation descended onto the western slopes the air quickly dryed out.

Similiar things happen in valley bottoms, I've known it to be snowing in the Thirlmere Valley whereas there was no snow in the Derwent valley some 4-5 miles further west.

We have a fascinating variety of weather helped largely in part by the varied topography - how much duller would our weather be if we had no mountain ranges..

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I find it interesting that despite it being just to the east of the Pennines, Sheffield actually has higher average annual rainfall than Manchester Airport. Could anyone provide a possible explanation for this? There isn't much in it, however.

Manchester is an exposed fairly low level site, Sheffield is a fairly high level site and parts of it, including the official observing site, are pretty well inside the actual Peak itself. It also, because of its elevation, gets 'additional' precipitation with winds from NE-SE, hence the relatively high level of snowfall it shows on its stats.

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The influence of altitude on precipitation is amply demonstrated by the difference in mean annual rainfall between where I live at 330m and where my mother lives, only 2 miles away as the crow flies but at 140m.

The 30 year mean here is 1070mm and at my mother's it's 920 mm. The greatest monthly difference is during the autumn and winter when there is more frontal rainfall and the least difference in mid summer when the rainfall tends to be more convective in nature.

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Interesting points, I wonder what would happen to the southeast if say Hertfordshire became the pennines, do you think say Essex would have even less than the current 600mm annual rainfall?

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They are small but the configuration is crucial, running perpendicular to general prevailing westerly flows.

As TWS mentioned the main ranges in NZ's South Island are similarly convenient in running in that direction, though they are much higher.

eg the driest place in NZ in 2010 was Clyde, just east of the Alps, with 389mm of rain in the year. The wettest place in 2010 was Cropp River, just west of the Alps, with 12374mm of rain in the year. So a site east of the Divide was 31 times drier than a site west of the Divide.

Another example was this previous weekend, it was 41C in Timaru (east coast) with a hot dry northwesterly whilst 100 miles directly west on the West Coast in Haast it was hosing down and 19C.

So I don't think the Pennines are all that remarkable. There are probably many small ranges across the world that have a similar effect, provided they are in the mid or high latitudes.

Topography makes weather interesting for me. I like the contrasts in coastal California. You can cross their coastal ranges very quickly on the highway, and what a difference there can be in summertime just an hour away!

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Manchester is an exposed fairly low level site, Sheffield is a fairly high level site and parts of it, including the official observing site, are pretty well inside the actual Peak itself. It also, because of its elevation, gets 'additional' precipitation with winds from NE-SE, hence the relatively high level of snowfall it shows on its stats.

It's always puzzled me, how "mountains" less than 1000m can produce such rainshadows. (New Zealand's are over 3000m). But the Welsh mountain rainshadow; why does it only seem to work properly in the winter months? In the last week we have had next to no rain here yet the mid-Wales hills have had 200mm+, and the Severn has flooded- it's even been sunny on a couple of days. Yet the same setup in July or August usually means cloud and drizzle here, fog and heavier drizzle in Wales, and leave the sun for the far SE. July 2010 saw this for weeks on end.

And why isn't N Shropshire/S Cheshire even drier than Essex/Suffolk/London, seeing as it's sheltered by the Peak District from NE'lies as well?

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Wales is a prime example for us here. 99% of the time all heavy rain is reserved for Wales while the West & East Midlands see the scraps and light rain. However sometimes weirdly heavier precip can make it across the mountains fine with the evening of 17th December 2010 a good example with fairly heavy snow showers making it here.

However in general here is very dry compared to Wales and shares rainfall totals similar to that of the East of England and SE England.

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It's always puzzled me, how "mountains" less than 1000m can produce such rainshadows. (New Zealand's are over 3000m). But the Welsh mountain rainshadow; why does it only seem to work properly in the winter months? In the last week we have had next to no rain here yet the mid-Wales hills have had 200mm+, and the Severn has flooded- it's even been sunny on a couple of days. Yet the same setup in July or August usually means cloud and drizzle here, fog and heavier drizzle in Wales, and leave the sun for the far SE. July 2010 saw this for weeks on end.

And why isn't N Shropshire/S Cheshire even drier than Essex/Suffolk/London, seeing as it's sheltered by the Peak District from NE'lies as well?

I think in the summer you keep the cloud but lose most of the rain.

I wonder if the perception that it works properly only in winter is influenced by the fact that the sun normally shines for a greater percentage of the possible time in summer. So, for instance, if a month dominated by that setup was to yield 33% of the possible total sunshine, you would get approximately 85 hours of sunshine in February (rather above average) or 165 in July (rather below average), giving the impression of summer months being duller with this setup even though in absolute terms they ended up about the same.

July 2010 was certainly an exceptionally dull month in western Britain but it was also an exceptional case.

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I think in the summer you keep the cloud but lose most of the rain.

I wonder if the perception that it works properly only in winter is influenced by the fact that the sun normally shines for a greater percentage of the possible time in summer. So, for instance, if a month dominated by that setup was to yield 33% of the possible total sunshine, you would get approximately 85 hours of sunshine in February (rather above average) or 165 in July (rather below average), giving the impression of summer months being duller with this setup even though in absolute terms they ended up about the same.

July 2010 was certainly an exceptionally dull month in western Britain but it was also an exceptional case.

165 hours up the road at Shawbury this July would mean the second sunniest July for 10 years, beaten only by 2006. The "keeping the cloud but losing the rain" was certainly true last July; the sunshine totals for Shawbury (86.1) and Cwmystwyth in upland mid-Wales (80.3) being very similar; but the respective rainfall ones were 74.6 and 209.3- a huge difference!

Another reason for that perception might be the number of times that it's sunny here at 6am in summer, overcast by 11, then we see the sun again for half an hour before it sets; maybe a third af the day is sunny, but not when you need it! This area gets an awful lot of that weather.

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That sounds like a convection-related issue- under relatively stable atmospheric conditions with strong sunshine it's quite common for convective cloud to bubble up, encounter a cap and spread out into stratocumulus. Tyne and Wear often has that problem as well- I remember an extreme case in June 2002 when it seemed that most of the month's sunshine occurred before 9am.

Thus, there probably is a lot of truth behind what you're saying- maybe under some synoptic circumstances, convection helps to keep cloud going in summer for most of daylight hours, whereas the same setup in winter would see the cloud dissipate to the east of the mountains.

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That sounds like a convection-related issue- under relatively stable atmospheric conditions with strong sunshine it's quite common for convective cloud to bubble up, encounter a cap and spread out into stratocumulus. Tyne and Wear often has that problem as well- I remember an extreme case in June 2002 when it seemed that most of the month's sunshine occurred before 9am.

Thus, there probably is a lot of truth behind what you're saying- maybe under some synoptic circumstances, convection helps to keep cloud going in summer for most of daylight hours, whereas the same setup in winter would see the cloud dissipate to the east of the mountains.

I was going to post rainfall and sunshine data from some dull westerly summer months from here and Mid Wales, decided not to because of taking up too much space, but guess the other one I instantly thought of besides July 2010 and August 2008? Yep, June 2002! Not very wet, not very cold, not very sunny is how I can best describe that month.

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Last July was no doubt the dullest I`ve ever seen,there were a few chilly NW-lys too giving the odd heavy shower but very little sun that day.

68mm of rain.

Does look like June 2002 was a non eventful month that gave 58mm.

This was the wettest day just under 1 inch and very muggy day.

Most unexpected chart.

http://www.wetterzentrale.de/archive/ra/2002/Rrea00120020606.gif

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I find it interesting that despite it being just to the east of the Pennines, Sheffield actually has higher average annual rainfall than Manchester Airport. Could anyone provide a possible explanation for this? There isn't much in it, however.

Manchester Airport is only about 69m asl, wheras Sheffield is on average 140m above sea level so in turn elevation helps. It's also noteworthy about where I live, I live about 14 miles from Manchester Airport and it can be a blizzard here but rain at the airport, sometimes I have the same weather as West Yorks yet I am classed as in Greater Manchester. So with a difference of about 350 feet between here in the Pennines and the airport it can make a huge difference.

My family live between 200-240 feet lower then where I live, just a few towns away and the snow melts quicker than here, about a week earlier. So you have to go about 200 feet higher and the weather can change quite dramatically here in the UK.

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