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Old 11-22-2010, 02:53 AM
 
Location: motueka nz
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Originally Posted by deneb78 View Post
Definitely think the orographic effect is the main reason for higher rainfall amounts on the western side of the mountain slopes here in the PNW. It's also why you have a temperate rainforest here with huge trees such as in Pacific Rim National Park on Vancouver Island as well as Olympic National Park in Washington State.
Even locally rainfall amounts can change by extreme amounts due to rain shadow effects. A good example here in BC is Victoria, BC vs Port Renfrew, BC. Victoria has an annual rainfall of over 608mm (23.9 inches) a year but nearby Port Renfrew, BC only 80km (50 miles) away has an annual rainfall of 3,671mm (144.5 inches). This is all due to the rainshadow that the nearby Olympic mountains cause on the systems that come in from the Pacific.
NZ has some pronounced rainshadow areas, with one region in the south having a twenty fold difference over 130 k(85 miles). In my own region we get 1400mm(56 inches) at our place, while the hills 25km(15 miles) away get 3000-5000mm(120-200 inches). It can get pretty wet here at times, but the rain tends to be short and sweet, no drizzle. Some rain would be nice now though!
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Old 11-22-2010, 03:24 AM
 
Location: Yorkshire, England
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Originally Posted by wavehunter007 View Post
From what I understand...the lower annual rainfall in western Europe is mostly to do with blocking and orographic issues. The modest rainfall in the lowlands of western Europe, which seems to be open to lows coming in from the Atlantic...is related to the infrequency of rain-bringing disturbances in latitudes 45 to 55 North (the approximate position of NW Europes Atlantic lowlands). Middle latitde cyclones seem to go north and south of this zone...but not across the zone. Relatively speaking...
That's more or less right, the depressions that give somewhere like London its rain rarely hit the major cities directly. Most of them pass by NW Scotland at about 57-58N then veer up the Norwegian Sea. From what I've read a lot of places in the PNW often record 0 sunshine hours in a week in winter. London and even Manchester will generally have some sunny breaks in the rain even when the weather is cyclonic.
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Old 11-22-2010, 07:13 PM
 
Location: The western periphery of Terra Australis
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Originally Posted by wavehunter007 View Post
That’s right from the way I understand it:

The drier summers of the PNW in Canada/USA compared to NW Europe is the result of the northward displacement of the North Pacific High. The cool waters generated by the cell’s circulation might also be an auxiliary factor. While the center of the North Pacific High is about 38 North in the summer – similar to the Azores/Bermuda High in the Atlantic…the isobars on its landward side bulge in a northeasterly direction toward the PNW. It’s not uncommon in July and August for the arm of the North Pac high to extend to Tatoosh Island near 48 north. The northward arm of the Pacific High not only stabilizes the air near the PNW…but acts to block most middle latitude cyclones from reaching the coast in July and August.

The Azores/Bermuda High is too far west (closer to the USA) to keep middle latitude storms/fronts/jets away from NW Europe in summer.

From what I understand...the lower annual rainfall in western Europe is mostly to do with blocking and orographic issues. The modest rainfall in the lowlands of western Europe, which seems to be open to lows coming in from the Atlantic...is related to the infrequency of rain-bringing disturbances in latitudes 45 to 55 North (the approximate position of NW Europes Atlantic lowlands). Middle latitde cyclones seem to go north and south of this zone...but not across the zone. Relatively speaking...
I think the dominating ridges of high pressure or the 'blocking highs' of the Atlantic play some role in making Europe comparatively 'dry' (in amount), but this is mainly in summer (when rainfall tends to be convective). The main reason for a place like London being relatively dry is, as has been said, mostly orographic and due to a moderate rainshadow. People often underestimate the influence hills can have on climate (precipitation and other factors too). Even hills as low as 200 metres influence rainfall. Actually, the wettest parts of Britain: the exposed highlands of Western Scotland, the Lake District, Snowdonia in Wales.etc compare quite favourably with wetter parts of the PNW. From memory some parts of the Lake District are as wet as the Olympic Peninsula and parts of BC. The fact is despite their modest totals, London, Paris etc are still basically moist climates with high cloudiness, high frequency of rain and constant low pressure from an almost permanent trough. Depressions/fronts hit England on an almost weekly basis. It's just alot of that rain has already been wrung out by the time they reach those cities.

As for the PNW being drier in summer, it has to do with both the temperature of the ocean in summer, and of course the latitude. In Europe the summer band of westerly fronts don't really pick up until you reach Northern France, which is already the latitude of Seattle.
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