To understand our weather we must look into what is happening in North America as we get most of our weather from that direction. Due to the rotation of our planet and the Coriolis effect in the Northern Hemisphere winds are blown to the right across the North Atlantic to our shores. The Coriolis effect increases in strength the nearer you get to the North Pole and is responsible for large cyclones or depressions. The weather in the North Atlantic is influenced by the 23.5° tilt in the axis of the Earth as the seasons are caused by variations in the angle of the sunlight hitting the Earth and are one of the major causes of Atlantic storms. During the Winter the UK is leaning away from the Sun so sunlight hits it at a greater angle thus spreading its energy over a much wider area which keeps temperatures low so winters will always be cold. During the Summer it is leaning towards the sun.





 


Jet Stream: They are important because they control our weather. The meteorologist Oishi Wasaburo detected the jet stream in the 1920’s from a site near Mount Fuji but his work went largely unnoticed outside Japan. It was only in 1944 during the Second World War when the high altitude B 29 bomber was used in the Pacific that the jet streams were actually recognised. Jet streams are narrow fast flowing "rivers" of air. They are formed by temperature differences in the upper atmosphere between the cold polar air and the warm tropical air. This abrupt change in temperature causes a large pressure difference which forces the air to move. In our latitude the jet stream is generally found at around 35,000 feet and is called the Polar Front Jet Stream. The polar front jet stream as its name implies separates the cold polar air to the north and the warm sub-tropical air to the south. With the temperature contrast of these air masses greatest in the winter time the jet stream is stronger at this time of the year reaching 300 miles per hour but have been measured at over 400 miles an hour in south-west Scotland. Jet streams are typically thousands of miles long hundreds of miles wide and a few miles deep. The strong winds along the jet stream generally blow from west to east due to the rotation of the earth. Jet streams move north and south following the boundary between warmer and colder air. These boundaries are also where weather fronts generally develop so when a front passes overhead bringing wind and rain it is quite likely that a jet stream is passing undetected too. The wind direction in the jet stream can change from the normal west to east to almost north to south. This is one of the methods that the Earth uses to transport excess heat from the equatorial regions towards the poles and in turn bring cold polar air southwards. It also helps to steer our Atlantic weather depressions from their normal eastward movement. At times it can even block their movements altogether. Jet streams can strengthen up or even die out so. The winds in the jet stream do not necessarily blow at a constant speed or in a straight line. Within this fast moving air there are accelerations and decelerations as the air speeds up slows down or in fact changes direction. It is at these points in the atmosphere that high and low pressures starts to form and either moves quickly in the wind flow or develops into a bigger depression or anticyclone. These positive or negative acceleration points are very important to the weather forecaster and these occur at the entrance and exits of the jet stream. By looking at a simple diagram of a jet stream it is possible to pick out the areas below which a depression or anticyclone is most likely to form. Weather forecasters use the polar jet to predict medium-term temperatures because it acts as a barrier between cold Arctic air and warmer tropical air. If the polar jet moves north then areas just south of it will be warmer and drier than normal. If it moves south then areas just to the north will be cooler and wetter.


UK Weather - Monitoring climatic conditions in the North Atlantic and weather over the UK

Daily Jet Stream Analysis - 20 May:  The Polar Jet Stream is not over the UK today so our weather should be more settled though a minor stream of high altitude air near the Outer Hebrides might cause winds to increase over the top of Scotland and near the Orkney and Shetland Islands as it travels on to Norway. At midday not much change over the country. Actually the jet stream will not be over the UK for at least the next 5 days which will let in high pressure and temperatures will begin to rise to 20 -25 C though jet stream activity will increase near the West Coast of Ireland from Tuesday 22 May. Massive precipitation is still building up over Northern Italy near Genoa and Bologna The Alps and down the Adriatic near Greece. Overnight and on Monday morning the country will still be free of the high altitude jet winds though it looks like the jet stream activity near The Alps and Northern Italy will cause a branch of the jet stream to travel up over France to near Le Havre which looks like affecting the English Channel.  















    

Current North Atlantic Surface Pressure Chart

The Met Office

Today: Mainly dry with some warm sunny spells, the best of these in the north and west. A chance of showers in parts of the south. Cooler and mainly cloudy in northeast and central England in a northeasterly breeze.


Tonight: Low cloud will spread inland into central areas. There could be some showers in the southeast. Dry with long clear spells in the west and across much of Scotland.












Canadian Weather:  Nova Scotia: Clear skies, light winds and near zero temperatures will give frost to Eastern Cape Breton tonight. At 4 am ADT Halifax is sunny with scattered cloud and a temperature of 9.9 C together with N winds of 11 km per hour. Ontario: An approaching warm front is expected to bring precipitation to Northern Ontario regions this evening. Most of the precipitation will be rain, however for areas near the Hudson Bay the surface temperature will still remain near or below freezing. As a result periods of freezing rain are most likely this evening and continuing overnight before tapering off early Sunday morning. In general 5 mm of ice accumulation is expected with the freezing rain. Strong winds of 25 knots per hour over Lake Superior and the Northern part of Lake Huron. High pressure in the area. Ice pressure warning in affect for the NW Labrador Sea and for the East Davis Strait. Build up of icebergs. Gale off Cape Farewell in Southern Greenland where a low pressure system of 996 hPa is coming out into the North Atlantic.


 



 

Precipitation Chart with a pop-up Temperature Chart - Courtesy of Icelandic Meteorological Office

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The Cairngorms

Latest Mountain Weather Information

UK Weather for the 20 May: A warm front is moving into East Anglia Essex and SE England today from the Continent with an area of high pressure extending up the country. For example at 12.00 hrs Clacton-on-Sea Dagenham Hornchurch and Windsor are all reporting a temperature of 15 C and it is extending as far west as Plymouth in Devon where it is 14 C Showers are building up over the Channel Islands and near the Cherbourg Peninsula and appear to be moving crossing the English Channel to affect coastal areas along Devon and Cornwall from Torquay to Falmouth. Winds are increasing in the south-eastern corner. At 11.00 hrs the Sandettie Lightship off Dover is reporting NNE winds of 28 mph with a temperature of 9.7 C and air pressure of 1009.7 hPa and Langdon Bay in Kent is reporting a temperature of 12 C with NNE winds of 25 mph. Further NE in Lincolnshire Donna Nook near the East Coast is reporting N winds of 23 mph. A gale with a low pressure system of 998 hPa is building up west of the Outer Hebrides. High pressure of 1018 hPa is over Western Norway which is helping eastern Scotland. Turriff is reporting 12 C and Aberdeen is reporting a temperature of 10 C   

       




      

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Ben Nevis

The Lake District

Snowdon

We get most of our weather from where the cold Labrador Sea meets the warm Gulf Stream off the coast of Newfoundland

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