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
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
Geoffrey Wildman 38 Union Street Grantham Lincolnshire NG31 6NZ. Email Disclaimer
Notice
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
We get most of our weather from where the cold Labrador Sea meets the warm Gulf Stream
off the coast of Newfoundland