JETSTREAM
'Jet streams' were first discovered during the Second World War. Pilots were regularly flying between United Kingdom and the United States of America and they noticed that it was quicker to fly to the UK, reporting tailwinds of over 100 miles per hour. These winds blew in narrow ribbons and were named 'jet streams'. If you want to see the current Jet Stream for this region then just or look below:
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.
The strong winds along the jet stream
generally blow from west to east due to the rotation of the earth. That is why,
especially in winter time, flights from the USA often land early in this country
as they are blown along by these very strong winds. (Incidentally it is also the
reason for some "bumpy" rides with clear air turbulence). Planes never land
early going the other way. Jet streams move north and south too, following the boundary between warmer
and colder air. 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. 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. This is the fundamental way that forecasters use jet streams to try to
predict whether and where a rain-bearing depression will form, and if it forms
whether it will develop into a full blown storm which may cause structural
damage as it rushes in from the Atlantic, or whether it will just be a little
blip in the fine weather that rushes along at 60 miles per hour.
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 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 southwest
Scotland). Jet streams are typically thousands of miles long, hundreds of miles
wide and a few miles deep.