'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'.
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 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.
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 Jet stream also helps to steer our Atlantic
depressions from their normal easterly movements. If the jet stream moves
south of the UK then the cold wet depressions will then get "trapped" over the
UK as happened in June / July 2012. While the North Atlantic jet stream generally points roughly due east, straight across the Atlantic, it often meanders north and south. These meanders are the signature of so-called Rossby waves, which are similar to the familiar waves on the surface of water. However, instead of water moving up and down, Rossby waves consist of large air masses, often thousands of kilometres across, moving north and south. When an air mass moves north it starts to spin clockwise (when viewed from above), forming what’s called an anticyclonic weather system – a high-pressure system associated with settled weather. When air moves south it spins anti-clockwise and becomes cyclonic – a low- pressure system which is prone to wet weather.
A Rossby wave can therefore lead to a string
of alternating high - and low-pressure systems, with the jet stream snaking around them from west to east. Like water waves, Rossby waves generally move relative to an observer on the ground, and this movement leads to changes in the weather from week to week. In fact, the Rossby waves
themselves always move towards the west, which means they are always swimming upstream against the eastward-flowing jet. If conditions are right and the wave speed matches that of the jet, the wave will remain stationary. Then the high- and low-pressure systems are no longer moving relative to the ground, and a persistent weather regime is born. Summer 2007 was a good example of this: a low-pressure system remained stationary over the UK and led to widespread flooding, while just downstream a high-pressure system brought heatwaves and drought to the Mediterranean and Eastern Europe. The UK had the trough of the wave and Eastern Europe had the peak. When waves on the ocean surface become too large they overturn and break, resulting in very turbulent motion. When Rossby waves break, the resulting weather situation is known as blocking.
In this case the turbulent flow often becomes dominated by an anticyclonic air mass cut off from its origin in the subtropics. This high-pressure system blocks the normal passage of the jet stream, and a regime of dry, settled weather sets in. When this happens in winter, blocking leads to a bitterly cold spell, as the mild westerly winds are replaced by winds bringing cold continental air from the east. When it happens in summer the result is drought and heatwaves, and blocking contributed to the events seen this summer in Russia. At the same time, downstream of Russia, a Rossby wave trough remained and interacted with the monsoon system to bring flooding to Pakistan
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.
- The position of the jet stream over the UK determines the type of weather
we experience.
- If the polar front jet is situated significantly to the south of the UK we will
experience colder than average weather.
- If the polar front jet is situated to the north of the UK we will experience
warmer than average weather.
- If the polar front jet is situated over the UK we will experience wetter and
windier than average weather.
- If the polar front jet has a large amplification then cold air will travel
further south than average and warm air will travel further north than
average.
- The direction and angle of the jet stream arriving at the UK will determine
what source of air (i.e. cold, dry, warm, wet, from maritime or continental
sources) the UK experiences.
