How It Works - UK (2020-02)

ENVIRONMENT

050 How It Works orksdaily.com

A

ir in the atmosphere is constantly on the

move. Composed of several layers – from

just above ground level to high in the sk y

• air particles are subjected to var y ing pressures
  and energ y distribution. Whether in slight
  movements to carr y heat from the equator
  towards the poles or through strong currents
  called jet streams, air is on a continuous journey
  often impossible to see.
  Earth’s atmosphere is div ided into five main
  layers, with the troposphere ly ing closest to the
  surface. Ranging from six to 20 kilometres in
  thickness, the troposphere consists of two
  layers: the upper layer, which is known as the
  free atmosphere, and the lower layer, known as
  the planetar y boundar y layer. The bottom layer
  begins at the Earth’s surface and stretches up
  around 1,000 metres. This may sound like a big
  space, but it is actually relatively thin. The air
  mov ing within the layer can be altered greatly
  by changing conditions in the landscape and
  wind-altering pressure above the layer.
  Jet streams are usually found in the upper
  layers of the atmosphere. In the form of thin
  bands of strong w ind, they are created bet ween
  t wo air masses of different temperatures. The
  var ying densit y between masses creates a
  horizontal pressure difference. Wind tries to flow
  from high to low pressures, but the planet’s
  rotation forces the air from west to east, flowing
  around the masses rather than between them.
  Four main jet streams circulate our planet; t wo

How the atmosphere creates Earth’s

air currents high in the sky

The wind

superhighway

north of the equator and t wo south of the

equator. The ones closest to the centre of the

globe are called the subtropical jet streams, and

those closest to the poles are the polar jet

streams. Occasionally the subtropical and polar

jet stream meet, forming a boundar y bet ween

the t wo extremely different air masses. This

boundar y is called the polar front.

The greater the contrast in temperature

across the polar front, the stronger the jet

stream. Ty pically, this is at its strongest during

the winter months. The area stores large

amounts of potential energ y, which is often

converted into kinetic energ y in the form of

extratropical cyclones.

Followthejetstreamsjourneyingaroundtheglobe

How do air currents move?

Creating UK climates

The location of jet streams are heavily analysed when predicting incoming

weather. In the UK, the position of the polar jet stream determines factors such

as where storms will arise. Warm winds that travel over water usually collect

some, holding onto it until moving over colder climates, when it condenses into

rain. When the polar jet stream is above the country, the weather is usually

mild as a result. Contrastingly, if the air current crosses through the land, this

is when storms are brought along. Heavy snow is often seen in the south of

the UK when the jet streams are carried below the country. Continental

Europe gets the majority of the stormy weather in this circumstance, but

wind from low-pressure systems heading towards the south pull in the cold

air from Europe.

The period of time a weather type will persist is dependant on the type of jet

stream in motion. The faster-moving zonal streams will carry air quickly,

passing the storm over in a short time. In more meridional currents, the

stretched, latitudinal path slows progression, increasing the time taken for the

weather front to pass.

When air reaches altitudes of 15 to 20km at the

equator, it begins to move towards the two poles

© Getty

w howitw

When the polar jet

stream travels over

the UK, weather is

wetter and windier

than average

© Getty

Polarjetstream

Highin theatmosphere,thepolarjetstream

is theworld’smostpowerful.It movessouth

in thewinterandnorthin thesummer.

Meridionalflow

Jetstreamsthattakea

slower,meandering

movementaroundtheglobe

aredescribedasmeridional.

Corestrength

Thecentreofjetstreamshavethe

greateststrength.Surroundingairmoves

withthecurrent’sdirection,butatlesser

speeds.Comparedtoa river’sflow,they

areoftenreferredtoas‘riversofair’.

Polar jet stream Subtropical jet stream