How does Earth’s atmosphere create air currents?

Explore the wind superhighway, high in the sky

Air in the atmosphere is constantly on the move. Composed of several layers – from just above ground level to high in the sky – air particles are subjected to varying pressures and energy distribution. Whether in slight movements to carry 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 divided into five main layers, with the troposphere lying 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 planetary boundary 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 moving 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 wind, they are created between two air masses of different temperatures. The varying density 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; two north of the equator and two 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 boundary between the two extremely different air masses. This boundary is called the polar front.

The greater the contrast in temperature across the polar front, the stronger the jet stream. Typically, this is at its strongest during the winter months. The area stores large amounts of potential energy, which is often converted into kinetic energy in the form of extratropical cyclones.

How do currents move?

(Image source: Future PLC – Illustration by The Art Agency/ Barry Croucher)

  • Polar jet stream – High in the atmosphere, the polar jet stream is the world’s most powerful. It moves south in the winter and north in the summer.
  • Subtropical jet stream – This strong current travels at lower latitudes and higher elevations to the polar jet stream.
  • Coriolis effect – The rotation of Earth on its axis means that it turns faster at the equator than at the poles. This deflects air currents towards the north and south, stopping jet streams flowing at the centre.
  • Core strength –  The centre of jet streams have the greatest strength. Surrounding air moves with the current’s direction, but at lesser speeds. Compared to a river’s flow, they are often referred to as ‘rivers of air’.
  • Meridional flow – Jet streams that take a slower, meandering movement around the globe are described as meridional.
  • Zonal flow – The straighter the route taken by a jet stream, the more zonal it is said to be.

This article was originally published in How It Works issue 133, written by Ailsa Harvey

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