Jet streams and the upper westerly winds

Overview

Upper westerly winds, jet stream and rossby waves.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What are the upper westerly winds?

   Model answer

   They are the broad flow of air in the upper troposphere over the mid-latitudes of both hemispheres, moving from west to east. They occupy roughly the upper half of the troposphere between about 30 and 60 degrees latitude, blow far faster than surface winds, and steer the weather systems beneath them. The jet streams are the fastest cores embedded within this flow.

2. 2

   Explain why the winds high above the mid-latitudes blow from the west, in terms of the pressure gradient and Coriolis forces.

   Model answer

   Air in the tropics is warm and expands vertically, while polar air is cold and contracts, so at any given height above the ground pressure is higher over the tropics than over the poles. This creates a pressure gradient force aloft that pushes air polewards. As soon as the air moves, the Coriolis effect deflects it to the right of its path in the Northern Hemisphere, turning a poleward flow towards the east; in the Southern Hemisphere the poleward flow is deflected to the left, which also turns it towards the east. The air turns until the Coriolis deflection balances the pressure gradient force and the wind blows almost parallel to the height contours, which in both hemispheres means a west-to-east flow.

3. 3

   What is a jet stream?

   Model answer

   A jet stream is a narrow ribbon of extremely fast wind near the tropopause, typically 9 to 16 kilometres above the ground. It is only a few kilometres deep and a few hundred kilometres wide but thousands of kilometres long, and its core commonly blows at 100 to 250 kilometres per hour, occasionally exceeding 400 kilometres per hour in winter. It flows from west to east in the mid-latitudes and marks the position of the sharpest horizontal temperature contrast in the atmosphere.

4. 4

   Name the two main jet streams in each hemisphere and say where each is found.

   Model answer

   The Polar Front Jet Stream lies at about 50 to 60 degrees latitude at the tropopause above the polar front, where the Ferrel and Polar cells meet. The Subtropical Jet Stream lies at about 25 to 30 degrees latitude at the tropopause above the descending limb of the Hadley cell, where the Hadley and Ferrel cells meet. The polar front jet is the stronger and the more variable in position, and it is the one that matters for British and Irish weather.

5. 5

   Why does a jet stream form exactly where two circulation cells meet?

   Model answer

   The boundary between cells is where two bodies of air of very different temperature lie side by side, so the horizontal temperature gradient is at its steepest. A steep temperature gradient means the pressure surfaces aloft slope very sharply across a short distance, which is a very steep pressure gradient force. That force accelerates the air, and the Coriolis deflection turns it until it runs parallel to the boundary, concentrating the flow into a narrow, very fast core.

6. 6

   Why are the jet streams stronger in winter than in summer?

   Model answer

   The speed of a jet depends on the size of the horizontal temperature contrast that drives it. In winter the polar regions cool dramatically while the tropics change comparatively little, so the temperature difference across the mid-latitudes is at its greatest, the pressure gradient aloft is steepest and the jet accelerates. In summer the polar regions warm, the contrast weakens and the jet slows and often shifts polewards.

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