Jet Streams – a river in the sky

A big temperature contrast across America has led to a powerful jet stream driving low pressure systems towards the UK over the coming days, as shown in this short video.

So what is a jet stream?

Over the Earth’s surface there are large-scale wind circulations. The global circulation can be described as the world wide system of winds by which the necessary transport of heat from tropical to polar latitudes is accomplished. In each hemisphere there are three cells (Hadley Cell, Ferrel Cell and Polar Cell) in which air circulates through the entire depth of the troposphere.

jet-streams

Figure 1: The global circulation cells, which drive the winds on Earth.

The Earth wants to be at a temperature equilibrium and so the three different cells enable the heat at the Equator generated by the sun to be transported to the poles. The earth’s rotation means that air does not flow directly from the equator to the poles – instead it is deflected to the right (in the northern hemisphere – the opposite is true in the southern hemisphere). This gives the UK prevailing west to south-westerly winds.

In addition as the warm air moves pole wards and the cool air moves towards the equator a strong temperature (and pressure) gradient will form around 40-70°N between the Polar and Ferrel Cells. At this location the polar jet stream is formed. The jet stream is a narrow band of fast flowing wind (greater than 60 knot and can exceed 200 knot in winter) at an altitude of about 12 km.

Why is an understanding of the jet stream important?

As the height of the jet stream is at approximately the same height that aircraft fly at it is important that accurate forecasts of the winds can be produced so that the flights can take the most efficient route; trying to go around the jet if they are heading into it or making use of the strong tail wind by being in the middle of the jet core.

windjetFigure 2: A global wind field for 250hPa (~FL340) on 9th January 2017 at 00z, the speed is in knots and wind fletches indicate both the speed and direction of the wind flow.

Lauren Reid- Business Development Manager

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