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WARNING - Low Level Jetstream in the South this Weekend


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Shamelessly lifted from another website, but best to get this out to as wide an audience as possible:-


This musing is of most interest to pilots who plan to fly late this evening or early on Saturday morning, but I hope that those of you who love meteorology will be fascinated by these features too.

After a hot day during the spring and autumn months temperatures can fall quickly. This results in a inversion forming (where temperatures actually rise with height as opposed to the usual falling with height).

Now, inversions are nothing unusual and they will form on most cool nights.

What is different about the type of inversion that we will see tonight is the sharpness of the temperature gradient across it (~8C tonight). This sharp gradient tends to occur at very low levels, usually within the lower few hundred feet of the atmosphere.

With the colder air at the surface being dense, and the warmer air above the surface being less dense, this gradient increases wind speeds markedly across the top of the inversion.

Conditions across southern England are near perfect tonight for the formation of a sharp inversion. This will lead to an increase in wind speed in the first few hundred feet from the surface, perhaps to be in excess of 20kts; we call this a low level jetstream (LLJ).

Now 20kts is not a particularly fast speed, but if a change in speed from say 1kt at the surface to 20kt at 100ft occurs, then this can cause particular problems for those need to fly within this zone. This puts balloons at particular risk, as it is these which need to fly when the winds are lightest, usually in the early morning and late evening.

As the ground warms in the morning the inversion will collapse. We then sometimes feel this at the surface as a period of a few minutes of gusty winds, and those of you in southern England may feel that between around 8am and 9am on Saturday morning.

For me I wouldnt like to launch into what seemed like calm air into a 20knt jetstream at 100ft!

Safe flying this weekend guys!


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Thanks for the heads-up Tricky.

Although it looked alright at home, when I got to the field (which is higher up) it was blowing quite a bit at ground level never mind 100 feet :(

I got up 'specialy early to enjoy the still air :?::? There's always tomorrow morning :)

Cheers, Alan

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Thanks for the warning, but I think he got it wrong.

The best way to check wind speed and direction at different heights is to look at the nearest sounding chart for the time you intend to fly these not only show the inversion but also the wind strength at different heights.

See: http://rasp.inn.leedsmet.ac.uk/RASPtabl ... bleGM.html

By the way: A Jet stream is a narrow band of fast moving air and not caused by or associated with an inversion. the air mass above the inversion may well have a different wind speed or direction, but as far as I am aware this is not caused by the inversion.

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