Meteorologist and sailor Chris Tibbs explains how a better understanding of the jet stream can improve your navigational planning
What is the jet stream?
The jet stream is a ‘river’ of strong wind high in the atmosphere which is caused by the unequal heating of the spinning earth. It naturally forms in longwaves, typically 2,500-5,000 miles in length, that are slow-moving or at times stationary. Embedded in the longwaves are faster moving shortwaves, or ripples, that will develop and deepen depressions.
How can it affect sailors?
Sailors in the 2017 ARC and 2017 RORC Transatlantic Race both experienced a rather unusual weather pattern that had most participants beating into headwinds for a while. It was not the tradewind sailing that we have come to expect.
Most boats in the ARC had a period of beating, and for the boats following a week later in the RORC race, it was hard on the wind from the off. Many yachts followed a more northerly route sailing across the Atlantic into strong headwinds and large seas, causing some to retire through damage.
This was due to a large depression sitting west of the Canary Islands, blocking the route west and preventing the trade winds from reaching south (except for a narrow band very close to the African coast).
At the same time, back in the UK, we had a period of unseasonably cold weather with early snowfalls across much of the country. Although far apart in distance, these two features were closely linked by the jet stream.
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In the UK we are familiar with the jet stream steering depressions across the Atlantic in the summer months. When the stream passes over the north of the UK, the lows also pass close by, giving unsettled and often windy weather.
When the jet stream migrates north, the UK will usually get more settled weather under the influence of high pressure. In winter months when the jet stream migrates north we will generally have dry but cold weather in the UK and, when it is south, mild and stormy conditions.
By the end of November and early December a large loop was visible in the jet stream (see the 500mb chart, above) showing a low mid-Atlantic and a large high to the west of the UK.
The 500mb charts show the atmosphere at about 5,000m (in reality a little higher); they are particularly useful as we can see the stronger wind of the jet stream found close to 10,000m, and also roughly where surface features are likely to be.
By comparing the 500mb charts with surface charts (below) forecasters can get a better understanding of just what is going on up in the atmosphere.
On the more familiar surface charts we can see this large high to the west of Ireland. On the east side of the high there were strong and cold northerly winds – not only did this bring cold temperatures but as the sea water is relatively warm, moisture evaporating from the ocean created cumulonimbus clouds dumping snow showers over the UK (and bringing the customary travel chaos).
Further south a low out in the Atlantic had developed in the southern loop of the jet stream. Over time we can see that the low becomes ‘cut off’ from the jet stream and therefore it loses the steering effect of the high, blocking any passage to the north-east. With no driving jet stream it became stationary over the mid-Atlantic, preventing the trades from becoming established.
This left a dilemma for the sailors; sail through, or north of, an almost stationary low pressure system giving strong headwinds? Or dive south to try and hook up with weak trade winds?
The southerly option gave a period of light and variable winds to negotiate – fine for the cruising boats but frustrating for the racers.
Watching the racing fleets we saw CQS dive south early and get line honours in the RORC race, while for the racing class in the ARC, Talanta won Class A after following a tough northerly route.
Close behind, Scarlet Island Girl took Class B (and overall) having followed a less extreme route diving south mid-way across.
It proved to be a particularly difficult year for balancing speed, comfort, and risk of gear failure while still trying to cross in the best possible time.
First published in the April 2018 edition of Yachting World.