WEATHER briefing
CHRIS TIBBS ON THE JET STREAM
ailors in the ARC and RORC
Transatlantic Race both experienced
a rather unusual weather pattern last
year 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 that followed a more
northerly route across the Atlantic were sailing
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.
Wide-ranging effects
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, top right) 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 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,
Ta l a n t a 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.THE JET STREAM
The 500mb chart
shows the jet
stream moving
north over
Greenland before
reappearing as a
northerly wind over
the UK. Mid-Atlantic
a low pressure
system is becoming
cut off from the
streamS
The surface chart
shows the intense
high pressure west
of Ireland and the
slow moving low
west-north-west of
the Canary IslandsThe 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.