Download Weather Part 3 - Local Effects

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Weather Training Notes. Part 3
Local Effects
Forecasts usually show wind-speed based on 10m above ground, that’s around the top third of your
mast. They also don’t take into account local effects, no matter how well documented they are, such
as seasonal winds like ‘Le Mistral’ nor sea-breezes. So knowing your local area can really help.
Part 1 of this series looked at sources of forecast data and the various weather models used. In Part 2, we identified factors
affecting confidence in a forecast. In this Part 3, we will look at local effects on weather - so you can adjust a general
forecast to the particular area where you are sailing.
Impact of Land Masses
1. Wind Shadows
Any land masses will slow wind down (by friction) and tall trees
or buildings will affect it more than low-lying open land. This
can lead to obvious wind-shadows, as the wind is forced around
or above the object, for example the wind-shadow caused by
the mountain on Guadeloupe can extend several miles, and
navigational decisions at this point can win or lose the RORC
Caribbean 600 race!
Whereas if you were seeking a quiet anchorage this windshadow could be a great spot!
Area of windshadow, also can see the funnelling
of wind around the island of Guadeloupe.
2. Wind Bend
Generally in lighter winds, the wind will bend around a headland more (creating significant changes in wind
directions) whereas strong winds rush through channels.As you head up a river or approach a large port, large
buildings may create wind-holes followed by sharp gusts where the wind has funnelled around the side. Islands
can also cause wind to funnel – for example through the straights of Messina between Sicily and ‘the toe’ of
Italy which is known as an ‘acceleration zone’.
3. Convergence – Divergence
In the northern hemisphere, as the wind is slowed down by friction, it goes to the left (backs). This is caused by
the effect of the Earth spinning (the Coreolis effect). So as you sail closer to land not only may an offshore wind
speed drop it will appear to change direction by 10-15
degrees.
Imagine a stretch of water with the wind blowing
parallel to the coast. If you stand with your back to the
wind, on the left hand coast the wind blowing over land
will slow down and back left when compared to the
wind over the sea, creating divergence. This results in
less wind pressure in this area.
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Conversely, on the right-hand shore, the wind blowing over the land will slow down and head left, towards the
sea, creating convergence – and an increased wind pressure.
To find out more about how this can affect your sailing or racing, consider joining our Weather Course for Sailors
on Saturday 5th July 2014.
4. Air-Mass Stability
Stability is a term not often used (see Weather Part 2 for more discussion on stability), but a more stable air
mass has fewer gusts. However, in an unstable air mass we can expect gusty conditions in addition to rain. Gusty
conditions may scare novice sailors, rip sails, cause broaches and so on, so knowing what to expect can change
your sailing plan.
Land masses, which can be at a very different temperature to the water, as well as their height and shape affect
the wind flow, can lead to instability. Cold air masses are also usually more unstable, clouds and fronts also
cause instability and lead to more shifty conditions.
Formation of Sea-Breezes
Sea-breezes are most likely to be seen in summer, when cloudless, sunny mornings allow the land to warm up.
Water takes longer to warm up than land, which heats up quickly in the morning sunshine creating a
temperature differential. Land doesn’t hold its heat very well, so by afternoon it has heated up the air above it,
which rises. In turn this sucks in the cooler air from over the
water, thus causing a breeze to develop from the sea
towards the land.
If there is an offshore gradient (higher altitude) wind, then
this helps this cycle to develop further – at a high altitude
the breeze is offshore but at sea level it is onshore. This
phenomena can explain why little cumulus clouds can
develop along the coast line. Forecasting the strength and
direction of a sea breeze is very difficult as it is affected by
the exact temperature difference between land and water,
as well as the strength and direction of the gradient wind, as well as the shape of the coastline!
If you have enjoyed reading these notes and would like to know more, join our new Weather For Sailors Course
or Saturday 5th July 2014, run by Libby Greenhalgh a weather professional and the official Team GB Sailing
weather forecaster.
©SailingLogic 2014
023 8033 0999
[email protected]
Return to Knowledge Centre