Breezes and other winds of interest to glider pilots - the whys
tutorial is not intended as a thesis for a PhD. It aims simply to
give practical advice.
heats up during a sunny day but sea temperature hardly changes.
no general change in airmass if the land heats enough to become warmer
than the nearby sea, a pressure gradient occurs and a sea breeze might
be pedantic, there is a component caused by this on-shore
wind that is added to any existing wind.
Coriolis Effect (technical - don't worry too much) makes the sea
breeze veer to the right later.
This explanation is not STRICTLY scientifically correct but is close enough.
component can :
The first two cases simply imply
convection becomes weak or non-existent near the coast and this effect
moves inland during the day. A significant on-shore wind might
Add to an existing on-shore
wind and make it stronger
or: if no pre-existing
wind, will produces a simple sea breeze that pushes further and further
inland as the day progresses
or: if pre-existing
wind was off-shore, then the on-shore sea breeze component in opposition
might set up a convergence, a "sea
the increase in wind speed could be useful for eg paragliders hill soaring
on or near the coast.
||The third case,
ie when the on-shore sea breeze component opposes the pre-existing wind
is perhaps the most interesting from the glider pilots' point of view.
It can permit flight with
wings more or less level, for extended distances by taking advantage of
the sea breeze convergence
of synoptic charts and the Met Office F 214 will show the winds at 2,000,
5,000 etc before any sea breeze sets in so can give valuable pointers
of wind during the morning
clear skies, winds tend to drop overnight and at dawn, and it is often
calm on the ground. (stagnant air near the ground)
happens when air near the surface becomes colder and more dense.
the convection starts surface and upper air begin to mix.
puff of wind or sudden improvement in visibility is often a good sign that
the thermals has started
in mind that calm conditions early do not necessarily imply that there
will be no wind later in the day.
on-shore "general" wind might in effect be there all the time, but "hidden"
for the time being due to that stagnant air near the surface..
Air Penetration with on-shore gradient wind
instances might be when a NE wind blows across East Anglia,
a SW wind affects Southern England or a NW into Lancashire.
thermals will not occur (exception early Spring - see below) until many
distance (of non convective air) increases once the sea breeze becomes
like Lasham can be severely affected in SW winds yet further from the coast,
eg at Dunstable, excellent conditions might occur.
example will help explain why sea
air penetration spoils conditions close to upwind coasts.
from the coast, surface dewpoint Tdew = 10°C
Trigger for convection Tdry = 16°C.
then forms inland at 16°C with base 2,400 ft Bradbury Rule
Cloudbase = 400 x (Tdry - Tdew)
inland, by mid afternoon away from coasts, Tdew is (more or less)
unchanged at 10°C but by now Tdry = 22°C.
base inland thus 12x400 = 4,800 feet
the further assumption the sea temperature is 14°C
the beach, the air temperature might hardly rise above 14°C (bracing
in the old seaside brochures), that is, well below the trigger for convection.
greater the distance inland, the more sea air will heat up and small cumulus
will form a short distance inland but these are likely to be at quite low
factor that might keep the cloud base even lower near the coast could be
that the influx of sea air might result in a higher dewpoint.
further from the coast, the sea air has negligible effect and proper
two pictures were taken on the same flight in East Anglia.
wind was northerly at around 10 knots.
left photo was near Fakenham, the right one some 60 kms further inland
to the south near Bury St.Edmunds
example had assumed typical summer temperatures. Early in the
season, some of the best thermic conditions occur in very cold air
that can readily be heated from the surface. The sea itself might
be warm enough. Sea temperatures, in the North Sea for example, hardly
drop below about 7°C even at the end of the winter. So
thermals can occur over the sea itself, and sea air penetration is not
Breeze Convergence (Popularly
known as a Sea Breeze Front) might occur with off
colder sea breeze air cuts under the land air and can produce a long line
of lift. This is not continuous and often has gaps.
does not usually move inland steadily but in a series of jerks. It
is very easy to get trapped on the wrong side (in the non-thermic air)
photos at the top of this page show three good examples of sea breeze
convergence (all in East Anglia) Notice the "stepped" cloud base.
|Pseudo Sea Breeze Front
a term coined in the early 1970s to describe an unusual phenomenon.
It came to prominence in
a gliding competition (in which I happened to be flying). North Sea
stratus covered East Anglia and the East Midlands. Further
west, the sun shone from dawn. The cold conditions under the stratus
in effect acted like a cold sea and a "sea breeze front" moved westwards
and undercut the cumulus further west. I was lucky (smart?) enough
to recognize what was happening. Others were not so lucky.
Indeed, one pilot had a minor accident when he - without realizing the
change in wind direction - landed downwind.
Rules of Thumb for Sea Breeze Convergence
guidelines are based on experience and have been known for many years.
The science behind them is obscure, but the rules work!
areas for sea breeze fronts:
wind component less than 10 knots
of convection (including any cloud tops) between 3,500 and 10,000 feet
means that hot, blue days are unlikely to get sea breezes except puffs
air very close to the beaches
be warmer than sea but the difference amazingly can be as little as 1°C
sea breeze front could move up to 50 kilometres inland
Coast in winds between NW and NE
in W'ly or SW'lies.
England in W or NW'lies.
soaring stay under the higher base cloud. (clearly a different
scenario for PG's soaring sea breezes on cliffs)
Surface wind. Bear
in mind possible changes. It might be much stronger than when you
took off. It might be in a very different direction. Watch
for the signs, eg direction of smoke.
Coastal fog and low cloud
can come in very suddenly. Cliff soaring can be full of hazards!
was amazed recently when on the east coast to watch a bank of sea fog reduce
the visibility form 10 kilometres to around 100 metres in less than a minute
(although this was not in fact a sea breeze effect)
movement of the front is not a steady progress but often in a series
of jumps. Don't get trapped on the wrong side if thermal soaring
by 1600 BST, the front will have progressed inland by about 40 kms.
- do have fun and enjoy one of Nature's "more imaginative ideas"