Sea Breezes and other winds of interest to glider pilots - the whys and whens

This tutorial is not intended as a thesis for a PhD.  It aims simply to give practical advice.

Land heats up during a sunny day but sea temperature hardly changes.

Assuming 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 set in. 
To be pedantic, there is a component caused by this on-shore wind that is added to any existing wind.

The Coriolis Effect (technical - don't worry too much) makes the sea breeze veer to the right later.

Remember:  This explanation is not STRICTLY scientifically correct but is close enough.

This on-shore component can :
  • 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 breeze front"
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 develop.

However, 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 

Study 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

Changes of wind during the morning

Under clear skies, winds tend to drop overnight and at dawn, and it is often calm on the ground.  (stagnant air near the ground)
This happens when air near the surface becomes colder and more dense. 
As the convection starts surface and upper air begin to mix.
spacer

A puff of wind or sudden improvement in visibility is often a good sign that the thermals has started
Bear in mind that calm conditions early do not necessarily imply that there will be no wind later in the day.
An 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..

Sea Air Penetration with on-shore gradient wind

Typical instances might be when a NE wind blows across East Anglia,
or a SW wind affects Southern England or a NW into Lancashire.

Good thermals will not occur (exception early Spring - see below) until many kilometres inland.
This distance (of non convective air) increases once the sea breeze becomes established. 

Places like Lasham can be severely affected in SW winds yet further from the coast, eg at Dunstable, excellent conditions might occur.

Sea Air Penetration with on-shore gradient wind

Typical instances might be when a NE wind blows across East Anglia,
or a SW wind affects Southern England or a NW into Lancashire.

Good thermals will not occur (exception early Spring - see below) until many kilometres inland.
This distance (of non convective air) increases once the sea breeze becomes established. 
Places like Lasham can be severely affected in SW winds yet further from the coast, eg at Dunstable, excellent conditions might occur.


A hypothetical example will help explain why sea air penetration spoils conditions close to upwind coasts.  Assume that:
Away from the coast, surface dewpoint Tdew  = 10°C    Trigger for convection Tdry = 16°C. 
Cumulus then forms inland at 16°C with base 2,400 ft  Bradbury Rule Cloudbase = 400 x (Tdry - Tdew) 
Well inland, by mid afternoon away from coasts, Tdew is (more or less)  unchanged at 10°C but by now Tdry  = 22°C. 
Cumulus base inland thus 12x400 = 4,800 feet
Make the further assumption the sea temperature is 14°C
On 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.
The 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 base.
Another 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.
spacer
However,  further  from the coast, the sea air has negligible effect and proper cumulus occur.

These two pictures were taken on the same flight in East Anglia.
The wind was northerly at around 10 knots.

The left photo was near Fakenham, the right one some 60 kms further inland to the south near Bury St.Edmunds
spacer
That 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 of consequence.
A Sea Breeze Convergence (Popularly known as a Sea Breeze Front) might occur with off shore winds
The 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.
It 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)

The photos at the top of this page show three good examples of sea breeze convergence (all in East Anglia)  Notice the "stepped" cloud base.
spacer

Pseudo Sea Breeze Front is 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.

Empirical Rules of Thumb for Sea Breeze Convergence

These guidelines are based on experience and have been known for many years.  The science behind them is obscure, but the rules work!

  • Off-shore wind component less than 10 knots
  • Depth of convection (including any cloud tops) between 3,500 and 10,000 feet
  • This means that hot, blue days are unlikely to get sea breezes except puffs of air very close to the beaches
  • Land must be warmer than sea but the difference amazingly can be as little as 1°C
  • A strong  sea breeze front could move up to 50 kilometres inland
Favoured areas for sea breeze fronts:
  • South Coast in winds between NW and NE
  • East Anglia in W'ly or SW'lies.

    • NE. England in W or NW'lies.

Hazards and advice
  • If Thermal 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! I 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)
  • Inland 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

    • Typically, by 1600 BST, the front will have progressed inland by about 40 kms.

Finally - do have fun and enjoy one of Nature's "more imaginative ideas"