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  • The mechanism behind a Sea Breeze


    johnholmes
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    Sea breeze article Tuesday 6 June 2007

    Below is an attempt to explain the mechanism behind sea breezes with the Walney Island case as an example.

    Sea breezes occur around our coasts when there is just a light wind, blowing off the land. However, it is complicated in that the term sea breeze can be used when a very light air drifts off the sea early morning and becomes stronger off the sea during the day. But for the ease of trying to explain it I will stick to the first instance, calm or a light wind blowing off the land at first.

    In this case the 'gradient' will be off the land. By gradient we mean the direction the isobars show. The first requirement is for these to be quite widely spaced thus making any surface flow very light. Remember the wider apart the isobars are the lighter the surface wind and vice versa of course.

    There is no need to go into any great explanation about this here. Just remember that if you see a weather chart with isobars spaced widely apart, during the late spring to late summer, along with very little cloud showing on the satellite picture, using the visual picture, then sea breezes may occur on the coast.

    The sea breeze is formed as a result of the differential heating which occurs between the land and the sea. On a day of light winds with plenty of sunshine the land heats up more quickly than the sea.

    Just one little bit of physics here: temperature and pressure are related through the Gas Laws, so when the temperature increases, all other things being equal, then the pressure decreases. This causes the air over the land to rise and in cases where there is even only limited instability, say no more than about 5,000ft, above the land it starts to flow out to the sea.

    Why does it flow thus?

    As the slight fall of pressure occurs over the land there is a difference then with the pressure over the sea which is then a little higher. Thus, air tries to flow from high to low pressure, and a very slight movement of air from sea to land commences. At the same time above the surface this will set up a circulation, the reverse of what is occurring at the surface, thus a circulation develops as shown in the diagram below. As the land continues to heat up this circulation becomes more pronounced. As time goes on another meteorological law comes into play. This is called the Coriolis effect. Put simply this is to do with the spin of the earth, and in the northern hemisphere, appears to try to turn any wind flow to the right of its actual flow. Thus the sea breeze will veer with time. So for example on a west coast, sea diagram below, the initial breeze will be from a south westerly point and will veer to a west north west direction during the day. This is due to the Coriolis effect.

    post-847-1181339703_thumb.jpg

    The breeze brings in somewhat cooler air from the sea and is strongest around the time of maximum hearing overland and dies away in the evening as the temperature inland falls.

    Sometimes if the conditions are right in the atmosphere Cumulus cloud will form on the sea breeze and in a few instances this cloud can develop into showers with Cumulonimbus developing. A case of a convergence zone with the trigger to it being the sea breeze development.

    In some areas, like Cornwall or Devon, with two coasts, in their case, roughly north and south, it is possible for these two sea breezes(termed sea breeze fronts) to meet and themselves cause a line of showers or even thunderstorms to develop.

    The largest example of a sea breeze type of development in the world is the Monsoon which occurs across parts of Asia. Caused by the huge differences that develop between the inland areas of the Indian sub continent and the cooler Arabian Sea.

    Below is a diagram of how different coastlines will give a sea breeze.

    post-847-1181339776_thumb.jpg

    If you are on the south coast it will start from about SE or ESE and veer during the day to about SW. On a north facing coast then, about NW or WNW and veer to about NE during the day.

    Try it out the next time you are on the coast and the situation gives a sea breeze, or look at the weather reports from one of the seaside resorts.

    That is a short and relatively simple explanation of the general requirements of a sea breeze.

    Let us now look at the specific example at Walney Island.

    Observations from Walney as the sea breeze developed. Notice how the temperature dropped as soon as the sea breeze developed, between 1300 and 1400.

    post-847-1181339842_thumb.jpg

    The GFS chart for 12Z below shows that there was a 'gradient' of about NE over the area and relatively light in strength.

    post-847-1181339882_thumb.jpg

    The upper air ascent that was probably the most relevant shows only very limited convection but sufficient to allow a small circulation to develop between sea and land as described in the first diagram.

    post-847-1181339942_thumb.jpg

    1600Z surface chart from XC Weather

    post-847-1181339989_thumb.jpg

    and a few comments with the relevant isobars on the 15Z surface chart

    As the words on the chart are pretty small I'll paraphrase them below.

    The 15z surface chart shows isobars at 2mb intervals, for this scale of chart it is the best indicator, although 4mb is often used.

    The red arrows show the surface wind direction in adjacent areas to Walney Island. All, within the effects due to topography, about what one would expect given the surface isobars.

    The general flow due to the surface isobars is shown in orange.

    The blue circles show where sea breezes have developed.

    post-847-1181340122_thumb.jpg

    I hope this quick look at sea breezes with an example helps explain the basics behind a sea breeze.

    an addition to the above on Sunday 10 June 2007

    I made mention that in some circumstances a sea breeze could be a trigger for showers or even thunderstorms. We had a very good example yesterday with the sea breeze from the south coast setting up a CONVERGENCE ZONE which in turn sparked off some showers. Without this additional trigger the relevant ascent would probably not quite have given any showers.

    Below are some charts and data to try and illustrate the events.

    the first chart shows the development of the convergence zone from the initial sea breeze along the south coast.

    post-847-1181484226_thumb.jpg

    the second one is the skew-t for Herstmanceux, relevant to the area.

    post-847-1181484279_thumb.jpg

    Observations at 1820 BST

    Gatwick=21/17 with south west 7mph(first number is the temperature-second the dewpoint)

    Biggin Hill=21/12 with east at 2mph

    Farnborough=22/17 with south at 8mph

    Heathrow=24/12 with ENE 5 mph

    Fairford=21/14 with ENE 3mph

    Lyneham=21/16 with SSE 8mph

    I hope this gives an illustration of a sea breeze creating a convergence zone which in turn gave showers that otherwise would not have occurred.

    John Holmes

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