Sand and the colour of the sea
(sand colour analysis)

   
 
   
 
   
Holding the mouse over any picture will give the beach location


Whitesands Bay

Porthminster, St Ives

Traigh Scurival, Barra

I've often wondered why the sea is the colour it is, and why it varies from place to place, but why places tend to show elements of similar colours (though different brightnesses) whatever the weather. If you can find a text book on sea colour it will tell you that water is blue because of light refraction and those with more plankton tend to be greener (and the cooler the water the more plankton there is). So the Meditteranean is blue, while the north Atlantic is green. But they also say that in shallow water the colour of the sand and quality of light will make difference. I've assumed that it's to do with the colour of what's underneath the water more than the quality of the light itself. But of course the light varies too. We all know that blue and yellow make green, so the light under a blue sky combined with yellow sand should give a green sea.

The pictures above show nine samples of sand from different beaches in West Cornwall at the same scale, and under the same light. Not 100% the same light scientifically but good enough for the moment.

Most of these beaches look pretty near "white" in bright sunshine, except for Marazion which looks muddy. The particle sizes at Marazion are much smaller (nearer mud) and it is much darker. The others are all fairly light, but vary greatly in particle size, the amount of shell and the overall tone. Whitesands Bay, Sennen and Porthmeor seem to be the lightest.

You can see more samples of sand colours in the "Sand Colour Chart", from this page's 'parent'


Godrevy

Whitesands Bay

Traigh Scurival, Barra



To many, what follows may seem a bit nerdy (and it is). It's probably totally incorrect and misleading as well, but I've put it here anyway. I decided to look further into colour by taking a closer look at the photos of beaches, and seeing what the differences were, beyond the obvious. Any self respecting beach and sea colour experts there may be out there will probably say this is rubbish - I'd like to hear from you if you do know better or are interested.

Averaging the colours

Using an image editing program I 'sampled' the colour of each photo to see the average characteristics in terms of "red, green, blue", and "hue, saturation and lightness". The picture below shows small pictures of various beaches against their "averages".

The first thing that struck me was how much they look like a Farrow and Ball paintchart - that's probably irrelevant but it may relate to how they come up with their colours. It's also apparent that they are different, but in some cases the average doesn't look quite as you might imagine.

Saturating the average colours

I then looked to see what the colours were like when the saturation and lightness was increased, since wet beaches are more 'colourful' than dry ones and sunshine can make them almost dazzling. Underneath the white that we perceive there will be a 'hue'.

So, presented below are photos of three examples, Porthminster in St Ives, Whitesands Bay at Sennen, and Traigh Scurrival on Barra, in four different modes.

  • As was,
  • Averaged,
  • Increased saturation of the average, and
  • Increased saturation of the whole photo, to show the effect.

Porthminster, St Ives    Whitesands Bay,Sennen    Traigh Scurrival, Barra

Original





"averaged colour"





Saturation increased





Original with increased saturation

The first thing that struck me this time was how much the saturated ones looked like a DIY shop ownbrand paint chart from the 1980s, compared with the more sophisticated Farrow and Ball colours.

Going back to sea colours, if it's a dull day the sea will probably look dull anyway, but if the sun's out (as it usually is in the UK?) we'd expect the sunlight to get through the water (as long as the sea is cleaner than most people think it is in the UK) and reflect off the sand and colour the water accordingly. It seems to me that it does. Blue light mixed with orange, yellow and green make very different colours.

How lights and pigments mix is different - mixing lights is known as subtractive while mixing pigments is known as additive. Red and green light make white light, while red and green pigment make brown. What we're dealing with here is something in-between (I think) as it's light filtered through a slightly pigmented medium (water) and reflected off another (sand). The water will scatter and absorb different light colours, and the sand will reflect and different colours depending on its make up. It's too complicated for me to explain it (or understand it at all fully), but I have the feeling that the resultant 'hue' will be somewhere between the light of the sky (which will include some invisible ultraviolet and infra red) and the colour of the sand.


Experimental colour mixer (Won't work in Firefox yet - sorry)

Mixing the colour of the beach with the light from the sky

To test this I wrote a web application that mixes colours according to hue, saturation and lightness. It's not that simple because web browsers work with red, green, and blue which don't lend themselves to simple formulae to mix colours, so I had to convert to "HSL" and back to "RGB" again. Also, different browsers work differently, and this currently works in Internet Explorer 7 and 8, partially in Chrome and Safari, and not at all in Firefox - I haven't tested Opera or any other browsers yet.

The 'app' also doesn't work properly all the time anyway and sometimes doesn't change the colours when it should, and sometimes freezes. I'm not sure why. If that happens it's best to re-load it using the 'update' icon at the top of the web browser or click Function Key 5 (F5 at the top of your keyboard).

If you can get it to work, then if you set the right hand square to a bright blue towards mauve you'll end up near high UV light (try something near Hue 290, Saturation 90 and Lightness 85) and use on the beach colours on the left by clicking on a sand, and you'll see a different sea colour as you change the beach. Besides the sand 'hsl's on the left I've put in four 'skies' that may or may not correspond to the light hitting the sea.

Of course you can use this to mix any lights you want once you've worked out how to work with Hue, Saturation, and Lightness.


Barra


Porthminster

Colour make up of the beaches

So far we've dealt with the "average" colour, ignoring the fact that the sand is made of of lots of different grains. Below are four charts where I've analysed two beach photos in terms of the individual pixels, in terms of the hues that are contained within each image.


Pure hues - Barra

Actual colour make up - Barra

The brightly coloured charts show the distribution of the hues in each beach, with the height showing the relative amount of that hue in the beach. So Barra has much more of a mix of hues in its makeup, while Porthminster is concentrated with orange and yellow hues and a few greens. Exmouth (made of red sandstone) is pretty much all read and orange.


Pure hues - Porthminster

Actual colour make up - Porthminster

The right hand duller charts show each of the pixels in the 150 by 150 photographs at the top of the page, reordered according to the hue. So any pixel that is red or orange is towards the left and anything blue and mauve towards the right. But in this case the saturation and lightness haven't been changed so you get a scruffy continuum through the colour circle. The effect reminds me a bit of paintings by Kurt Jackson who works in Cornwall. But that has to be co-incidence, though the colours will be there. It's apparent that the bulk of the Porthminster colours are yellow and orange, while Barra has the appearance of a bluey grey.


Exmouth

Exmouth
It's obviously the geology that leads to the make up that the beaches have. Barra in the Outer Hebrides is an area of Gneiss and very old metamorphic rocks. Porthminster Beach in St Ives is a granite area. Exmouth is under red sandstone cliffs.

But how does this relate to sea colour? The colours I like most have been at the far end of Cornwall and on Barra - people often suggest that my straight photos have been manipulated, when I haven't changed the colours. Both of these places are surrounded by sea, and do have more light reflected around. I don't know the extent to which the light is 'different' in quality but there's certainly more to interact with the sand and that's likely to give more interesting colours.

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© Gordon Stokes, 2011