Thinking further after yesterday's post, the obvious thought struck me - there's still light on a cloudy day. The evidence is in the picture above. (We rang the bells in the Town Hall tower!) I remembered that this is due to SCATTERING. I didn't study this in my Engineering degree but have just learned a lot by Googling it. Clouds are an example of Mie Scattering - the water droplets are large compared to the wavelengths of light so all wavelengths have an equal chance of being scattered. This mix of all wavelengths gives clouds their white colour. However, the gas molecules in the air are much smaller than the wavelength of light, so they cause Rayleigh Scattering. Here the smaller the wavelength, the more the scattering. Blue has a small wavelength and so is scattered a lot by the air molecules, hence the sky looks blue. The scattered light reaching us on the surface of the Earth is then called diffuse sky radiation. So it started in one place - the Sun - but the light was then scattered and appears to be coming down from all over the sky.
This was the view of Berwick from our pitch on the caravan park. It looks like a medieval Italian town with the red roofs and its Elizebethan walls. Note the blue sky! But it takes me back to Dan's big question last month: "Violet has a shorter wavelength than blue, so it must be scattered more. Why isn't the sky violet?" Google revealed that the answer is that it might be but that the physiology of your eye means that the cone cells detect blue wavelenghts far better than violet.
