Is the Sky Above Bilsdale too Blue?

My memory of this morning’s bimble over Urra Moor is of a glorious autumn day. Spoilt only by the fusillade sounding like the whole sky itself was being blasted and not just the pheasants. I knew the sky was cloudless and blue. I did not appreciate that it was pale at the horizon and deepened higher up. The photograph did that for me. I now suspect my iPhone of over-enhancing the colours.

Urra Moor is the highest point in the North York Moors, at 454 metres. My walk to its summit, Round Hill, had started from Clay Bank. It is not a mountain but high enough to make the sky look big.

Why does the blue fade? Air molecules scatter blue light more than red, which is why the sky is blue at all. Look straight up and the light reaches you after a short trip through the air. Look at the horizon and the picture changes. Light from the horizon passes through more of the atmosphere and is scattered again and again, which mixes in white light and weakens the blue¹Why the sky is blue and sunsets red, Met Office. https://weather.metoffice.gov.uk/learn-about/weather/optical-effects/why-is-the-sky-blue.

How much more air? At sea level, the path towards the horizon is about 38 times the length of the path straight up. Haze adds to the effect. More wet haze along your line of sight gives a brighter, paler sky²Air mass (solar energy), Wikipedia. https://en.wikipedia.org/wiki/Air_mass_(solar_energy).

Is my iPhone lying? Could be. The general opinion is that phones do not simply record what is there. The processing includes steps that identify parts of the scene, and sky areas may receive controlled contrast enhancement. And my phone doesn’t even have AI yet.

So the phone probably touched up the sky. The gradient is a different matter. Physics predicts a pale horizon and a deeper blue overhead, and my eyes would have shown me both had I looked. The phone may have deepened the blue. It did not invent the fading.

• 1
  Why the sky is blue and sunsets red, Met Office. https://weather.metoffice.gov.uk/learn-about/weather/optical-effects/why-is-the-sky-blue ↩︎
• 2
  Air mass (solar energy), Wikipedia. https://en.wikipedia.org/wiki/Air_mass_(solar_energy) ↩︎