To understand why the sunrise you see before a storm is red, you also need to understand why the sky is blue.
When you look up on a cloudless day (as it is today as I write this), the sky looks like a cool blue dome. This sky’s colour is caused by the way sunlight interacts with what is in our atmosphere – mostly nitrogen and oxygen.
Sunlight appears white but is actually made up of a spectrum of colours (all the colours of the rainbow). As sunlight passes through the atmosphere, it strikes the gas molecules and particles in the air, ‘scattering’ the light. The blue part of the light is scattered – giving the sky its blue colour.
However, the thickness of the atmosphere changes its colour. When the sun is low in the sky, as is the case in a sunset or sunrise, the light passes through much more atmosphere before it gets to our eyes. All the blue light has been scattered away by the time it reaches us, leaving only the red and orange light. The reddish colours are made even more striking when the light passes through a high pressure air zone – the high pressure trapping more dust and small particles in the air, scattering even more of the blue light.
So, why a beautiful sunrise before a storm? The saying “Red sky at morning, shepherd’s warning” holds some truth in parts of the world where the prevailing winds are west to east (as they are in Northern Europe, much of USA and New Zealand – see this interactive map). Generally speaking, weather patterns (such as storms or high pressure systems) will approach from the west in these areas. Therefore, when the sun is rising in New Zealand, the morning light is passing through the weather pattern that has just passed overhead. If this is high pressure, then the sunrise will be red. And what follows a strong high pressure weather system can often be a storm.
You have been warned.
Answer by Dr Stu
Question from @sineira via Twitter
Footnote: This interaction between sunlight and the air is called ‘Rayleigh Scattering’ and, in physics terms, is a part of light spectrum being refracted. You can watch a video that explains it all here (which could pass the time while you baton down the hatches).