Why are sunrises colourful anyway? 

Duane Hamacher, a Monash University astronomer, explains the sky’s colour as a result of Rayleigh Scattering. Essentially, when light (from the sun) passes through Earth’s atmosphere, it is scattered by atmospheric molecules. Sunlight is actually ‘light’ from the whole spectrum of colours, however for the purpose of ease, we can view it as blue and red hues. Blue light is scattered more than red light as it has a shorter wavelength, meaning there are more opportunities for it to ‘collide’ with atmospheric molecules. 

During the day, sunlight travels a shorter distance (through the atmosphere). This means that over a short distance, blue light is scattered a lot so that we see the sky as blue. This is why the sky appears blue on a clear day.

At sunrise or sunset, sunlight has a much longer distance to travel. This means that sunlight is scattered over a longer distance. Since red light is scattered at a lower rate than blue light, red light is more prominent. This is why red hues light up the sky at sunrise and sunset.

Sometimes, there will be purple hues at sunrise or sunset. This is because there is still enough blue light, which combines with vivid red to produce purple.

Essentially, clouds are a great way to predict the colour vibrancy of a sunrise. Clouds that are high in the sky will produce a better, more vibrant sunrise or sunset since they intercept more vivid sunlight. (Thomas Heaton) Low clouds block light and will produce a more dull sky.  Clouds at medium height are less predictable and may have a positive or negative impact on colour and vibrancy, depending on other weather conditions.

A cloudless gap at the horizon will generally produce a better sunrise or sunset, as this allows better passage of light since there are no (or few) low clouds.

Thin, broken clouds produce a wider array of textures and colours as the light passes through, especially when there is a variety of cloud formations. (David Peterson) Cloud cover between 30-70% is ideal, as it provides a window of oppurtunity for light to shine through. (Gregory Dunbar) If cloud cover is low, i.e. less than 30%, there isn’t any oppurtunity for red wavelengths to be intercepted. If cloud cover is too high, e.g. greater than 70%, light cannot pass through.