From the ephemeral red of a sunset to the elusive, multi-coloured southern lights, what is the science behind colours in the sky – and can those colours tell us anything about the weather?
Red skies
Have you heard the saying “red sky at night is a shepherd’s delight, red sky in the morning is a shepherd’s warning”?
This red sky proverb means a red glow at dusk is an indication that the next day will be fine and sunny, while red at dawn indicates stormy weather.
While many proverbs are challenged for lacking the scientific precision of modern meteorology, science can explain why this one endures.
The colour of the sky – whether blue, red, or anything in between – is determined by how sunlight is scattered through the earth’s atmosphere.
Blue light is scattered further than red light, which is why, at noon on a fine day, the sky is blue.
At sunrise and sunset, however, the sun is low on the horizon and has to travel a much greater distance through the atmosphere to reach us.
Blue light is scattered even further through this greater distance of atmosphere, leaving more of the red and yellow parts of the light spectrum to reach our eyes.
If other particles are present in the air, such as dust and smoke, this scattering difference between blue and red light is even greater.
This means a smoky sunset can look even redder than usual.
In areas such as southern Australia where weather systems usually move from the west to the east, the red sky proverb often applies due to this scattering effect.
A red sunset indicates a high pressure system (finer weather, clear skies) is approaching from the west, and is likely to move overhead meaning fine weather is on the way.
Sometimes, this is enhanced by cloud to the east, which can suggest a recent weather system is moving away or clearing.
A red sunrise indicates the clear skies and settled weather (high pressure system) now sit to the east of us, and are likely to move further away to the east, often to be replaced by incoming low pressure and rainy weather from the west.
And as the sun rises in the east, sometimes high cloud in the west will add to the spectacle, further indicating an approaching weather system bringing bad weather.
Stormy skies
While storm clouds generally appear dark blue or black, colours seen in thunderstorms depend on the ice content in the clouds as well as on the angle of the sun and how the sunlight penetrates the storm.
Sometimes, you can see brown, green or red in storm clouds.
Around sunrise and sunset, red and orange light can reflect off clouds making them appear to have a red tinge.
Clouds can appear brown before a storm, particularly in the interior of Australia, when the updraft has picked up a lot of dust from the ground and particles of dust and dirt are in the air.
That dust or dirt in the air can provide the ‘nuclei’ onto which water vapour condenses to form clouds, raindrops and hailstones.
The reason some thunderstorms appear green is still the subject of debate, but it seems that the green component of sunlight is scattered preferentially in stronger thunderstorms that contain large amounts of hail/ice and water droplets.
It’s widely believed, and supported by anecdotal evidence, that the eerie green glow requires the presence of ice/hail in the storm – but that doesn’t necessarily mean the storm will deliver hail on the ground.
Lights in the sky
While the aurora doesn’t tell us much about terrestrial weather – though you need the right sky cover to catch a glimpse – it is a visual manifestation of space weather.
Space weather refers to events beyond the earth’s atmosphere that impact the near-earth space environment.
The main source of space weather is the sun, which is so hot that high-energy plasma escapes from its gravitational field forming what we call the solar wind.
Auroras – known as the aurora australis or the southern lights in the Southern Hemisphere – are born from coronal mass ejections (CMEs) which occur when eruptions on the sun release enormous clouds of plasma into the solar wind.
These clouds travel away from the sun, sometimes towards earth, at speeds of up to 2000km per second.
If they reach earth, they can cause a type of space weather known as a geomagnetic storm.
Under the right conditions, particles from the geomagnetic storm spiral toward the upper atmosphere of the north and south poles along the earth’s magnetic field lines and collide with neutral atoms of oxygen and nitrogen.
The atoms then release the energy from these collisions in the form of light.
Light is released by the atoms at various wavelengths, which determine the emitted colour.
How energetic the collisions are, where they occur in the atmosphere and which atoms and molecules are involved are all factors in determining what colour is emitted.
Oxygen releases greenish-yellow or red light, while nitrogen releases dark red or blue light. These colours can mix – so purple, pink and white light can also be seen.
Auroras are best seen at high latitude regions, such as Tasmania or southern Victoria.
The stronger the geomagnetic storm, the brighter the auroras will be, meaning there is a chance more locations could see them.
Dark and clear skies away from light pollution and cloud cover will provide the best conditions for viewing auroras during a geomagnetic storm.
Miriam Bradbury is a senior meteorologist at the Bureau of Meteorology.
Learn more about Bureau of Meteorology weather services including forecasts and warnings, severe weather, thunderstorms and space weather at www.bom.gov.au.
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