- Optical Effects
Optical effects occur when light interacts with the atmosphere to create vibrant and colourful displays
Optical effects are the result of tiny particles in the atmosphere. During its passage through the atmosphere, sunlight is reflected, refracted, and scattered by water droplets, ice crystals and dust particles.
Sunlight or visible light consists of a spectrum of colours: red, orange, yellow, green, blue, indigo, and violet.
Rainbows result from an optical illusion caused by millions of water droplets acting like tiny prisms. Each component colour of visible light travels at a different rate in the water droplets, and refracts (bends) at different angles. The water droplets split sunlight into the seven colour components and reflect them to the viewer’s eye.
A rainbow appears as an arch stretching to the sky, as the water droplets lie above the observer’s level of sight. From a high viewpoint such as an aircraft, it is possible to see a complete circular rainbow formed by raindrops both above and below the observer’s level of sight.
Halos appear as faint coloured rings encircling the sun. They are formed by the refraction of light through ice crystals in high-level clouds and sometimes form under relatively clear skies.
Iridescent clouds also known as cap clouds, are quite common in Singapore. They form when a humid layer of air above a cumulus cloud is pushed upwards by the cloud growing and rising. When the atmosphere is stable, air that is lifted expands and cools, and the water vapour in it condenses into droplets, resulting in a thin layer cloud known as a cap cloud. Bright colours are sometimes observed surrounding cap clouds, due to diffracted sunlight
Crepuscular rays are pillars of light that pierce through gaps in the cloud layer, extending like beams radiating from a point. When these rays are directed downward, they are known as ‘Jacob’s Ladders’. They usually occur at sunrise or sunset, when the sun is partially obscured.