Atmospheric rainbow

An atmospheric rainbow is an arc or circle of spectral colours and appears in the sky when an observer is in the presence of strong sunshine and rain.

• Atmospheric rainbows:
  • Are caused by sunlight reflecting, refracting and dispersing inside raindrops before being seen by an observer.
  • Appear in the section of the sky directly opposite the Sun from the point of view of an observer.
  • Become visible when millions of raindrops reproduce the same optical effects.
• Atmospheric rainbows often appear as a shower of rain is approaching, or has just passed over. The falling raindrops form a curtain on which sunlight falls.
• To see an atmospheric rainbow, the rain must be in front of the observer and the Sun must be in the opposite direction, at their back.
• A rainbow can form a complete circle when seen from a plane, but from the ground, an observer usually sees the upper half of the circle with the sky as a backdrop.
• Rainbows are curved because light is reflected, refracted and dispersed symmetrically around their centre-point.
• The centre-point of a rainbow is sometimes called the anti-solar point. ‘Anti’, because it is opposite the Sun with respect to the observer.
• An imaginary straight line can always be drawn that passes through the Sun, the eyes of an observer and the anti-solar point – the geometric centre of a rainbow.
• A section of a rainbow can easily disappear if anything gets in the way and forms a shadow. Hills, trees, buildings and even the shadow of an observer can cause a portion of a rainbow to vanish.
• Not all rainbows are ‘atmospheric’. They can be produced by waterfalls, lawn sprinklers and anything else that creates a fine spray of water droplets in the right conditions.