§   Caused by sunlight being blocked by high-rise objects (such as cumulus clouds or high mountains), creating strips of alternating bright and dark rays. The rays appear to converge on the sun.

§   At sunrise or sunset, crepuscular rays would appear to converge at the antisolar point. 

§   An optical phenomenon caused by refraction of sunlight by ice crystals

§   The sky is typically covered by cirrostratus or thick cirrus when it occurs

§   The most common halo is a luminous ring of 22 degrees radius surrounding the sun, with the space within the ring appearing less bright than that just outside

§   On average, solar halo is observed in Hong Kong on about 3 days in a year

§   An optical phenomenon caused by diffraction of sun or moon light by very tiny cloud droplets

§   Usually appears in altocumulus and altostratus clouds close to the sun or moon

§   Includes colours such as light green, light blue and light pink

§   The way light is diffracted depends on the size of cloud droplets, therefore the colours and shapes of iridescent clouds change rapidly with time



§   A type of man-made clouds and are mainly formed by jets

§   Water vapour from aircraft exhaust condenses in the surrounding cold air, producing water droplets and ice crystals, leaving a trail aloft

§   An optical phenomenon caused by refraction of sunlight by water droplets

§   Blue light has a shorter wavelength and is refracted more than light with a longer wavelength such as red light, therefore blue light is located in the inner part of the rainbow, while red light is located in the outer part

§   Each of the colours in the rainbow comes to the eye from a different set of water droplets

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