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The Sun


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Sunrise and sunset: sunrise and sunset are mostly noted for the associated changes in the colour of the sky. There are general stages that are associated with the times from dawn to sunrise and sunset to dusk. The heights of the clouds have an important influence on the length of times that cloud reflections occur. The higher the cloud, the longer the cloud bases will be able to reflect light. In fact, high clouds will reflect light for periods of up to 30 minutes whilst low clouds will typically reflect light from their bases for around 5 to 10 minutes. The reason for such a vast difference is that lower level clouds near the horizon are much closer to the observer ( around 1 to 15 kilometres away) than clouds at higher levels, which may be up to a few hundred kilometres away. The cloud bases of higher clouds therefore reflect light well before sunrise and remain so until just before sunrise. The same situation still applies for sunset but in the opposite order.
Crepuscular and anticrepuscular rays: Occasionally, when the sun is rising or setting, it is possible to see the sun’s rays looking like they have been blasted through a colander. It looks as though the sun’s rays are emanating from a central point. We are being fooled by the optical illusion of da Vinci’s point of perspective; at distance all things converge to a central point. In this instance, the central point of light is displaced due to the redirection of the solar beam. Crespuscular rays occur when parallel solar beams are funneled down and redirected through clouds or mountains, giving the impression that they are distinct beams emerging from a light source. Anticrepuscular rays are only slightly different. Occasionally, when crepuscular beams are visible, they can cover the entire sky. This does not mean that the beams continue to fan out as they do from the (seemingly) original light source. Anticrepuscular beams are the tail end of these beams, culminating at their own point of perspective. As the light beams pass overhead, they converge on the horizon at exactly 180 degrees from the point where crepuscular rays seem to emerge.
Silver lining: When sunlight is diffracted by large water droplets along the edges of a thick cloud, the "silver lining" effect can be seen.




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