How to tell when the sunset will be purple?
The reason the sky can be purple at sunset and sunrise is the same reason the sky is blue and it is called the Rayleigh scattering. It is the scattering of light on atoms smaller that the frequency of the visible light. To have a good view of it, you need moisture in the atmosphere but relatively little at ground level. This occurs when e.g. a warm front is advancing and warm moist air climbs over relatively colder and dryer one. Hence the saying: “Red sky at night, sailor’s delight. Red sky in the morning, sailor’s warning.” If the sky is red at night, it may come from the heat of the day having lifted moisture in the upper atmosphere. If, on the other hand, the sky is still red in the morning, it may be because a warm front is on its way. But the true sailor can see the front in other aspects such as the telltale mare tails (cirrus clouds with their “tail” all in the same direction) and a halo around the sun as alto stratus veil it.
