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Understanding why rainbows occur after many storms is no mystery. One must consider what causes a rainbow, what direction a storm might take, and when a storm is likely to pass.
Double rainbows actually have different sun rays cause them. They do not reflect primary rainbows.
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Do Rainbows Really Indicate a Storm Is Over?
The answer requires an understanding of how rainbows form, and of weather patterns. Then some insight into why rainbows are so often associated with the end of precipitation can follow.
A rainbow is a naturally occurring phenomenon which is caused by light bending in a process called refraction. When light passes from one medium, like air, into another medium, like water, it is subject to bending. The amount of bending depends on the indices of refraction, values associated with the mediums, and with the angle the light makes with the boundary between the two mediums. The greater the angle the light makes with the surface, the greater the bending.
When a light ray passes from one medium to another, the different colors refract at different angles, since the indices of refraction differ slightly with different frequencies of light. This causes the light to separate into its color components. This is what happens in a prism.
When light encounters a surface between two mediums, it is also subject to reflection. In the case of air and water, the reflection of light in air encountering water is weak, but a much stronger reflection occurs when the light is in water and encounters air.
Sunlight contains almost all frequencies, the only exceptions are those absorbed by the atmosphere of the Sun and the atmosphere of the Earth. These missing frequencies cannot be determined without magnification, and will not be apparent in a rainbow. They occur for very narrow bands of frequencies and tend to wash out in rainbows.
In a cloud, light enters into water that has a rounded surface. The light separates into its components. At the back of the water, a large percentage of the light reflects. The colors, due to the curvature of the surface, cross each other reversing the color order from top to bottom. Some of the light exits the water back into the atmosphere at the front of the water components. Because the different colors exit from different places the rays do not correct, but continue to separate as the light moves through the atmosphere.
In a double rainbow, the light undergoes two internal reflections, reversing its order twice. This is why a secondary rainbow will have colors in the opposite order than the colors of the primary rainbow.
A rainbow requires the observer and the Sun to be on the same side of the cloud. The Sun must be at the back of the observer.
Many storms occur because of thermal heating. These are predominately an afternoon phenomenon, when the Sun is in the west. Other storms are associated with frontal movements. Still others occur as air is forced over hills or mountains.
As air moves from the equator it forms jet streams that move from west to east, usually one in each hemisphere. These jet streams guide the weather over much of the inhabited parts of the planet, causing the weather movements from west to east.
Since storms are more likely in the afternoon, the setting sun is most likely to be involved with rainbows. There usually are too few storms in the morning to have the rising sun contribute as often. This places the sun in the western sky, and if the movement of a storm is from the west to the east, the rainbow does indeed occur after the storm. Can it occur before a storm? It can, but this is a much rarer event.
Halo arcs are similar to rainbows in that refraction separates the light passing through a cloud component into its spectrum. However, the process differs in that the sun and the cloud are on the same side of the observer. The way the light is separated is that when light passes through an ice needle cloud, it passes through a transparent six-sided crystal. These crystals act as prisms. Since there is no internal reflection, halo arcs do not exhibit the ray crossover that occurs within the droplets when a rainbow is formed.
In some halo arcs, the light appears as a white dot or band, but coloration is very often observed. The ingredients needed are clouds in the cirrus family and the sun.
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