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Have you ever looked at the sky and seen something similar to a rainbow around the sun? If so, have you had before your eyes the phenomenon of solar halo, antelia or aro iris, which constitutes, along with the northern lights and electrical discharges, one of the most beautiful and superb manifestations that the sky offers us. The appearance of this "celestial ornament" can be observed with relative frequency in our latitudes throughout any season of the year, although it is more common to observe it at higher latitudes. Seeing it may be surprising and inexplicable to us, therefore, at Ecologista Verde we want to let you know what is a solar halo and why is it produced.
By the way, be very careful when observing it: always try to cover the Sun to avoid possible damage to the eyes! We can help ourselves to do this by hiding it with the outline of a tree, building, lamppost, etc., and, if not, with our own hands. It is also advisable to wear sunglasses.
What is a solar halo
The solar halo, iris or antelia ring is a atmospheric (or photometeor) optical phenomenon that appears in the sky in the form of a ring, circle or ring colored and luminous with the Sun located in the center of it. These are the characteristics of the solar halo:
- The interior of the halo is faint red, changing to orange and yellow outward, ending with the outer edge in a blue-white color, although sometimes it culminates in violet.
- In addition, it can be seen that the portion of the sky inside the ring is somewhat darker than the rest.
- The diameter of the solar halo is 22 degrees in radius (note that from the horizon to the zenith or highest point in the sky there are 90 degrees) and it will always maintain that size regardless of its position in the sky.
- On occasions, it is possible to verify the existence of a second larger solar halo and, therefore, external, with a radius of 46 degrees, concentric to the one already described of 22 degrees and less bright. It is called the greater halo, but they are very rare even at high latitudes.
- Other optical phenomena that appear regularly together with the solar halo are the parhelios, also called "false suns". They are two bright spots of light, either white or colored. They are usually very bright, red on the inside and blue or whitish on the fuzzier outside. One of the parhelios is positioned to the left and another to the right of the Sun, at the same height above the horizon, although often only one of them can be seen.
- Halos can also form around the Moon, but these, as we will explain later, are always seen as white.
Why is there a solar halo?
Next, we tell you why it is produced and how a solar halo is formed.
- Clouds of the cirrostratus type and, in particular, cirrostratus nebulosus are the ones that give rise to the majority of halo phenomena and to the most splendid ones. This is due to the fact that these high-type clouds are made up of tiny suspended ice crystals, since they are located between 5 and 13 kilometers high, where the surrounding temperatures are usually less than -40 ºC. Here you can learn more about the types of clouds.
- When the sun's rays strike the small ice crystals, they are refracted, that is, their trajectory deviates as in a glass prism and they separate in the colors of the spectrum, causing us to perceive a colored halo in the sky. They can even be caused by the reflection of sunlight, however in this case they will look white. In the case of lunar halos, we said that they also looked white, well, the reason is simple: perceiving colors at night is difficult for the human eye, hence we can only distinguish white.
- Depending on how the ice crystals are distributed in the cloud, that is, according to how they are oriented and if they are present in greater or lesser quantity, we will see the halo with greater or lesser intensity.
Difference between rainbow and solar halo
Although both the rainbow and the solar halo are optical phenomena that we can contemplate in the atmosphere, we should not confuse them with each other. Some differences between the rainbow and the solar halo fundamental are:
- The rainbow is much more frequent and known than the solar halo or antelia.
- The rainbow is observed on the horizon and always in the opposite direction to where the Sun is located.
- The solar halo forms in the presence of small ice crystals, while the rainbow forms in the presence of water droplets (the Sun must illuminate a curtain of rain).
- The most common is that the colors are sharper and more numerous in the rainbow.
- From the outside to the inside, the color gradation is just the opposite; thus, the colors go from red to purple for the rainbow and from blue and white to red for the solar halo.
- Solar halos commonly occur before the arrival of a warm front, while rainbows form when the rains have already started.
Now that you know all this about the solar halo or antelia and something about the rainbow, we encourage you to discover much more about the latter in this other post by Green Ecologist on How the rainbow is formed and in this video that we leave you here below.
If you want to read more articles similar to What is a solar halo and why does it occurWe recommend that you enter our Nature Curiosities category.
