What is a solar halo and why does it occur

Have you ever looked up at the sky and seen something similar to a rainbow around the sun? If so, you have had before your eyes the phenomenon of the solar halo, antelia or aro iris , which constitutes, together with the northern lights and electric 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. It may be surprising and inexplicable to see it, therefore, at Green Ecology we want to let you know what a solar halo is and why it occurs .

What is a solar halo

The solar halo, iris is an atmospheric optical phenomenon that appears in the sky in the form of a ring, circle or colored and luminous ring 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 towards the outside and 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.
  • Occasionally, 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 assiduously along with the solar halo are the parhelios, also called “false suns”. These are two showy 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.

Next, we will tell you why it is produced and how a solar halo is formed .

  1. Cirrostratus-type clouds and, in particular, cirrostratus nebulosus are the ones that give rise to the majority of halo phenomena and 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 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 .
  2. 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 were also white, well, the reason is simple: perceiving colors at night is difficult for the human eye, hence we only distinguish white.
  3. 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.

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 fundamental differences between the rainbow and the solar halo 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 grading is just the opposite; thus, the colors range 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.

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