Occasionally, we see a phenomenon called a halo around the moon or the sun, which typically displays an iridescent disk around the outer circumference of each celestial body. This phenomenon is common in the coldest regions of the world, such as Antarctica, Greenland, Alaska, and Siberia, but it can also be seen in other locations with ideal weather conditions. A lunar halo can indicate certain situations and is related to atmospheric phenomena occurring in the upper atmosphere.
In this article we are going to tell you everything you need to know about the lunar halo, its characteristics, origin and what it means.
What is the lunar halo
In moderate atmospheric conditions where this phenomenon can be observed, in temperate regions , light clouds crystallized by the cold, called cirrus clouds, can form. This atmospheric phenomenon occurs when small ice particles are suspended in the air directly in the troposphere, and these particles refract when they receive sunlight, creating a spectrum around the moon or the sun.
One of the most striking features of ring formation is its iridescence, creating an effect as if it had its own "light," consisting of red (inside the ring) and blue-green on the outside. However, sometimes it appears as if a complete rainbow is forming. This phenomenon may be related to other events such as solar halos.
The color usually seen is white, sometimes reaching a completely pale tone due to the backlighting caused by the color of the sky. The physical phenomena that cause this are reflections and refractions in the ice crystals.
These halos typically form in the highest clouds in the atmosphere, called Cyrus clouds, which can reach altitudes of 20.000 meters. Returning to the topic of halos, one of the most common types is the halo formed by refraction, which causes light to pass through hexagonal crystals.
Types of lunar halo
This phenomenon usually occurs in the troposphere, the lowest layer of the atmosphere and where most of the weather events on Earth occur. As if that were not enough, much of the different types of cloud layers that exist tend to form and accumulate in this layer.
In recent years, this layer of Earth's atmosphere has undergone certain changes, becoming increasingly colder across most of its extent (10 km altitude), reaching -65°C in most areas. Because of this, dust particles and ice crystals tend to accumulate in this layer, an essential ingredient in the formation of these types of clouds.
In the case of the halo, the ring is formed when moonlight manages to refract through small ice crystals. However, if we compare them with solar halos, there is a key difference, because this type of halo is only visible when the clouds are high enough (close to the satellite).
If all these features were present, a typical hexagonal ice crystal would form, deflecting the moonlight at an angle of inclination of 22° , thus forming a complete ring with a diameter of 44°.
Another requirement for observing this phenomenon is that the moon must be in its full phase, as the halo is difficult to see when the moon is in other phases. This optical effect in clouds can be better understood by studying similar phenomena such as those that occur in the Brocken spectrum.
Origin and formation
Any halo, iris ring, or ring is known to be an optical effect that produces a disc or ring around the moon (or sun) with an iridescent character on the outer side of the projection disc—that is, the tone of the light. This effect changes depending on the viewing angle and is similar to that observed in CDs and DVDs, which are widely used today.
The pointed iridescent effect is caused by multiple translucent surfaces on which phase changes and interference from light refraction are perceived, lengthening or shortening the wavelength depending on the angle and distance of each observer from the object.
The light projected in this rainbow effect is modulated or graduated in one way or another due to the interference that occurs when the light passes through; in addition, depending on the angle of vision, different colors will be projected at greater or lesser intensities, producing the described effect; they are, as already explained, a process similar to the appearance of a rainbow.
The best places to observe the lunar halo are Alaska, Atlantis, Greenland, and northern Scandinavia, as well as northern Russia and Canada (near the North Pole). However, scientifically speaking, this phenomenon can be seen anywhere, provided the appropriate atmospheric conditions exist, even where there are storms.
The ice particles in the clouds, in the region of the troposphere, when they are in suspension create a range of colors around the Moon or the Sun depending on the situation. Typically, red tones are observed in the inner area of ​​the ring and green or bluish in the outer area. In a way, it can be like a full rainbow, that is, round.
The most common lunar halos are yellowish and, in some cases, white. They originate from terrestrial regions or from other planets with atmospheres. The optical effect is the reflection and refraction of light through the small crystals mentioned earlier, which make up high-altitude cirrus clouds (that is, high-altitude clouds with small crystals). This is also related to the phenomenon of the parhelion (sun dog ).
Conditions for the lunar halo to occur
In any case, the halo represents a rare luminous phenomenon, as several conditions must be met, such as a cold, iridescent atmosphere, as well as sufficient crystals to deflect the light.
The intensity of moonlight varies depending on its position, which explains why each observer perceives a different image from their vantage point. The light is refracted in multiple directions as it passes through or strikes the glass, and the sum of all these refractions forms the projected ring.
The low temperatures required to form a halo logically affect the weather to some degree, so it is explained that a halo indicates a change in the environment. On the other hand, the same details of a common cold can indicate certain effects on the health of some people, being able to produce respiratory diseases or similar.
I hope that with this information you can learn more about the lunar halo and its characteristics.
