ATMOSPHERIC OPTICAL PHENOMENA

Atmospheric optical phenomena are those phenomena of the atmosphere that can be explained in terms of optical laws. Some of the atmospheric elements, such as moisture, serve as a prism to break a light source down into its various component colors. The resulting phenomena can be spectacular as well as deceptive.

A halo is a luminous ring around the Sun or Moon. When it appears around the Sun, it is a solar halo; when it forms around the Moon, it is a lunar halo. It usually appears whitish (caused by reflection), but it may show the spectral col-ors, from refraction (red, orange, yellow, green, blue, indigo, and violet) with the red ring on the inside and the violet ring on the outside. The sky is darker inside the ring than outside. Halos are formed by REFRACTION of light as it passes through ice crystals. This means that halos are almost exclusively associated with cirriform clouds. Refraction of light means that the light passes through prisms; in this case, ice crystals act as prisms. Some reflection of light also takes place.

Halos appear in various sizes, but the most common size is the small 22-degree halo. The size of the halo can be determined visually with ease. Technically, the radius of the 22-degree halo subtends an arc of 22. This simply means that the angle measured from the observation point between the luminous body and the ring is 22. Halos of other sizes are formed in the same manner.

Coronas 

A corona is a luminous ring surrounding the Sun (solar) or Moon (lunar) and is formed by DIFFRACTION of light by water droplets. It may vary greatly in size but is usually smaller than a halo. All the spectral colors may be visible, with red on the outside, but frequently the inner col-ors are not visible. Sometimes the spectral colors or portions of them are repeated several times and are somewhat irregularly distributed. This phenomenon is called iridescence.

The rainbow is a circular arc seen opposite the Sun, usually exhibiting all the primary colors, with red on the outside. It is caused by diffraction, refraction, and reflection of light from raindrops or spray, often with a secondary bow outside the primary one with the colors reversed.

A fogbow is a whitish circular arc seen opposite the Sun in fog. Its outer margin has a reddish tinge; its inner margin has a bluish tinge; and the middle of the band is white. An additional bow, with the colors reversed, sometimes appears inside the first.

Mirages are images of objects that are made to appear displaced from their normal positions be-cause of refraction. These images may be only a partial image of the object, and they may appear in either an upright or an inverted position, de-pending upon the atmospheric condition that ex-ists at the time of observation. Mirages occur when adjacent layers of air have vastly different densities because of great temperature differences. Whether these layers exist side by side and horizontally or vertically determines the type of mirage.

Mirages are often seen in desert areas where air near the surface becomes very hot. Cool air overlies this hot layer resulting in a large difference in the densities of the two layers. Three types of mirages result from the refraction of light rays through layers of air with vastly different densities.

INFERIOR MIRAGE. The inferior mirage, the most common of the three, appears as a mir-rored image below the object being viewed by the observer. In this case, you can associate the word inferior with beneath or below.

SUPERIOR MIRAGE. In the superior mirage, the mirrored image appears above the object being viewed. In this case, associate the word superior with above or over.

LATERAL MIRAGE. Since the positions of above and below represent superior and inferior mirages respectively, the lateral mirage then appears to the side of the object being viewed.

Looming is similar to a mirage in that it brings into view objects that are over a distant horizon. Looming occurs when there is superrefraction in the lower atmosphere which makes reflected light travel a path similar to the curvature of Earth. Objects over the horizon may be seen when light reflected from them takes this path. Looming is somewhat rare and is normally observed over flat surf aces, such as oceans and deserts.

Scintillation is caused by variations in at-mospheric density near the horizon. It produces the appearance of rapid changes in the position, brightness, and color of distinct luminous objects, such as stars. Stars flickering and changing color near the horizon shortly after sunset are good ex-amples of scintillation and area reasonably com-mon phenomenon.

Crespuscular rays are another common phenomena. They are simply sunbeams that are rendered luminous by haze and dust particles suspended in the atmosphere. They are seen before and after sunrise and sunset as they pass through small breaks or openings in or around clouds. The sunbeams are actually parallel but appear to diverge from the Sun.

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