Optical and acoustical phenomena in the Arctic
The Aurora Borealis, or Northern Lights, shines above Bear Lake in Alaska.
Contents
Optical Phenomena
In the Arctic, people frequently see unique and beautiful optical phenomena. The atmospheric conditions that can lead to the development of these phenomena are the suspension of ice crystals in the atmosphere, the presence of strong surface inversions that refract light rays, the reflection of light by cloud bottoms, and interactions between the solar wind and gases in the upper atmosphere. Optical phenomena observed in the Arctic include the Aurora Borealis, Halos (sun dogs and fog bows), Coronas and Anticoronas, Water Sky and Ice Blink, Superior Mirages (looming, towering, fata morgana), and Optical Haze.
 Auroral arc. (Source: NSIDC) |
 Corona formation. (Source: NSIDC) |
 Curtain with rays. (Source: NSIDC) |
Aurora Borealis
Also called the northern lights, the spectacular color displays of the aurora borealis appear on clear, cold nights in the arctic sky during periods of solar activity. The aurora borealis is centered around the geomagnetic North Pole, but displays may be observed up to 5,000 kilometers (km) away. The height of the auroras is about 70 km to 200 km above the earth's surface.
The amazing color displays and formations are produced by the solar wind, a stream of electrons and protons coming from the sun, as it collides with oxygen and nitrogen atoms in the upper atmosphere. High-altitude oxygen, about 200 km up, produces rare, all-red auroras, while lower-altitude oxygen, about 60 km up, is the source of the most common auroral color, a bright yellow-green. Blue light comes from ionized nitrogen molecules. The nitrogens also create purplish-red and red colors in the aurora.
 Multi-colored bands. (Source: NSIDC) |
 High-altitude Aurora. (Source: NSIDC) |
 Twilight display. (Source: NSIDC) |
Halos
A halo occurs around the sun when light isrefracted as it passes through ice crystals. When a thin uniform cirrostratus cloud deck containing ice crystals covers the sky, the halo may be in the form of a complete circle as seen below in the middle.
 Example of sun dog. (Source: NSIDC) |
 Example of halo. (Source: NSIDC) |
 Example of fog bow. (Source: NSIDC) |
Sun Dogs
There are many types of halos. One of special note is the parhelion or "sun dog." Sun dogs are luminous spots on both sides of the sun that occasionally occur with a halo (left).
Fog Bow
A fog bow is caused by a process similar to that causing rainbows, but because of the very small size of the water droplets, the fog bow has no colors.
Coronas and Anticoronas
Example of an anticorona. (Photograph contributed by K. Steffen. Source: NSIDC) A corona is a ring of light that surrounds the sun or the moon, sometimes forming a luminous disk with the sun or moon at its center. It may be colored with blue on the inside and red on the outside. Coronas are produced when light waves from the sun or moon are slightly deflected around cloud droplets, causing convergence of light that appears in circles around the sun or moon.
The anticorona or "glory" occurs at the point opposite to the light source and consists of one or more colored rings that appear around the shadow cast by an observer on a cloud or in fog. As seen in the photo, the anticorona can on rare occasions be observed from an airplane. The observer in the airplane can see the shadow of the airplane on the cloud and the "glory" rings around the shadow.
Water Sky and Ice Blink
Example of water sky. (Source: NSIDC) 
Example of ice bink. (Source: NSIDC) Water sky refers to the dark appearance of the underside of a cloud layer when it is over a surface of open water.
Ice blink refers to a white glare seen on the underside of low clouds indicating the presence of ice which may be beyond the range of vision.
When other means of reconnaissance are not available, water sky and ice blink can assist travelers in navigating the ice of the polar seas, since they give a rough idea of ice conditions at a distance.
Superior Mirage
The towering mirage. (Source: NSIDC) A superior mirage occurs when an image of an object appears above the actual object, due to the refraction or bending of light waves from the object down toward the eyes of the observer. Downward refraction occurs because air closer to the ground is colder, and therefore more dense, than air aloft.
Superior mirages can take the form of looming, towering, and inversion, depending on the particular density structure of the air column.
The superior mirage. (Source: NSIDC) In looming, distant objects appear to float above the horizon, and objects that are below the horizon may come in to view. In towering, rays from the upper portion of an object are bent more than those from the lower portion. This results in the object appearing to be stretched as well as elevated.
In a superior mirage with an inverted image, the gradient in atmospheric density is so sharp that rays from the lower portion of an object are bent considerably more than rays from the upper portion of an object, causing a mirage of the object to appear inverted above the object. The object appears as it normally would at the same time, because some light continues to travel to the eye directly from the object.
Fata Morgana
The fata morgana is a complex mirage in which distant objects are distorted as well as elongated vertically. For example, a relatively flat shoreline may appear to have tall cliffs, columns, and pedestals. The phenomenon occurs under much the same meteorological conditions as the superior mirage with inversion, and contains features of both towering and inversion.
Optical Haze
Also called shimmer, optical haze occurs in a layer of air next to the ground where small-scale convective currents develop. In this layer warmer air ascends and colder air descends. The difference in how the warm and cold air refract light causes a blurring of objects seen though the layer. Optical haze occurs quite frequently in the Arctic in the same meteorological conditions as the inferior mirage, and often makes it difficult to identify details in the landscape.
Acoustic Phenomena
People frequently report supernormal audibility in Arctic regions. As with optical phenomena, this phenomenon occurs when the vertical density structure of the air causes refraction, but instead of light being refracted, sound waves are refracted. The air near the surface tends to be colder and more dense than air higher up, causing sound waves to tend to bend down toward the surface rather than up away from the earth as they do in more temperate latitudes where air temperature typically decreases with height. These meteorological effects can aggravate introduction of man-made noise pollution in the Arctic.
The range at which sound can be heard depends on the vertical air temperature gradient (and hence density gradient), the speed and direction of the wind, and the extent to which sound energy is absorbed by the earth's surface. For instance, soft snow absorbs sound energy very efficiently, effectively muting the transmission of sound. In contrast, a hard-crusted snow surface absorbs little energy and a smooth ice surface is an almost ideal reflector of sound. Given the right conditions, conversations can sometimes be heard up to three kilometers distant.
| Disclaimer: This article contains information that was originally published by the National Snow and Ice Data Center. Topic editors and authors for the Encyclopedia of Earth have edited its content and added new information. The use of information from the National Snow and Ice Data Center should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content. |
