Atmospheric optics is the scientific study and explanation of the unique and stunning optical effects in the sky. It encompasses the principles of physics that help answer common questions like why the sky appears blue, why certain clouds are white while others appear dark, and how rainbows are formed. This article will explore various atmospheric phenomena and will help gain a deeper understanding of their nature and underlying causes.
Atmospheric Optical Phenomena Definition
Atmospheric optical phenomena are created when sunlight or moonlight interacts with different atmospheric components like clouds, dust, water vapour and gases. The interaction of solar wind with certain atmospheric gases can also cause these optical phenomena.
Difference between Atmospheric Optics and Meteorological Optics
Atmospheric optics studies the optical properties of the atmosphere and its products beyond naked-eye observation. Meteorological optics focuses on observable patterns with the naked eye. However, the terms are occasionally used interchangeably.
Atmospheric Optical Phenomena Causes
Atmospheric optical phenomena occur when light passing through the atmosphere is obstructed and/or deflected. Air molecules, aerosol particles and various hydrometeors, such as cloud droplets and raindrops, can cause these obstructions. Deflection can happen through different mechanisms: reflection, refraction, diffraction, scattering and absorption. For example, mirages are caused by refraction, rainbows by reflection and refraction, and the sky’s colour by scattering.
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Types of Atmospheric Optical Phenomena
Blue Skies and White Clouds
Nitrogen in the atmosphere selectively scatters shorter wavelengths (violet, blue and green) more efficiently than longer wavelengths (yellow, orange and red). The sky appears blue because our eyes are more sensitive to blue light.
Clouds appear white due to countless droplets scattering all visible wavelengths in all directions. Dark clouds appear when clouds grow thicker, larger and taller, and little light penetrates the base of the cloud. Raindrops at the cloud base are too large to scatter efficiently, resulting in energy absorption.
Afterglow
Afterglow is the whitish and pinkish arch of light in the twilight sky, created by dust in the high stratosphere that catches the hues of the twilight arch below the horizon. It consists of bright segments and purple light and is often observed during or after volcanic eruptions when large amounts of dust and ash are deposited into the high atmosphere.
Crepuscular Rays
When dust, smoke and other dry particles in the atmosphere scatter light from the rising or setting Sun, it creates sunbeams called crepuscular rays. These rays can be seen streaming through gaps in clouds. They typically appear during twilight when the Sun is just below the horizon.
Haze
Haze is an atmospheric optical phenomenon caused by dry particles, such as dust and smoke, that reduce the sky’s visibility. Haze can stem from various sources, including farming, traffic, industry, volcanoes and wildfires.
Halo
A halo is an atmospheric optical phenomenon caused by light interacting with ice crystals in the atmosphere. These ice crystals are located in the upper troposphere and act like prisms and mirrors, refracting and reflecting light to create the halo’s optical illusion.
Sundog
A sun dog is a bright spot beside the Sun caused by sunlight refracting through ice crystals in the atmosphere. It belongs to the halo family and can appear anywhere, but it is most visible near the horizon.
Rainbow
A rainbow is a colourful circular shape that appears in the sky due to the reflection, refraction and dispersion of light in water droplets. This optical phenomenon occurs when sunlight passes through water droplets in the atmosphere. The outer part of a rainbow appears red, while the inner side appears violet. Rainbows caused by sunlight always appear in the section of the sky opposite the Sun’s location.
Mirage
A mirage is an atmospheric optical phenomenon caused by the refraction of light as it passes through layers of air with different temperatures and densities. This phenomenon is most commonly associated with the appearance of a sheet of water in a desert or on a hot road. The heated air near the ground causes light to refract and bend, creating the illusion of water or objects in the distance. Mirages are most common during the hottest times of the day when temperature gradients are strongest.
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Cloud Iridescence
Cloud iridescence refers to the presence of colours within a cloud resembling the hues in oil films. This atmospheric optical phenomenon typically takes place near the Sun or Moon. Iridescent clouds occur due to the diffraction of light by tiny water droplets or ice crystals, resulting in a colourful display. While larger ice crystals may not cause iridescence, they can give rise to halos, a separate phenomenon.
Aurora Borealis
An aurora is a spectacular atmospheric optical phenomenon resulting from disturbances in the magnetosphere caused by the solar wind, which alters the trajectories of charged particles that precipitate into the upper atmosphere and emit a variety of colourful light patterns.
Brocken Spectre
A Brocken spectre is the enlarged shadow of an observer seen on a cloud when the sun shines from behind them as they look down from a height into mist or fog. The shadow is often triangular and appears larger than it actually is. The phenomenon may also include a bright area called Heiligenschein and halo-like rings of rainbow-coloured light called a glory around the head or aperture silhouette of the spectre if the cloud consists of water droplets backscattered.
Factors Affecting Atmospheric Optical Phenomena
The interaction of light with individual particles can often explain the geometry and colouration of atmospheric optical phenomena. For instance, rainbows result from light interacting with spherical particles, and halos from light interacting with hexagonal ice crystals. However, these phenomena are produced by numerous particles dispersed throughout the atmosphere, so the optical thickness of the light path heavily influences their appearance. The study of atmospheric optical phenomena encompasses the aesthetic aspects of radiative transfer, cloud physics and aerosol science, and it plays a significant role in climate and weather.
Frequently Asked Questions – FAQs
What is the meaning of atmospheric optics?
Atmospheric optics is the scientific study and explanation of the unique and stunning optical effects visible in the sky.
What is an example of an atmospheric optical phenomenon?
A rainbow is one of the famous examples of an atmospheric optical phenomenon.
Why do smoke and dust particles reduce visibility?
Smoke and dust particles in the atmosphere absorb some of the light, while the remaining light gets scattered and fails to reach the observer. If the quantity of pollutants increases, the amount of light absorption and scattering also increases, resulting in reduced clarity and altered colour of the objects perceived.
How are atmospheric optical phenomena formed?
Atmospheric optical phenomena occur when light passing through the atmosphere is obstructed and/or deflected, often in a specific manner. Air molecules, aerosol particles and various hydrometeors, such as cloud droplets and raindrops, can cause these obstructions.
What are the factors affecting atmospheric optical phenomena?
The interaction of light with individual particles can often explain the geometry and colouration of atmospheric optical phenomena.
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