On a sunny day, the sky appears blue because solar wavelengths are scattered most by particles in our atmosphere – an effect known as Rayleigh Scattering.
Air molecules are smaller than the wavelengths of light, so they scatter it evenly in all directions – more so at shorter wavelengths such as blue than longer ones such as violet – hence why skies appear blue!
Earth’s Atmosphere
The Earth’s atmosphere appears blue because short violet wavelengths of sunlight scatter more strongly than longer red wavelengths due to air molecules being smaller than wavelengths of light and being subjected to bumping into each other, redirecting energy in different directions; when this happens with blue and violet light it results in the energy becoming spread out more widely, giving sky its characteristic hue.
From a physical viewpoint, color has nothing to do with the wavelengths of light that are scattered by our atmosphere; as Isaac Newton proved with his prism. All colors in the visible spectrum can exist simultaneously in sunlight; only those wavelengths that scatter more widely make their way to our eyes through atmospheric filters.
Sunlight travels through Earth’s atmosphere similarly to light traveling through clear water or air, and interacts with atmospheric molecules through Rayleigh scattering; this interaction causes much of its white light from being scattered into blue and violet wavelengths, with only some remaining as original white light from its source.
Blue light scattered by clouds depends on several variables in the atmosphere – density, temperature and composition are all factored in. Thus, as atmospheric density and thickness increase, so too will its amount of blue scattered light; conversely a thinner atmosphere will make for more white clouds in the sky.
Dust and pollution, rain, and even the presence of water vapor are also major influences in altering the color of the sky, and forest fires or volcanic eruptions tend to produce redder skies than usual; while heavily polluted cities experience bright orange sunsets due to an abundance of human-made aerosols.
Imagining life without atmosphere would be quite different: daytime and nighttime would appear the same shade of gray. Our satellite Moon, similarly without atmosphere, appears this way both daytime and nighttime regardless of weather conditions; day and nighttime appear similarly dark on its surface.
Blue-Green Microbes
On a typical day, the sky appears blue due to particles in the atmosphere scattering (reflecting) sunlight through Rayleigh Scattering. Light enters through the ground and travels upward, passing over various particles such as dust, pollution and water droplets before arriving back at its source – giving rise to Rayleigh Scattering and Rayleigh-induced Rayleigh scattering effects that scatter it back down through various particles such as dust, pollution and droplets in its path before returning through them with various wavelengths reflected back at them until eventually ending up back at source – giving off that distinct blue hue due to Rayleigh Scattering effects combined with longer wavelengths such as red, Yellow and Orange wavelengths leaving overall the overall effect being one that makes the sky appear bluer!
Many lakes and rivers contain blue-green bacteria commonly referred to as “blue-green algae,” however these organisms are actually called cyanobacteria rather than algae. Cyanobacteria belongs to Kingdom Monera and Division Eubacteria and are classified as Gram-negative bacteria. Like algae, they use photosynthesis to transform carbon dioxide and water into oxygen; getting their name from their distinctive blue pigment known as phycocyanin which works together with chlorophyll a to capture light for photosynthesis; they also possess yellow pigmented carotenoids while certain species possess red pigment phycoerythrin pigmentation for photosynthesis.
Electron microscope studies of cyanobacteria reveal cell walls with superficial similarities to bacteria’s. Furthermore, both types use ribosomes to synthesize organic molecules at an extremely fast rate of metabolism and have high metabolism rates; both types are also sensitive to certain antibiotics.
Blue-green algae have long been an integral component of Earth’s ecosystem, producing oxygen while turning CO2 into food sources for themselves and other organisms – contributing significantly to today’s high oxygen levels in our air.
Blue-green algae blooms may provide many advantages in certain situations; however, their spread can create problems elsewhere. When introduced into bodies of water, blue-green bacteria can form dense mats on its surface that look similar to scum while creating an unpleasant taste and smell; it may also release toxicants into the environment that cause respiratory and skin irritation as well as liver damage; when consumed directly through ingestion these toxicants could also cause stomachache, vomiting and diarrhea.
Rayleigh Scattering
As sunlight passes through the atmosphere, it is reflected and scattered in various directions – this gives rise to its blue hue. Light scatters more due to smaller particles (atoms and molecules) in the atmosphere than it absorbs; when photons of sunlight hit an air molecule it causes it to vibrate more than it absorbs – this causes blue and violet wavelengths of light to be scattered more widely across our atmosphere than red and green wavelengths resulting in predominantly blue lighting in its path through space.
This is why the sky appears much brighter overhead and fades into a pale yellow or white at the horizon; sunlight has had to travel further through the atmosphere, scattering more blue and violet wavelengths out of sight.
Rayleigh Scattering was named for British physicist John William Strutt, 3rd Baron Rayleigh who first described this process and made an important observation: wavelengths of light vary significantly but emit at the same frequency. When different frequencies of light are scattered by airborne molecules and atoms, their electric fields become polarized – an effect known as polarization. Polarized light waves contain electric fields with opposing positive and negative charges which alternate in strength. When atoms and molecules vibrate, their vibratory motion polarizes them to emit light at different frequencies, with those emitting more intensely scattered as blue skies having frequencies close to their natural resonance frequencies of air molecules.
Violet light has shorter wavelengths than blue light and therefore scatters more easily through air molecules. Our eyes don’t detect violet light though, making it invisible. In fact, the only reason the sky doesn’t turn purple is due to lack of violet air molecules present; multiple scatterings also tend to weaken light waves so they aren’t as powerful.
The Sun
The Sun emits vast quantities of energy as light and other forms of electromagnetic radiation, as well as large quantities of matter as particle radiation; mostly protons and electrons with high energies; this particle radiation gives our atmosphere its color. With a spectral class G2V designation, which indicates its classification as a main sequence star which generates most of its energy through nuclear fusion of hydrogen nuclei into helium, our Sun emits vast quantities of energy every second!
As solar radiation leaves the Sun and enters Earth’s atmosphere, it interacts with gases like nitrogen and oxygen that make up this layer of the atmosphere. Since these molecules are smaller than wavelengths of visible light, they scatter them across various directions – more so for blue wavelengths than for red or green ones; hence why the sky appears bluer during sunlight.
However, the sky doesn’t only appear blue due to nitrogen and oxygen; other atmospheric gases like carbon dioxide and methane also exist but don’t contribute to making the sky so vivid; instead they have shorter wavelengths than blue light that reflect back towards our eyes instead of scattering. What gives the sky its hue is sunlight striking Earth being scattered by blue wavelengths back toward our eyes instead.
Atmospheric scattering of sunlight explains why we see various hues when the Sun shines; however, during sunrise and sunset the angle at which light enters our atmosphere changes; more short wavelength orange and red colors reach us through their wavelengths than otherwise would.
But why does the sky turn orange or red at sunset? There are multiple factors at work here; sunlight reaching our eyes increases nearer to the horizon due to greater atmospheric diffusion; secondly, atmosphere nearer the horizon contains materials which absorb and scatter light differently.