The sky appears blue because molecules in the atmosphere scatter light of all wavelengths; shorter-wavelength light being scattered more heavily than longer wavelengths; which explains why it appears blue during daylight hours.
At sunrise or sunset, when the sun moves closer to the horizon, more blue light scatters away while more red light reaches your eyes.
Rayleigh Scattering
One process which influences the range of wavelengths emanating from the Sun that actually reach our planet’s surface is scattering by molecules in our atmosphere; this phenomenon is known as Rayleigh Scattering after 19th-century English physicist Lord Rayleigh.
As waves travel through the atmosphere, their energy can be distributed among molecules in various ways and be scattered in various directions, altering its frequency slightly while losing some of its energy and changing frequency slightly. Typically, intensity of scatter energy inversely proportional to fourth power of wavelength; hence why blue wavelengths of sunlight tend to be scattered more heavily than shorter red wavelengths giving sky its blue hue.
Violet and indigo wavelengths, which have the shortest wavelengths in the visible spectrum, scatter even less than blue light rays, leading to an appearance of mixed blue and violet tones during daylight hours. Our eyes’ increased sensitivity towards blue-green colors may also contribute to this effect.
Mie Scattering explains why clouds appear white: when light strikes a cloud, its water droplets and ice crystals are significantly larger than sunlight wavelengths, scattering its light evenly in all directions resulting in its appearance as white light.
Sky blue coloration comes from Rayleigh Scattering and some absorption by gases in our atmosphere, while dust and water vapour enable some blue light to reach us through Rayleigh scattering. If these substances weren’t present, there would be no blue at all! Luckily for us, they make up part of what enables some blue light from reaching us during sunsets; similar mechanisms explain why it appears yellow during sunrises due to needing to travel further through our atmosphere than usual when shining directly down onto earth’s surface.
Water Vapor
Water vapor in our air is an integral component of Earth’s atmosphere, essential to life on this planet and essential to climate systems around it. Water vapor plays a large part in both greenhouse effect and hydrological cycles – it is one of the primary greenhouse gases and an amplifier of climate warming caused by CO2.
When sunlight hits air molecules in the atmosphere, its light is scattered in all directions by air molecules. This scattering effect is most apparent at short wavelengths (blue end of spectrum), with less effective scattering occurring with longer wavelengths like red, orange, and yellow wavelengths – giving rise to what makes our sky appear blue.
As air rises and cools, its contents cool enough for water vapor to condense into droplets that form clouds – one way in which our global water cycle operates. When these clouds clear away entirely, leaving only deep blue sky behind.
Cloudy skies reveal air that is still very warm, not rising quickly enough to cool, because its density doesn’t permit condensation by the adiabatic process of condensation to occur. Otherwise, it would sink to lower atmospheric layers due to increased heat making air lighter with lower density than lower temperature air.
So when the air does not rise and cool down, water vapor can collect in pockets called fogs, consisting of masses of water droplets visible to the naked eye – giving sky its color.
As sunlight reaches Earth’s atmosphere, it interacts with various gases and particles present. Water vapor scatters light more strongly than any other particle; as a result, its light is more easily detectable, appearing bluer.
As soon as the sun is low during sunrise or sunset, its light must travel further through the atmosphere before reaching your eyes – this means more blue light is scattered away instead of reaching our eyes directly, leading to paler skies nearer the horizon.
Dust
The color of the sky changes with time of day. As the sun descends lower in the sky, more light must travel through atmosphere to reach our eyes causing its scattering to change significantly causing wavelengths shorter than blue to be scattered more widely resulting in redder hues at sunset and sunrise.
Philip Pullman’s His Dark Materials series, as seen on HBO and BBC in The Golden Compass and The Amber Spyglass adaptations, depicts Dust as an inexplicable force which some powerful people fear. Dust acts like life energy connecting living things and keeping them from becoming brutish automatons; Pullman uses Dust more subtly than Christianity does Original Sin to criticize organized religion.
Dust is responsible for producing various hues in the sky, while at the same time creating the whiteness of clouds and dust hazes. When clouds or dust particles exceed sunlight wavelengths by Rayleigh Scattering (evenly dispersing), resulting in white colors. Conversely, when waves hit smaller particles that match or equal these wavelengths they scatter less and create blue colors instead.
Dusty or polluted skies depend on the particle sizes and distribution in the atmosphere. A layer of fine dust might produce a blue hue; but passing through areas laden with dirt would produce grayish skies.
Martian skies tend to have an orange or red hue due to iron-rich dust being released by meteoroids and dust storms, as well as due to their thin atmosphere which contributes to higher temperatures there. Dust particles also have some influence over Earth skies but less so due to typically being much smaller particles in our own atmospheres.
Clouds
Rayleigh scattering is responsible for giving the sky its unique blue hue. As sunlight passes through the atmosphere, its beams get scattered by molecules and particles in the air; wavelengths of different colors scatter differently with blue light being dispersed more widely, creating that iconic sky-blue appearance.
Water vapor and dust both scatter light similarly, but clouds tend to disperse their illumination more evenly across all directions – producing whiter-looking light which reaches your eyes.
Clouds are visible accumulations of tiny water droplets or ice crystals suspended in Earth’s atmosphere, either as thin wisps or chunky lumps. To form, an area of air must become saturated with enough moisture vapor that it no longer remains gaseous and begins to condense into liquid or solid forms – this may occur due to cool weather but also from nearby bodies of water evaporating, or sudden wind shifts creating turbulent circulation and swirling air currents that create swirling eddies in airflow.
Water vapor in clouds requires something to condense around, such as salt particles, smoke molecules or dust grains – to become visible. Such nuclei may exist naturally in the atmosphere or be introduced through human activities like slashing forests or burning forests and dispersing dust into the atmosphere – for this to occur. Once sufficient nuclei have been introduced into the air, sufficient amounts of condensable water vapor can condense into visible clouds which appear either white in clear conditions or grayer under more adverse ones.
At sunset, the hue of the sky depends on whether it is clear or clouded with dust and pollution, which scatters light in such a way as to reduce blue illumination reaching our eyes (known as Mie scattering). When the sun is low in the sky and visibility is good, reds and yellows of the spectrum become more apparent than otherwise.