Answer · confidence: partial
Why does sticky summer air make the sky look milky even when it's clean?
Research question: Why does a humid summer day make the sky look milky even when the air is fairly clean?
Humid air causes tiny airborne particles, such as dust, pollen, and pollution, to absorb water 10. Hygroscopic growth changes particle size and refractive index 22, and it strongly affects how much light they scatter 1; at Ny-Ålesund the scattering enhancement factor at 85% relative humidity reached as high as 6.6 2. Once particles grow larger than the wavelength of light, Mie scattering takes over, scattering all colors about equally, so the blue sky becomes weaker and whiter 181511. The facts do not directly compare humid days with equally clean air, so the 'fairly clean' part of the question is only partly addressed. The newest fact is dated 2026-04-27 10, about five months before October 2026.
Checked 2026-10-14. Only documented and supported facts are used.
Dig deeper on this questionSources
[1] documented
Aerosol particles undergo hygroscopic growth in the ambient atmosphere, which strongly affects their light scattering properties.
"Aerosolparticlesexperiencehygroscopicgrowth suredin-situscatteringcoefficients. TheRHdependencyof in the ambient atmosphere. Their optical properties – espe- the recorded f(RH, λ) can be well described by an empir- ciallytheaerosollightscattering–arethereforestronglyde- ical one-parameter equation."Effects of relative humidity on aerosol light scattering in the Arctic (2010)
[2] documented
At the Ny-Ålesund site, the scattering enhancement factor at 85% relative humidity could reach values as high as 6.6, indicating a significant increase in light scattering due to humidity.
"As an extreme, the NYA site showed the largest f(RH=85%)(whichcouldrangeuptovaluesof6.6)comparedtotheothersites."Effects of relative humidity on aerosol light scattering: results from different European sites (2013)
[3] documented
During periods not influenced by sea salt, the mean scattering enhancement factor at 85% relative humidity was higher (3.41) than during sea salt influenced periods (2.86).
"The mean values for non-sea salt influenced periods were higher (mean f(RH=85%)=3.41) than the sea salt influenced periods (mean f(RH=85%)=2.86)."Effects of relative humidity on aerosol light scattering: results from different European sites (2013)
[4] documented
The aerosol forcing efficiency changes significantly with humidity, increasing from -13 W/m² at dry conditions to -17 W/m² at 85% relative humidity.
"For high RH values, the hygroscopic effect should be taken into account since the aerosol forcing efficiency changed from (cid:3)13W/m2 at dry conditions to (cid:3)17W/m2 at RH(cid:2)85%."Study of the relative humidity dependence of aerosol light-scattering in southern Spain (2014)
[5] supported
Mie scattering, which occurs when atmospheric particles are larger relative to the wavelength of light, scatters all colors approximately equally, contributing to a whitish or milky appearance.
"At larger ratios scattering varies in a more complex fashion, as described for spherical particles by the Mie theory ."Diffuse sky radiation
[6] supported
Cloud droplets, which are larger than the wavelength of light, scatter all colors approximately equally, resulting in light that is not very wavelength-dependent.
"The flux of light is not very wavelength-dependent because the cloud droplets are larger than the light's wavelength and scatter all colors approximately equally."Diffuse sky radiation
[7] supported
The scattering of light by atmospheric particulates varies based on the ratio of particle diameter to the wavelength of the incident radiation.
"Scattering varies as a function of the ratio of particle diameters (of particulates in the atmosphere) to the wavelength of the incident radiation."Diffuse sky radiation
[8] supported
Diffuse sky radiation is solar radiation that has been scattered from the direct solar beam by molecules or particulates in the atmosphere.
"Diffuse sky radiation , is solar radiation reaching the Earth 's surface after having been scattered from the direct solar beam by molecules or particulates in the atmosphere ."Diffuse sky radiation
[9] supported
Light scattering by aerosol particles determines atmospheric visibility.
"The scattering of light by aerosol particles dictates atmospheric visibility, which is a straightforward indicator of air quality."Differentiating the Contributions of Particle Concentration, Humidity, and Hygroscopicity to Aerosol Light Scattering at Three Sites in China (2022)
[10] supported
Extra humidity causes tiny airborne particles such as dust, pollen, and pollution to absorb water.
"That extra humidity causes tiny particles already in the atmosphere — like dust, pollen, and pollution — to absorb water."The “why” behind the hazy skies
[11] supported
With higher air humidity, all colors are scattered and the blue of the sky appears whitish and brightened.
"Bei größerer Luftfeuchtigkeit werden alle Farben gestreut und das Blau erscheint weiß-aufgehellt."Opaleszenz
[12] documented
Ambient relative humidity and chemical composition jointly influence aerosol liquid water content.
"The latter two factors jointly influence the aerosol liquid water content (ALWC)."Differentiating the Contributions of Particle Concentration, Humidity, and Hygroscopicity to Aerosol Light Scattering at Three Sites in China (2022)
[13] documented
Particle hygroscopic growth under humid conditions produces a strongly non-linear relationship between visibility and aerosol concentration in Singapore.
"we show that in Singapore, particle hygroscopic growth results in a strongly non‐linear relationship between visibility and aerosol concentration under humid conditions."Visibility deterioration and hygroscopic growth of biomass burning aerosols over a tropical coastal city: a case study over Singapore's airport (2016)
[14] supported
When the humidity-swollen particles grow larger, they form a thin haze that reduces visibility and gives the sky a washed-out appearance.
"When these particles get larger, they create a thin layer of haze that reduces visibility and gives the sky that washed-out appearance."The “why” behind the hazy skies
[15] supported
When there is a lot of haze in the atmosphere, Mie scattering adds more white components to the scattered light, so the blue sky becomes weaker and whiter.
"Wenn viel Dunst in der Atmosphäre ist, kommen durch die Mie-Streuung weitere Weißanteile ins Streulicht. Damit wird die blaue Himmelsfarbe schwächer und weißlicher."Opaleszenz
[16] documented
Ambient relative humidity should be considered when forecasting visibility deterioration caused by haze aerosols, even in a tropical coastal city.
"Thus, even for a tropical coastal city, the consideration of ambient relative humidity is desirable when forecasting for visibility deterioration caused by haze aerosols."Visibility deterioration and hygroscopic growth of biomass burning aerosols over a tropical coastal city: a case study over Singapore's airport (2016)
[17] supported
Moisture levels naturally rise during spring and summer, and onshore flow can push humidity even higher.
"During the spring and summer months, moisture levels naturally climb, and persistent onshore flow over the last few days has made it even higher."The “why” behind the hazy skies
[18] supported
Rayleigh scattering, which produces blue color, applies to particles smaller than the wavelength of light; when particles grow larger than the wavelength, Mie scattering, which is independent of wavelength, takes over.
"Wenn die Partikel größer als die Wellenlänge werden, wird die Rayleigh-Streuung durch die von der Wellenlänge unabhängige Mie-Streuung abgelöst."Opaleszenz
[19] supported
Fog and mist look visible because light is scattered by Mie scattering, which produces the Tyndall effect and makes the otherwise colorless droplets visible.
"Nebel ist deshalb sichtbar, weil Licht aufgrund der Mie-Streuung gestreut wird, wodurch der Tyndall-Effekt auftritt und die eigentlich farblosen Tröpfchen sichtbar werden."Nebel
[20] supported
Haze is the term used for reduced visibility of roughly one to four kilometers, while fog requires visibility below one kilometer.
"Sichtweiten von einem bis etwa vier Kilometern gelten als Dunst ."Nebel
[21] supported
Pilinis et al. (1995) suggested that raising relative humidity from 40% to 80% increases the radiative forcing of global mean aerosol by a factor of 2.1.
"Pilinis et al. (1995) suggested that an increase ofthe relativehumidityfrom40 to80%willresult inanincrease oftheradiativeforcing ofglobalmeanaerosolbyafactorof2.1."A simplified empirical method for determination of aerosol hygroscopicity and composition (2010)
[22] supported
Hygroscopic growth changes the shape and refractive index of aerosol particles.
"Inaddition,thehygroscopicgrowthwillresultinchangesofparticleshape andrefractiveindexofaerosol(Covertetal.,1972)."A simplified empirical method for determination of aerosol hygroscopicity and composition (2010)