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Relative Humidity's Effect on Atmospheric Transmittance

Moisture in the air is expressed as a relative humidity value or dewpoint. As the moisture content of the air increases people say the "humidity goes up," which expresses in lay terms the increase in moisture per pound of air. A moist air mass interacts differently with electromagnetic radiation than dry air. This includes the visible spectrum called light and invisible wavelengths like ultraviolet and infrared.

Moisture and Clouds
- The relative humidity must generally be at 100 percent for clouds to form. Cloud cover is a visible sign of gaseous water vapor condensing into liquid form as millions of water droplets. Since clouds are composed of liquid instead of gas, they have a much greater effect on the transmittance of radiation to both the surface and space compared to a humid vapor-only air mass. Clouds 16,000 feet and higher are generally composed of ice crystals which have their own peculiar effects on radiation transmittance. About one-third of the total incoming solar radiation is bounced back to space by clouds.
Visible Effects
- Water vapor has a scattering effect on visible light traveling through the air. Skies tend to appear hazier on humid days because the moisture particles are larger than dry ones. High-level clouds such as cirrus are composed of ice crystals which bend and reflect light in different ways. Light passing through ice crystals makes rings around the sun and moon and can also create "sun dogs" and circumzenithal arcs, sometimes called "smiles in the sky."
Infrared Radiation
- Infrared radiation (IR) is invisible long-wavelength radiation which produces heat. IR detectors are commonly used for night vision surveillance because most objects emit some of this radiation. IR is absorbed and retransmitted by clouds, so if you compare two nights with similar wind, temperature and surface moisture, the cloudy night remains warmer. More radiation escapes to space on a clear night resulting in a cooler surface temperature. Seventy percent of all IR radiation which passes through clouds is transmitted to space while the remainder is absorbed by water vapor and other greenhouse gases in the atmosphere or reradiated to the surface at a different wavelength.
Ultraviolet Radiation
- Ultraviolet (UV) light is invisible like IR but of shorter wavelengths. It is the type of radiation most responsible for suntans. UV light is not affected by the relative humidity as much as IR so it can penetrate cloud cover to the Earth's surface and escape through it to space at night. This is why you can still get a sunburn on a cloudy day.

Nature