What is the effect of humidity and fog on millimeter wave propagation?
Humidity and Fog
Water vapor absorption arises from rotational transitions of the H2O molecule. The strongest resonance in the mmWave range is at 22.235 GHz, with a peak attenuation of about 0.2 dB/km at sea level with 7.5 g/m³ water vapor density. Additional peaks at 183 GHz and 325 GHz affect sub-THz propagation. Between these peaks, the water vapor contribution to total absorption is relatively small (0.1-0.3 dB/km).
Fog consists of suspended water droplets with typical diameters of 1-50 μm. Because these droplets are much smaller than mmWave wavelengths (3-10 mm), the attenuation follows the Rayleigh scattering regime where attenuation is proportional to the liquid water content (LWC) and approximately proportional to f². Typical LWC: moderate fog = 0.05 g/m³, dense fog = 0.5 g/m³.
For most mmWave terrestrial links shorter than 1 km: fog and humidity attenuation are secondary to rain attenuation in the link budget. However, for maritime and coastal environments where fog is frequent, fog attenuation can be the dominant availability limiter. For satellite links at mmWave: cloud attenuation (similar mechanism to fog) contributes 1-5 dB at Ka-band depending on cloud cover.
Frequently Asked Questions
How does fog compare to rain?
At 60 GHz: dense fog (visibility 100m, LWC = 0.5 g/m³) causes about 5-10 dB/km. Heavy rain (25 mm/hr) causes about 13 dB/km. Rain is generally worse for mmWave propagation because rain drops are larger (more efficient scatterers/absorbers) and rain rates can be very high during storms.
Does humidity cause signal fading?
Not directly as fading, but humidity variations cause changes in the refractive index of the atmosphere, leading to beam bending, multipath, and scintillation effects. In tropical climates with high humidity: these refractive effects can cause significant fading on long paths.
Can I ignore fog for indoor mmWave?
Yes. Fog does not form indoors. Indoor mmWave links are affected by: wall/obstacle penetration loss, human body blockage, furniture reflection, and multipath. Humidity effects are negligible for indoor distances (<50m).