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How can clouds and humidity interfere with optical communication signals?

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information theory Shannon's communication model
Consider a simple telephone conversation: A person (message source) speaks into a telephone receiver (encoder), which converts the sound of the spoken word into an electrical signal. This electrical signal is then transmitted over telephone lines (channel) subject to interference (noise). When the signal reaches the telephone receiver (decoder) at the other end of the line it is converted back into vocal sounds. Finally, the recipient (message receiver) hears the original message.

Clouds and humidity interfere with optical communication signals primarily through atmospheric absorption and scattering.

Atmospheric Absorption

The atmosphere contains gaseous constituents, including water vapor ( $H_{2} O$ ), that absorb optical energy. When the transmission wavelength nears the resonant frequencies of these gases, signal loss increases significantly. To mitigate this, engineers select specific transmission wavelengths that fall within low-loss "windows" in the infrared, visible, or ultraviolet regions of the spectrum.12

Atmospheric Scattering

Clouds, fog, and other airborne particles cause scattering, which is a major source of signal degradation. Scattering occurs when particles—such as water droplets in fog or clouds—have cross-sectional diameters comparable to the signal's wavelength. While rain is a dominant cause of scattering for lower-frequency microwave signals, fog is the primary cause of scattering for higher-frequency optical signals, ranging from infrared through visible light.12

These atmospheric sensitivities remain a significant challenge for the development of reliable outdoor free-space optical links.1

Are you interested in how these principles apply to specific technologies like LIDAR?