Visibility From Rh Calculator
Free Visibility rh Calculator for meteorology & atmospheric science. Enter variables to compute results with formulas and detailed steps.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
V = 3.912 / (sigma_dry * f(RH) + sigma_Rayleigh)
Where V is meteorological visibility in km, sigma_dry is dry aerosol extinction, f(RH) is hygroscopic growth factor, and sigma_Rayleigh is molecular scattering.
Worked Examples
Example 1: Humid Coastal Evening
Problem:RH is 92% at 18 C with aerosol concentration 60 at standard pressure and 550nm.
Solution:Growth=(1-0.92)^(-0.33)=2.32, dryExt=60*0.002=0.12, wetExt=0.278, total=0.290, V=3.912/0.290=13.5km
Result:Visibility: 13.5 km (8.4 mi) | Clear | Fog Risk: Moderate
Example 2: Pre-Fog Urban Conditions
Problem:RH has risen to 98% at 8 C with high aerosol loading of 120.
Solution:Growth=(1-0.98)^(-0.33)=3.68, dryExt=0.24, wetExt=0.883, total=0.895, V=3.912/0.895=4.37km
Result:Visibility: 4.37 km (2.72 mi) | Moderate | Fog Risk: Very High
Frequently Asked Questions
How does relative humidity affect visibility?
Relative humidity has a profound and nonlinear effect on atmospheric visibility. As RH increases, hygroscopic aerosol particles absorb water and swell in size, dramatically increasing their ability to scatter and absorb light. The relationship becomes especially steep above 80 percent RH, where small increases cause large visibility drops. At 95 percent RH, aerosol particles can grow to several times their dry diameter. This is why visibility often deteriorates rapidly in the hours before fog formation as the air approaches saturation.
What causes low visibility besides fog?
Several atmospheric phenomena beyond fog can significantly reduce visibility. Haze consists of dry aerosol particles from pollution, dust, or biomass burning that scatter light even at moderate humidity levels. Smog combines photochemical pollution with haze, particularly in urban areas. Blowing dust and sand in arid regions can reduce visibility to near zero during strong wind events. Volcanic ash plumes from eruptions can travel thousands of kilometers degrading visibility. Heavy precipitation including rain, snow, and sleet scatter light proportional to their intensity.
How is visibility measured at weather stations?
Weather stations measure visibility using several methods depending on automation level and required accuracy. Traditional observations involve a human observer estimating the distance at which known landmarks become indistinguishable. Automated stations use transmissometers measuring light beam attenuation over a fixed baseline of 10 to 75 meters, then extrapolate using the Koschmieder equation. Forward scatter meters send a light beam and detect the amount scattered at a specific angle, correlating with extinction coefficient. Modern lidar-based ceilometers can also provide visibility profiles through the atmosphere.
How does temperature affect visibility calculations?
Temperature affects visibility through several physical mechanisms. Lower temperatures reduce saturation vapor pressure, meaning less moisture is needed to reach high relative humidity and potential fog formation. Temperature inversions trap pollutants and aerosols near the surface, increasing extinction and reducing visibility. The Rayleigh scattering contribution increases slightly at lower temperatures due to higher air density. Temperature also controls photochemical reaction rates producing secondary aerosols like sulfates and organic particles, indirectly affecting visibility in polluted regions.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy