Aerosol Optical Depth to Visibility Calculator

The visibility estimate is based on the contrast threshold relationship: V = K / beta_ext.

When only aerosol optical depth (AOD) is available, the calculator assumes a well-mixed layer of height H: AOD ~= beta_ext · H. Combining both gives: V ~= K · H / AOD.

Optional wavelength scaling uses the Angstrom law: AOD(lambda) = AOD(lambda0) · (lambda/lambda0)^(-alpha). Humidity and correction factors act as multipliers for practical tuning.

1. Select a computation method from the dropdown.
2. Enter AOD and boundary layer height for the primary method.
3. If needed, add wavelength and Angstrom exponent for scaling.
4. Use humidity and correction factors only when justified.
5. Press Calculate to see results above the form.
6. Use the download buttons to export CSV or PDF.

1) What aerosol optical depth represents

Aerosol optical depth (AOD) is a dimensionless measure of how strongly airborne particles reduce light through scattering and absorption along a vertical column. Values near 0.05–0.15 often indicate clean air, while 0.3–0.6 suggests haze. Values above 0.8 commonly occur during smoke or dust episodes.

2) Why visibility depends on extinction

Human and sensor visibility is controlled by the atmospheric extinction coefficient, beta_ext, which expresses light attenuation per distance. The calculator uses a contrast threshold parameter K to convert extinction into meteorological visibility. This creates a consistent mapping for comparisons.

3) Linking AOD to near-surface conditions

AOD is column-integrated, so converting it to a local visibility estimate requires a vertical assumption. The primary method treats aerosols as well-mixed within a boundary layer height H. Typical daytime H ranges from 500 to 2000 meters, but stable nights can be much lower.

4) The role of wavelength and particle size

AOD changes with wavelength because smaller particles scatter shorter wavelengths more efficiently. The Angstrom exponent alpha typically ranges from about 0.3 (coarse dust) to 2.0 (fine smoke). When alpha is provided, the calculator scales AOD between measurement and target wavelengths.

5) Humidity growth and real-world haze

Many aerosols absorb water, increasing particle size and scattering at higher relative humidity. The optional humidity growth factor f(RH) lets you apply a practical adjustment when conditions are moist. If humidity effects are unknown, leave the factor at 1.0.

6) Choosing a method for your data source

Use “AOD + Boundary Layer Height” for satellite or sunphotometer AOD and a boundary layer estimate. Use “Extinction Coefficient” when you have measured beta_ext from visibility sensors or lidar. Use the “Empirical” option to tune results to local observations.

7) Interpreting the output categories

Visibility below 1 km is commonly considered very low, 1–5 km low, 5–10 km moderate, and above 10 km good. These bands help communicate operational impacts, but thresholds can vary by application and region.

8) Limits and best practices

AOD-to-visibility conversion is sensitive to vertical layering, aerosol type, and humidity. For the best results, use realistic H, provide alpha when available, and apply local tuning only when validated against ground visibility reports. Always treat outputs as estimates, not guarantees.

1) What AOD value indicates hazardous haze?

There is no universal cutoff, but AOD above about 0.6 often corresponds to noticeable haze. If the boundary layer is shallow or humidity is high, visibility can drop sharply at lower AOD.

2) Why do I need boundary layer height?

AOD is a column total. Height H converts that total into an average extinction per kilometer. A smaller H concentrates aerosols and reduces visibility more than a deeper mixed layer.

3) Which wavelength should I use for visibility?

Visible-light estimates often use 550 nm as a reference. If your AOD comes from another band, enter the measurement wavelength and provide alpha to scale it to the target wavelength.

4) What is a good Angstrom exponent value?

Fine smoke and urban pollution often fall near 1.2–2.0. Dust can be nearer 0.2–0.8. If alpha is unknown, leave it blank to avoid introducing a misleading assumption.

5) What does the Koschmieder constant K mean?

K represents the contrast threshold used to define visibility. A common default is 3.912, linked to a 2% contrast criterion. Different sensors or definitions can justify other values.

6) When should I use the empirical option?

Use it when you have local visibility observations and want a tuned mapping. Adjust the exponent b and bounds so the calculator reproduces typical conditions for your area.

7) Why can the calculator return very large visibility?

When AOD or extinction is very small, the mathematical visibility grows rapidly. This can exceed practical reporting limits, so apply realistic bounds or use measured extinction when possible.