## Haze Metrics Converter

Note: * 3 converters are given below*, dependent on input (VR, bext, and DV).

Caution: * The converters cannot be used for average values. * E.g. you cannot calculate average DV from average b

_{ext}; instead, you must calculate DV for each daily measurement, and then calculate an average DV from the individual DV values.

**Visual Range (V.R.)**:The greatest distance at which an observer can just see a black object viewed against the horizon sky.

**Units**: Distance, e.g. kilometers (km)

**Benefit**: This metric is most useful in situations where only the distance that one can see is important, such as for aviation.

**Drawback**: Does not account for changes in scene, such as loss of textures and discoloration as haze increases.

**Relationships**:

V.R. = K/b_{ext}

Where K is the Koschmieder Coefficient – the natural log of the contrast threshold of the human eye, K = 3 – 3.9.

Note that a unit conversion is required in converting between V.R. and b_{ext}.

**Light Extinction (b _{ext}**): The attenuation of light due to scattering and absorption as it passes through a medium.

**Units**: inverse distance, e.g. inverse mega meters (Mm^{-1})

**Benefit**: Light extinction can be directly related to gaseous and aerosol concentrations.

**Drawback**: Light extinction is non-linearly related to a person’s perception of changes in haze. For example, a 10 Mm^{-1} increase in b_{ext} will have a larger perceived impact on a scene at b_{ext} = 20 Mm^{-1} than at b_{ext} = 100 Mm^{-1}

**Relationships**:

b_{ext} = K/V.R.

b_{ext} = 10*exp(DV/10); where b_{ext} is calculated in Mm^{-1}.

Where K is the Koschmieder Coefficient – the natural log of the contrast threshold of the human eye, K = 3 – 3.9.

Note that a unit conversion is required in converting between V.R. and b_{ext}.

**Deciview** **(DV)**: A metric of haze proportional to the logarithm of the atmospheric extinction (b_{ext}).

**Units**: Unitless

**Benefit**: Under many circumstances a change in one deciview will be perceived to be the same on clear and hazy days.

**Drawback**: Deciview is not easily related to gaseous and aerosol concentrations

**Relationships**:

DV = 10*ln(b_{ext}/10); where b_{ext} is expressed in Mm^{-1}.

**Perceived Visual Air Quality (PVAQ): **An index that relates directly to how human observers perceive changes in visual air quality.

**Units**: Unitless

**Benefit: **Quantitatively, measures haze or visibility impairment to how human observers perceive changes in visual air quality.