# Zenith Sky Optical Thickness Infrared Spectrum

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**Zenith absorption, A** is the fraction of incident electromagnetic energy which is absorbed by the atmosphere at a given wavelength on a directly vertical path from the Earth's surface to space.
It is a dimensionless quantity ranging from

`A`= 0 (full transmission) to

`A`= 1 (full absorption).

**Zenith optical thickness, τ** can be calculated from the absorption as

`τ`= - ln(1-

`A`). Optical thickness is dimensionless and always ≥ 0, with larger values corresponding to stronger absorption.

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Click and drag on the graph to select an area to zoom (pinch to zoom on touchscreens)

Use the scrollbar below the graph to view a different wavelength range

Graphs can be downloaded from the menu at the top left