This information is derived by an observer. An example of augmentation would be adding the occurrence of a thunderstorm to the observation. The voice message is composed of the body of the ASOS observation and selected remarks. Selected remarks can include variable visibility, tower visibility, or Density Altitude when 1,000 or more feet above airport elevation. Note that wind direction in the voice message is given in degrees magnetic.
ASOS visibility is based on light scattering .
The visibility sensor projects a beam of light over a very short distance, and the light that is scattered is detected by a receiver. The amount of light scattered and then received by the sensor is converted into a visibility value. There are two ASOS present weather sensors. ASOS evaluates multiple sensor data and infers the presence of obstructions to vision. The Freezing Rain sensor measures accumulation rates as low as 0.01 inches per hour.