AO1 is an automated station without a precipitation sensor. AO2 is an automated station with a precipitation sensor. It is defined as a change in wind direction of 45 degrees or more which takes place in less than 15 minutes and has sustained winds of 10 knots or more throughout the wind shift. For example, TWR VIS 2. For example, SFC VIS 2.
Visibility is reported in statute miles.
This is only reported if the visibility is less than 3 miles and rapidly increases or decreases by mile or more during the observation period. Virga is an augmented remark describing precipitation not reaching the ground. It is reported if the ceiling in the body of the report is less than 3,000 feet and variable. It is a rise or fall in station pressure at the rate of 0.06 inches or more per hour which totals 0.02 inches or more. A trace is P0000.