Final answer:
A report modifier 'AUTO' indicates an automatically generated weather report from the Automated Surface Observing System (ASOS), which is used mainly in the United States.
Step-by-step explanation:
A report modifier in the context of weather observations refers to a descriptive code, indicating how the observation was taken. AUTO is a report modifier indicating that the report was generated automatically without human intervention. The system that collects this automated weather information is known as the Automated Surface Observing System (ASOS). ASOS is a system used primarily in the United States to provide continuous weather observations, which give data on temperature, wind speed and direction, humidity, pressure, and other relevant atmospheric conditions.