Explanation:

- Option 1: Opacity

- Opacity refers to the measure of transparency or how much light can't pass through an object or substance. It’s not related to moisture.

- Option 2: Abrasion

- Abrasion is the process of scraping or wearing something away. It has no link to moisture levels in the air.

- Option 3: Precipitation

- Precipitation is a meteorological term for any form of water, liquid or solid, falling from the sky, like rain or snow. It involves moisture but not as a measure present in the air.

- Option 4: Humidity

- Humidity measures the amount of water vapor present in the air. It’s about how moist the air is.