The purpose of an air line on a well is to:

The purpose of an air line on a well is primarily to allow for monitoring the water level in the well. An air line creates a connection between the atmosphere and the well, enabling the measurement of the water level through changes in air pressure. This method is especially useful because it helps operators determine how much water is available and can indicate the well's pumping condition without the need for direct contact with the water.

By allowing air into the well, it provides a reference point for gauging the height of the water column, which can fluctuate based on usage, rainfall, and other factors. This practice is crucial for maintaining the well's operation and ensuring that adequate water supply is managed effectively. The other options pertain to different practices or concepts that do not relate to the function of the air line in this context.