What is the first step to determining the altimeter when using a mercurial barometer?

Determining the station pressure is a critical initial step when using a mercurial barometer to assess altimeter settings. The station pressure is the atmospheric pressure measured at the location of the barometer, typically at the elevation of the weather station. This measurement forms the baseline for further calculations and adjustments because altimeters need to be calibrated to sea level pressure for accurate readings.

Once the station pressure has been established, it can be adjusted to standard sea-level pressure if needed, allowing for consistent and accurate altitude readings across different geographical locations. Understanding the local atmospheric conditions is essential for pilots and meteorologists to maintain safe operations, especially when flying at various altitudes. By first determining the station pressure, one can ensure that subsequent steps in calculating the altimeter setting are based on accurate data.