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

Determining the altimeter setting when using a mercurial barometer begins with establishing the station pressure. The mercurial barometer measures atmospheric pressure, which is essential in calculating the altimeter setting. The station pressure is the atmospheric pressure at a specific location, corrected for temperature and other factors. This value is crucial because the altimeter setting will allow pilots to read the actual air pressure at sea level, which is vital for safe altitude measurement in aviation.

Once the station pressure is known, it can be adjusted to provide the correct altimeter setting, accounting for local atmospheric conditions. This is foundational because the other options, such as temperature, wind speed, or humidity, are not primary factors in directly determining the altimeter setting but may influence atmospheric pressure measurements in other contexts. However, they do not serve as the initial step in configuring the altimeter itself.