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Air pressure in the atmosphere is commonly measured in several units, including inches of mercury (Hg), which is one of the traditional units used in weather forecasting and atmospheric sciences. The concept of measuring atmospheric pressure using a column of mercury dates back to the invention of the barometer, where the height of mercury in a tube reflects the weight of the air above it.

While inches of mercury is widely recognized, other units such as Pascals (Pa), atmospheres (atm), and millibars (mbar) are also used. Pascals is the SI unit for pressure and is increasingly utilized in scientific applications. Millibars, which correlate closely to hectopascals, are commonly used in meteorology for weather maps. Lastly, atmospheres are a convenient unit for expressing pressure in relation to the average atmospheric pressure at sea level.

The focus here on inches of mercury as a correct option reflects its historical and ongoing significance in practical applications, especially in the context of weather observation and reporting.