Inches of mercury (inHg) is a unit of pressure that is commonly used in the United States to measure barometric pressure. The unit is derived from the use of a mercury column in a barometer, which measures the pressure exerted by the atmosphere.

The height of the mercury column in a barometer is used to indicate atmospheric pressure. At sea level, the standard atmospheric pressure is 29.92 inHg, which is equivalent to 1013.25 millibars (mb) or 1013.25 hectopascals (hPa).

In aviation, inches of mercury is commonly used to measure the barometric pressure setting on altimeters. The altimeter setting is the atmospheric pressure at mean sea level, which is used to determine an aircraft's altitude above sea level. Pilots will often set the altimeter to the local altimeter setting in order to obtain an accurate altitude reading.

Inches of mercury is also used in weather forecasting to describe changes in barometric pressure. A rising barometer (increasing pressure) is an indicator of improving weather conditions, while a falling barometer (decreasing pressure) is a sign of approaching bad weather.

In recent years, some countries have transitioned to using the metric system for pressure measurement, with the use of units such as pascals (Pa), kilopascals (kPa), and millibars (mb) becoming more common. However, inches of mercury is still widely used in the United States and in aviation.

To convert inches of mercury to other units of pressure, such as pascals or kilopascals, conversion factors must be used. For example, 1 inHg is equivalent to 3386.39 Pascals (Pa) or 33.8639 hectopascals (hPa).

It's important to note that atmospheric pressure can vary significantly depending on factors such as altitude and weather conditions. Therefore, it's important to consult local weather reports and use accurate measurement instruments when using inches of mercury or other units of pressure in practical applications.