Inch of mercury facts for kids

Early American barometer calibrated in inches of mercury

Inches of mercury, often written as inHg or just Hg, is a way to measure pressure. Pressure tells us how much force is pushing on a certain area. This unit is still widely used in the United States for weather forecasts and in aviation (flying). It helps people understand how "thick" the air is in one place. While it's common in the U.S. and Canada, many other countries use different units for pressure.

Inches of mercury measures the pressure created by a column of mercury that is one inch tall. This measurement is taken at a specific temperature, 32 °F (0 °C), and under standard gravity. This means it's based on how much a certain amount of mercury weighs and pushes down.

One inch of mercury is equal to 3,386.389 pascals at 0 °C. Pascals are another unit for measuring pressure, used more commonly in science.

How it's Used in Aviation

When airplanes fly very high, above a certain height called the "Transition Altitude," their pilots set their altimeters to a standard pressure. An altimeter is a tool that measures how high an airplane is flying based on air pressure.

In the U.S. and Canada, this standard pressure is 29.92 inHg. In other countries, they use different units like 1,013.2 hPa (which is the same as 1,013.2 mbar).

Setting the altimeter to this standard pressure helps all planes flying at high altitudes use the same reference point. The readings from these altimeters are called flight levels. This helps air traffic controllers keep planes safely separated in the sky.

Related pages

• Torr (This unit is similar to inches of mercury but uses millimeters of mercury.)