inch of mercury

The inch of mercury survives on two instrument panels: the North American weather report and the aircraft altimeter. US barometers and broadcasts still speak in inches, where 29.92 is the standard atmosphere, a strong high pushes past 30.5, and a deep storm falls below 29. Aviation took the same scale and made it operational: before departure a pilot dials the current local altimeter setting in inHg so the instrument reads field elevation, and controllers issue updated settings en route as the weather changes underneath.

The distinctive rule sits at 18 000 feet. Above that altitude in the United States and Canada, every aircraft sets 29.92 inHg regardless of the actual weather, and altitudes become flight levels: FL350 means 35 000 feet of pressure altitude, not true altitude. Two aircraft at FL350 may both be wrong about their height above the sea by the same amount, which is exactly the point, because separation requires altimeters that agree with each other, not with the ocean. Forgetting to reset on descent back through 18 000 feet is a known enough error to have its own checklist line.

A third life is in vacuum work: automotive manifold vacuum idles near 18 to 20 inHg below atmosphere, and HVAC technicians pull refrigerant lines down watching the same scale. Conversions worth carrying: one inch of mercury is about 33.86 hPa, about half a psi, and a bit over one foot of water column.