Air Pressure Converter (hPa · inHg · mmHg · psi · atm)

Numerically identical by construction: meteorology used millibars for a century, then SI arrived and the hectopascal (100 Pa) was adopted precisely BECAUSE 1 hPa = 1 mb exactly — every historical chart stayed valid with a new label. So 1013 mb and 1013 hPa are the same reading; only the decade of the textbook differs. US surface observations and hurricane reports still mix both freely.

Institutional inheritance: early barometers were literal mercury columns measured with rulers, US instrumentation standardized on inches, and altimeter-setting practice (the Kollsman window) froze it in. 29.92 inHg IS 1013.25 hPa — the ICAO standard atmosphere — and the rest of the world's aviation sets altimeters in hPa (QNH). Crossing into US airspace, pilots convert with this page's factor: divide hPa by 33.864.

Sea-level-corrected: 1013 is the textbook average; everyday highs run 1020–1035 and strong winter anticyclones 1040–1055 (the all-time records sit near 1085 in Siberian cold domes); ordinary lows 990–1005, vigorous storms 960–980, and intense hurricanes below 920 (Wilma's 882 hPa is the Atlantic record). Rate of change matters more than level for local forecasting: a 3+ hPa fall in 3 hours flags an approaching system.

Altitude swamps weather: pressure drops ~1 hPa per 8.5 m, so Denver's raw station pressure (~830 hPa) would read as a catastrophic super-hurricane next to Miami's 1015 on the same calm day. Maps therefore reduce every station to equivalent sea-level pressure so only the weather signal remains. The same correction runs your altimeter in reverse — and explains why phone barometers double as floor counters: one building storey is about 0.4 hPa.