hPa

Hectopascal (hPa) is a unit of pressure used primarily in meteorology and aviation. It is equal to 100 pascals, since one pascal is one newton per square meter. Although the pascal is the SI unit, the hectopascal is a non-SI unit widely accepted for atmospheric pressure measurements. The millibar (mb) is a historical synonym for the same value, and 1 mb equals 1 hPa, so many datasets and maps use either designation interchangeably.

At sea level, standard atmospheric pressure is defined as 1013.25 hPa (and equivalently 1013.25 mb). In weather observations, surface pressure typically ranges from about 980 to 1050 hPa, with lower values indicating low-pressure systems and higher values indicating high-pressure systems. Pressure also decreases with altitude, so values at mountain peaks or aircraft cruising altitudes are much lower; for example, the summit of Mount Everest experiences around 337 hPa.

Pressure is measured with barometers, including mercury, aneroid, and digital sensors. In weather reporting and forecasting, hPa is commonly used to describe sea-level pressure fields, as meteorologists often convert observed pressures to mean sea level pressure for comparisons across locations.

Overall, the hectopascal provides a practical, non-SI unit for expressing atmospheric pressure in daily meteorological practice, closely aligned with historical and contemporary usage.