Millibar in the context of HPa

The pascal (symbol: Pa) is the unit of pressure in the International System of Units (SI). It is also used to quantify internal pressure, stress, Young's modulus, and ultimate tensile strength. The unit, named after Blaise Pascal, is an SI coherent derived unit defined as one newton per square metre (N/m). It is also equivalent to 10 barye (10 Ba) in the CGS system. Common multiple units of the pascal are the hectopascal (1 hPa = 100 Pa), which is equal to one millibar, and the kilopascal (1 kPa = 1,000 Pa), which is equal to one centibar.

The unit of measurement called standard atmosphere (atm) is defined as 101325 Pa.Meteorological observations typically report atmospheric pressure in hectopascals per the recommendation of the World Meteorological Organization, thus a standard atmosphere or typical sea-level air pressure is about 1,013 hPa. Reports in the United States typically use inches of mercury or millibars (hectopascals). In Canada, these reports are given in kilopascals.

The Siberian High (also Siberian Anticyclone; Russian: Азиатский антициклон (Aziatsky antitsiklon); Chinese: 西伯利亞高壓; Pinyin Xībólìyǎ gāoyā; Kazakh Азия антициклоны (Aziya antitsiklonı)) is a massive collection of cold dry air that accumulates in the northeastern part of Eurasia from September until April. It is usually centered on Lake Baikal. It reaches its greatest size and strength in the winter when the air temperature near the center of the high-pressure area is often lower than −40 °C (−40 °F). The atmospheric pressure is often above 1,040 millibars (31 inHg). The Siberian High is the strongest semi-permanent high in the northern hemisphere and is responsible for both the lowest temperature in the Northern Hemisphere outside Greenland, of −67.8 °C (−90.0 °F) on 15 January 1885 at Verkhoyansk, and the highest pressure, 1083.8 mbar (108.38 kPa, 32.01 inHg) at Agata, Krasnoyarsk Krai, on 31 December 1968, ever recorded. The Siberian High is responsible both for severe winter cold and attendant dry conditions with little snow and few or no glaciers across the Asian part of Russia, Mongolia, and China. During the summer, the Siberian High is largely replaced by the Asiatic low.

In atmospheric science, an atmospheric model is a mathematical model constructed around the full set of primitive, dynamical equations which govern atmospheric motions. It can supplement these equations with parameterizations for turbulent diffusion, radiation, moist processes (clouds and precipitation), heat exchange, soil, vegetation, surface water, the kinematic effects of terrain, and convection. Most atmospheric models are numerical, i.e. they discretize equations of motion. They can predict microscale phenomena such as tornadoes and boundary layer eddies, sub-microscale turbulent flow over buildings, as well as synoptic and global flows. The horizontal domain of a model is either global, covering the entire Earth (or other planetary body), or regional (limited-area), covering only part of the Earth. Atmospheric models also differ in how they compute vertical fluid motions; some types of models are thermotropic, barotropic, hydrostatic, and non-hydrostatic. These model types are differentiated by their assumptions about the atmosphere, which must balance computational speed with the model's fidelity to the atmosphere it is simulating.

Forecasts are computed using mathematical equations for the physics and dynamics of the atmosphere. These equations are nonlinear and are impossible to solve exactly. Therefore, numerical methods obtain approximate solutions. Different models use different solution methods. Global models often use spectral methods for the horizontal dimensions and finite-difference methods for the vertical dimension, while regional models usually use finite-difference methods in all three dimensions. For specific locations, model output statistics use climate information, output from numerical weather prediction, and current surface weather observations to develop statistical relationships which account for model bias and resolution issues.

Equivalent potential temperature, commonly referred to as theta-e ${\displaystyle \left(\theta _{e}\right)}$, is a quantity that is conserved during changes to an air parcel's pressure (that is, during vertical motions in the atmosphere), even if water vapor condenses during that pressure change. It is therefore more conserved than the ordinary potential temperature, which remains constant only for unsaturated vertical motions (pressure changes).

${\displaystyle \theta _{e}}$ is the temperature a parcel of air would reach, if lifted to the point of saturation, if all the water vapor in the parcel were to condense out, releasing its latent heat, and the parcel was brought adiabatically to a standard reference pressure, usually 1000 hPa (1000 mbar) which is roughly equal to atmospheric pressure at sea level.