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An aneroid barometer.
The distribution of air pressure across Earth is an extremely important feature of the atmosphere. It sets the winds in motion and determines whether air will rise or sink. Air pressure increases with both the density and temperature of the air, as dictated by the equation of state. Because pressure decreases with altitude, it is important to distinguish between surface air pressure and sea level pressure, the latter being a standard that allows comparisons between different locations. Mercury and aneroid barometers both measure air pressure, which is expressed in units of millibars, pascals, or kilopascals.
The nonuniform distribution of the atmosphere gives rise to horizontal pressure gradients. Although small compared to vertical gradients, horizontal differences in pressure can sometimes produce devastating winds. The vertical pressure gradient force is much larger but is usually offset by a nearly equal but opposite gravitational force, resulting in hydrostatic equilibrium. Under conditions of hydrostatic balance, the vertical pressure gradient is proportional to the density of the atmosphere, as shown by the hydrostatic equation. Spatial variations in the density of the lower atmosphere lead to horizontal pressure gradients in the upper atmosphere that initiate upper-level winds.
Horizontal winds respond to the combined effect of three forces: the pressure gradient force, the Coriolis force, and friction. These combine to form gradient and geostrophic flow in the upper atmosphere and flow across isobars near the surface. The distribution of pressure tends to organize into fairly large-scale patterns of anticyclones, cyclones, ridges, and troughs. Wind vanes, anemometers, and aerovanes each measure different components of the wind velocity.