Barometer Simultaneous barometric readings from across a network of weather stations allow maps of air pressure to be produced, which were the first form of the modern weather map when created in the 19th century. Isobars, lines of equal pressure, when drawn on such a map, give a contour map showing areas of high and low pressure. Localized high atmospheric pressure acts as a barrier to approaching weather systems, diverting their course. Atmospheric lift caused by low-level wind convergence into the surface brings clouds and sometimes precipitation. The larger the change in pressure, especially if more than 3.5 hPa (0.1 inHg),
Atmospheric pressure Siberian High often attains a "sea-level pressure" above , with record highs close to . The lowest measurable "sea-level pressure" is found at the centers of tropical cyclones and tornadoes, with a record low of (see Atmospheric pressure records). "Surface pressure" is the atmospheric pressure at a location on Earth's surface (terrain and oceans). It is directly proportional to the mass of air over that location. For numerical reasons, atmospheric models such as general circulation models (GCMs) usually predict the nondimensional "logarithm of surface pressure". The average value of surface pressure on Earth is 985hPa. This is in contrast to