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),
Turbine map at the NGV throat being about 0.85. Under these conditions, there is a slight scatter in flow between the percent corrected speed lines in the 'choked' region of the map, where the flow for a given speed reaches a plateau. Unlike a compressor (or fan), surge (or stall) does not occur in a turbine. This is because the flow through the unit is all 'downhill', from high to low pressure. Consequently there is no surge line marked on a turbine map. Working lines are difficult to see on a conventional turbine map because the speed lines bunch up. One trick