Interpret barometric pressure changes.
Weather forecasting from a barometer relies on the three-hour tendency, not the absolute reading: a fall of 3 hPa or more in three hours is the classic signal of an approaching depression, while a comparable rise indicates clearance behind a front. Station pressure also has to be reduced to sea level before it can be compared with published charts, since pressure falls by roughly 1 hPa per 8 metres of altitude.
Pressure tendency
Three-hour change = current pressure - pressure three hours ago; sea level pressure = station pressure x (1 - 0.0065 x elevation / 288.15)^-5.257
A fall of more than about 3 hPa in three hours usually precedes strengthening wind and rain. Rapid falls of 5 hPa or more in three hours are associated with storm-force development.
Because altitude alone lowers pressure substantially. Without the correction, a mountain station would always read as a deep low compared with coastal charts.
Not always — in winter it can trap fog and cold air near the surface. The tendency tells you more about change than the absolute value does.