[Peter ODonnell]

I should add that in weather forecasting, all pressures are converted to sea level. If you live some significant elevation above sea level, your actual pressure will be a lot lower than the sea level pressure on the weather map, and your home barometer may or may not be showing that. The best way to check is to wait for a time of slack high pressure when your local TV station pressure (whether given in inches or millibars) will be the same as yours even if you’re some distance away. The same “sea level” pressure, that is. To give some idea, the upper air reports given from radiosonde balloons are given not at standard heights but when the balloon gets to standard pressures of 850, 700, 500, 300 and 250 mbs. The 850 mb maps that are then derived from those observations average about 1,500 metres above sea level which is about 5,000 feet give or take. The 500 mb maps are at an average height of 5,500 metres which is close to 18k feet. So if you live in the plains states you might be at some elevation almost up to 850 mb, which means your actual outside air pressure is really 900 mb when the sea level reading is 1000 mb. If you live in the Great Basin you could be up around 750-800 mb. At the top of the highest peaks in the Rockies you are close to 600 mb and when you’re at cruising altitude in a jet plane the outside pressure is about 200 mb. No wonder they have to pressurize jet aircraft.

[Peter ODonnell]

I think to be more precise, millibars are neither metric nor Imperial measure. They are arbitrary units chosen so that reporting would be relative to some implied average. When chosen, it was probably not known that the average global pressure was actually 1013 rather than 1000.

The metric measurement of air pressure would be mm of mercury and there, 760 mm is similar to 30 inches.

The use of millibars is not some new thing that has crept into usage recently. Weather maps from the 1940s were drawn up in millibars too.

[Robert A Cook PE]

Thank you sir.

[Loud Mime]

Good point about the conversion to sea level. It’s important, and I forgot about that.

[Excellence]

The 500 mb maps are at an average height of 5,500 metres which is close to 18k feet.

So the weather system movements at 18k feet are the best indicator of system flows? Is that why it's always used, or is it more or less an average?