Because the USA refuses to follow the rest of the world but insists on staying in the 20th century. Air pressure is not measured in inches of mercury anywhere but in the USA and is not measured in millibars either. The World Meteorological Organisation changed the unit for air pressure to hectoPascals (hPa) more than ten years ago and every country in the world bar the USA uses that unit. The hectoPascal was chosen as the unit because the figure is the same as for millibars so barometers and altimeters did not need to be recalibrated.
Science has used metric units for many decades even in the USA as metric units of pressure can be used in mathematical formulae and calculations. Inches of mercury can't be used in this way.
Changing to metric is not difficult. Australia did it in the 1960s and 70s with little fuss and these days no-one thinks in imperial units. I certainly don't and yet I used them for the first 20+ years of my life. I have to convert imperial to metric these days to understand what is being said. The USA will catch up with the rest of the world eventually, they will have to or be left behind.
Science has used metric units for many decades even in the USA as metric units of pressure can be used in mathematical formulae and calculations. Inches of mercury can't be used in this way.
Changing to metric is not difficult. Australia did it in the 1960s and 70s with little fuss and these days no-one thinks in imperial units. I certainly don't and yet I used them for the first 20+ years of my life. I have to convert imperial to metric these days to understand what is being said. The USA will catch up with the rest of the world eventually, they will have to or be left behind.
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Because of the public's familiarity with it. It's been presented to them that way for years. It's in millibars on the map, because that is the US professional standard. It's the difference between products that are for public consumption, versus those for professional use.
The US government tried to legislate a conversion to the metric system back in the 1970's. It failed miserably, and was given up due to public outcry.
The US government tried to legislate a conversion to the metric system back in the 1970's. It failed miserably, and was given up due to public outcry.
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They don't and both can be used interchangeably.