what is a barometer reading?

Answer 1

Barometers measure the air pressure - if you like, the weight of the air pressing down on you. As you get higher in the atmosphere, there is less air above you and the pressure decreases.

The unit for air pressure is the hectoPascal (hPa). this is the unit in the standard metric system. In the centimetre/gram system, the unit was the millibar (mb). The hectoPascal was chosen because the figure is the same as the millibar figure and barometers did not need to be changed.