Trevor watched as the barometer on his front porch showed a change in air pressure.

What happened as Trevor was watching the air pressure change?

While watching the air pressure change, the change that occur within the barometer is that the barometer (if aneroid barometer) contains within it a partially evacuated (vaccumed) steel corrugated box. The top surface is held by a strong spring to stop it from collapsing.

If the air pressure changes, the surface moves up or down with a decrease or increase in air pressure respectively. This movement is transmitted and amplified by a connected system of jointed lever and chain that passes around a grooved wheel. This transmission is to a pointer that is deflected with these change and movements to give the reading on the scale. The scale is is often calibrated in millibars; higher pressure indicates a bad weather.