barometric pressure

Can someone explain barometric pressure? How do you know if the pressure is low by just looking at barometric pressure reading on a scanner?

Air has weight and therefore exercises pressure. Pressure itself is defined as a force acting on a given surface. The amount of pressure depends on a couple of variables as density (depending on temperature ) the air mass etc. Barometric pressure changes with altitude because at sea level the air column pushing on this given surface is "higher" and hence has more weight when compared to an air column situated on Mount Denali.

A sound engine is capable of creating low pressure, often referred to as vacuum. The level of low pressure the same engine can achieve does not change with altitude, it will always create the same low pressure level. But the difference in pressure to barometric pressure this engine produces will change with altitude. This is because the barometric pressure itself changes with altitude.

Often it is said that a good engine at sea level will create a vacuum of 18 - 21 inches of mercury. Barometric pressure there is around 30 inches of mercury, so the real pressure in the intake manifold will be around 10 inches - give or take. I live at 7000 ft above sea level where the barometric pressure is only 24 inches of mercury. The same engine will still be able to lower the pressure n the intake to the same 10 inches it achieves at sea level. But the difference from barometric will only be 14 inches because of this point of departure/atmospheric pressure - 24 inches -at my altitude.

Basically we are looking at 3 variables: 1st is the absolute barometric pressure, 2nd is the absolute pressure achieved in the intake manifold with the engine running and the 3rd is the difference between those two values.The later is what a vacuum gauge will read.

Unfortunately there is no clear terminology when talking about lower than aatmosphere pressure, even the term vacuum does not make too much sense. Vacuum would be the absence of any pressure but what we are dealing with in reality is just low pressure, not absence. Throw into this mix the fact that with lower than barometric pressure some weird units like inches of mercury, mm of water etc are used and you get an idea why people may have a hard time to get their heads around these pressure readings.

It may help when you look at it as what it is: pressure and difference in pressure, nothing else. There is one reference point that is 0 pressure and everything else is just above it, that's all. There is no such thing as negative pressure, at least down on earth