by papa-metro » Mon Dec 05, 2005 12:56 pm
29.92 is considered standard barometric pressure at sea level. IE, the weight of the air on top of us. It is measured inches of mercury and is stated by how far up a glass tube the atmosphere forces the mercury, therefore the terminology of high pressure and/or low pressure areas in the determination of weather.
an altimiter measures the same thing and adjusts it to height above sea level in hundreds or thousands of feet. of course, the higher you go the less air is on top of you ... less weight , or "pressure".
The variations that occur are reported by ATC in inches of mercury, with 29.92 being considered standard pressure (at sea level) Knowing that all aircraft in the area have the same reading from ATC guarantees that their stated altitude is correct, thus guaranteeing separation from each other.