Weather: Pressure Altitude
Pressure altitude is our height, real or imagined, above standard pressure (29.92”Hg).
Pressure altitude is the height above standard pressure, the line (or theoretical line) where the atmospheric pressure is 29.92’’ Hg. Or, said differently, pressure altitude is the altitude displayed in an altimeter that is set to 29.92.
Pressure altitude allows us to understand how the current air pressure compares to a standard day.
On average, around the world, atmospheric pressure is roughly 29.92’’, but local weather patterns cause this to fluctuate. Pressure in the atmosphere always decreases with height at a rate of 1 inch of mercury for every 1,000 feet. This means at 1,000 feet MSL the standard atmospheric pressure is 28.92’’.
However, a standard altimeter setting for an airport at 1,000 MSL would still be 29.92’’. That’s because the altimeter shows the difference between the pressure set in the Kollsman window and the actual atmospheric pressure. If we set the actual atmospheric pressure, the altimeter would indicate 0 feet. Generally, we are more concerned with our height above sea level than our height above ground. So, on the ground, we want our altimeter to indicate field elevation above sea level.
Weather stations at airports measure the pressure, correct for height by applying a standard lapse rate of 1’’ per 1,000 feet and then transmit the corrected value to pilots in text or radio reports.
When airplanes enter Class A airspace, they all set the altimeter to 29.92”. This ensures that even as airplanes move quickly over areas of changing pressure, they are using the same altimeter setting and will maintain separation from each other. When this switch is made at 18,000 feet, the airplane is said to be at a “flight level” rather than a simple altitude. An airplane at 17,000 feet is at 17,000 feet. An airplane at 19,000 feet, assuming the pilots have set 29.92”, is at FL190 (flight level one niner zero).
- What is pressure altitude?
- What is standard pressure at 3,000′ MSL?
- If the current altimeter is 29.90, and you are at 1,000 feet MSL, what is your pressure altitude?