Air pressure is the actual force exerted by the weight of the air above a given point, while pressure altitude is an altitude read on an altimeter when it is adjusted to a standard sea level pressure of 29.92 inches of mercury (inHg) or 1013.25 hectopascals (hPa).
Understanding Air Pressure
Air pressure, also known as atmospheric pressure or barometric pressure, is the force exerted by the weight of air on a given area of the Earth's surface. It's influenced by factors such as altitude and temperature. Air pressure decreases with increasing altitude because there is less air above pushing down.
Understanding Pressure Altitude
Pressure altitude is a theoretical altitude representing the height above the Standard Datum Plane (SDP). The SDP is a reference plane where the atmospheric pressure is equal to the standard sea level pressure of 29.92 inHg. Pressure altitude is used in aviation to calculate aircraft performance, particularly for takeoff and landing. It’s a crucial parameter because aircraft performance charts and calculations are based on standard atmospheric conditions. When the actual atmospheric pressure differs from standard, pressure altitude allows pilots to adjust their calculations accordingly.
Key Differences Summarized
Here's a table highlighting the key differences:
| Feature | Air Pressure | Pressure Altitude |
|---|---|---|
| Definition | Actual force exerted by the air. | Altitude above the Standard Datum Plane (SDP). |
| Measurement | Measured by a barometer (e.g., inHg, hPa). | Read from an altimeter set to 29.92 inHg. |
| Influence | Altitude, temperature, weather systems. | Actual air pressure. |
| Usage | Observing weather patterns. | Calculating aircraft performance. |
Relationship Between Air Pressure and Pressure Altitude
The two concepts are directly related. Pressure altitude is derived from air pressure. The lower the air pressure, the higher the pressure altitude. If you set your altimeter to 29.92 inHg, it essentially calculates your altitude based on a standard pressure lapse rate (approximately 1 inHg decrease per 1,000 feet increase in altitude). In reality, the atmospheric conditions are rarely standard. Therefore, pressure altitude is a corrected altitude, providing a standardized reference for performance calculations.
Why Pressure Altitude Matters
Aircraft engine and aerodynamic performance are significantly affected by air density. Since air density is directly related to pressure and temperature, pressure altitude helps pilots determine takeoff distances, climb rates, and other critical performance parameters. High pressure altitude means lower air density, leading to reduced engine power and lift.
Conclusion
In summary, air pressure is the actual force exerted by the atmosphere, while pressure altitude is a standardized altitude derived from air pressure, used primarily in aviation for performance calculations. Pressure altitude provides a valuable reference point, especially when actual atmospheric conditions deviate from standard.
