How to calculate pressure altitude?

To calculate pressure altitude, you can use the following formula:

Pressure Altitude = Standard Pressure Altitude + (Pressure Difference / Standard Pressure Gradient)

1. Determine the standard pressure altitude: The standard pressure altitude is the altitude at which the atmospheric pressure is equal to the standard pressure of 29.92 inches of mercury (inHg) or 1013.25 millibars (mb). This standard pressure altitude is typically 0 feet above sea level.

2. Measure the current atmospheric pressure: Use a barometer or obtain the current atmospheric pressure from a weather report in inches of mercury (inHg) or millibars (mb).

3. Calculate the pressure difference: Subtract the current atmospheric pressure from the standard pressure of 29.92 inHg or 1013.25 mb.

4. Determine the standard pressure gradient: The standard pressure gradient is 1 inHg or 34 mb per 1000 feet of altitude.

5. Plug the values into the formula: Substitute the standard pressure altitude, pressure difference, and standard pressure gradient into the formula to calculate the pressure altitude.

6. Calculate the pressure altitude: Add the pressure difference divided by the standard pressure gradient to the standard pressure altitude to determine the pressure altitude in feet above sea level.

For example, if the current atmospheric pressure is 28.92 inHg, the pressure difference would be 1 inHg (29.92 - 28.92). Using the standard pressure gradient of 1 inHg per 1000 feet, you can calculate the pressure altitude by adding the pressure difference divided by the standard pressure gradient to the standard pressure altitude of 0 feet.

Step 1: Obtain the Barometric Pressure (QNH)

- From an altimeter or weather report.

Step 2: Determine the Elevation (H) of the Pressure Altitude Reference Point

- This is typically sea level (0 feet or 0 meters).

Step 3: Calculate the Pressure Altitude (HPA)

Formula:

```
HPA = H + [(29.92 in Hg - QNH in Hg) x 1000 ft]
```
or

```
HPA = H + [(1013 hPa - QNH hPa) x 32.81 ft]
```

where:

- HPA is the pressure altitude in feet or meters
- H is the elevation of the pressure altitude reference point in feet or meters
- QNH is the barometric pressure at the reference point in inches of mercury (in Hg) or hectopascals (hPa)

Example:

- QNH = 29.50 in Hg
- H = 0 ft (sea level)

Solution:

```
HPA = 0 ft + [(29.92 in Hg - 29.50 in Hg) x 1000 ft]
HPA = 0 ft + [0.42 in Hg x 1000 ft]
HPA = 420 ft
```

Therefore, the pressure altitude is 420 feet.