What’s the difference between pressure altitude and geometric height?

• Pressure altitude is what aircraft instruments measure.
• Geometric height is the actual height above ground level.

On a hot day, air expands, so pressure altitude underestimates geometric height. On a cold day, it overestimates it. This is why pilots use the phrase:
“When it’s cold, look out below.”

APG accounts for this difference when calculating level-off heights to ensure true geometric clearance of obstacles.