What is the significance of the term "High to low, beware below" in aviation?

The phrase "High to low, beware below" serves as an important advisory in aviation regarding altimeter settings and altitude readings. This term underscores the potential for altitude reading errors when transitioning from high-pressure areas to low-pressure areas.

In aviation, accurate altitude awareness is critical for safety, particularly during approaches and landings when pilots are maneuvering at lower levels. When the altimeter is calibrated based on high atmospheric pressure settings, but the aircraft is then flying into a lower pressure area, it can lead pilots to mistakenly believe they are at a higher altitude than they actually are. This discrepancy could result in the aircraft being dangerously closer to terrain than intended, thus increasing the risk of controlled flight into terrain (CFIT) incidents.

The term is not about speed variations, pressure altitude changes, or navigation techniques; instead, it specifically highlights the need for vigilance concerning altitude accuracy in varying pressure conditions. Therefore, understanding this phrase is crucial for pilots to ensure they maintain proper altitude awareness and avoid potentially life-threatening situations.