Which term describes the boundary where cold air replaces warm air?

• A. The ratio of compressor discharge pressure to compressor inlet pressure.
• B. Glossy ice formed by slow freezing.
• C. The boundary between two air masses where cold air is replacing warm air.
• D. A low-power beacon installed at an ILS marker.

Answer: (D)

Air masses with different temperatures meet at boundaries called fronts. When colder air advances and replaces warmer air, that boundary is a cold front. Recognizing a cold front is important for pilots because it often brings clouds, precipitation, and wind shifts as the warm air is forced upward and cooled. These changes can affect visibility, turbulence, and overall flight planning. The other options describe unrelated things—machinery pressure ratios, a type of ice, or a navigational beacon near an ILS marker—so they don’t describe the meteorological boundary at issue.