How does icing risk relate to the freezing level in flight planning?

• A. Icing risk increases above the freezing level when dry air is present
• B. Icing risk is unaffected by the freezing level
• C. Icing risk occurs only at the freezing level
• D. Icing risk increases below the freezing level when visible moisture is present

Answer: (D)

Icing forms when aircraft encounter visible moisture in air that’s cold enough for liquid water to exist in a supercooled state and freeze on contact with the airplane. The freezing level is the altitude where the air temperature is 0°C. Below this level, air temperatures are at or below freezing, so any visible moisture in clouds, fog, or precipitation can be liquid water that remains supercooled and will freeze on surfaces, causing ice buildup. Above the freezing level, the air is warmer than 0°C, so the likelihood of supercooled water is much lower and icing risk diminishes, even if some moisture is present. That’s why icing risk increases below the freezing level when visible moisture is present.