ATPL Canadian Meteorology, Radio Aids to Navigation, and Flight Planning (SAMRA) Practice Test

The correct answer is based on the fundamental conditions that contribute to the development of in-flight icing. For icing to occur, two critical factors must be present: the temperature must be close to freezing, and there must be visible moisture in the atmosphere.

When the temperature is around the freezing point (0°C or 32°F), supercooled water droplets can exist in the atmosphere. These droplets remain in liquid form even though they are below the freezing point. If an aircraft enters a region where these supercooled droplets are present, they can freeze on contact with the aircraft's surfaces due to the lower temperatures experienced in flight.

Visible moisture is essential because it indicates the presence of water droplets in the air. If these droplets are supercooled and the aircraft's surfaces are at or below freezing, then icing will occur. This combination is crucial for pilots to understand, as it directly impacts safety and requires vigilance in meteorological assessments to prevent dangerous icing conditions while flying.

The other conditions mentioned in the incorrect options are not sufficient on their own to cause in-flight icing, which is why they do not provide the same level of risk or concern as the correct answer.