What are the two primary requirements for icing to occur?

The occurrence of icing in the atmosphere primarily requires super-cooled water droplets and an air temperature that is below freezing. Super-cooled water droplets exist in conditions where the temperatures are below freezing but the droplets have not yet solidified into ice. This state is crucial because it is these droplets that can freeze upon contact with surfaces, such as an aircraft wing, leading to icing.

When the air temperature is below freezing, the potential for these droplets to freeze increases significantly. If super-cooled droplets come into contact with a surface that is at or below freezing, ice will form rapidly, which can pose hazards, particularly in aviation. The other options do not accurately capture the specific conditions necessary for icing; for example, while humidity and cold temperatures can contribute to overall weather conditions, they are not as directly relevant to the icing process as the presence of super-cooled water droplets and sub-freezing temperatures.