Final answer:
Hoar frost is a type of frost with feathery ice crystals that forms on cold, clear nights with high humidity and calm air, on surfaces below freezing point. Snow persists on mountain slopes due to slow heat exchange and snow's insulating properties, allowing it to remain even slightly above freezing temperatures.
Step-by-step explanation:
Hoar frost is a type of frost that forms on cold, clear nights when the air is calm and humidity is high. It appears as delicate, feathery crystals that typically form on the ground or exposed objects such as branches and leaves. Hoar frost develops when the water vapor in the air directly deposits as ice, without first becoming dew. This process is similar to how snow forms, as both are the solid phase of water, originating from ice crystals.
For hoar frost to form, the surface temperatures must be below the freezing point of water, which is 0°C (32°F). Conditions favoring its formation include clear skies, which allow for more efficient radiational cooling of the ground, and calm winds that minimize mixing of air layers. If the air gets too warm or the wind picks up, the delicate hoar frost crystals will quickly sublimate or melt.
Regarding why snow remains on mountain slopes even at temperatures above freezing, it's because snow is an insulator and melts slowly. The gradual heat exchange between the snow and the warmer air is not sufficient to cause immediate melting. The larger the volume of snow, the longer it can persist even in conditions slightly above 0°C due to the amount of heat required for a phase change from solid to liquid.