Atmospheric stability refers to air that does not rise, which can lead to fog, mist or frost formation. Instability occurs when air rises, forming clouds and rain. Clouds are classified by form, height, and whether convection or fronts caused them. Convectional rainfall happens when warm land heats the air above, causing it to rise and condense. Frontal rainfall occurs at boundaries of warm and cold air masses. Orographic rainfall results when air is forced to rise over mountain barriers, cooling and condensing. Hail forms through a process of freezing and melting within cumulonimbus clouds. Dew and radiation fog form on clear nights through surface cooling and condensation. Advection fog