What is frost?
Frost is formed when a solid surface is chilled below the dew point – that is, the atmospheric temperature below which water droplets begin to condense and form water droplets. If a surface is colder than freezing – ice and frost will form on it, even if the air temperature is humid and not as cold. To put it simply, when water vapour in the air condenses into liquid water, you get raindrops and dew, and when it condenses in colder conditions you get frost and ice.
A ground frost refers to the formation of ice crystals on the ground, or any items outside such as garden furniture, wood, cars and trees. Ground frost occurs on items that have a surface temperature that is below the freezing point of water. It is not unusual for ground frost to be present without an air frost – this happens when the ground cools more quickly. Ground frost produces a covering of delicate white ice crystals wherever it forms, and these crystals can be slippy underfoot – especially on wooden surfaces like decking.
Hoar frost is composed of tiny ice crystals and is formed by the same process as dew, typically on clear, cool winter nights. It only occurs when the temperature of the surface reduces to below freezing point by radiation cooling. This type of frost is more than a thin layer, and often grows so thick that it looks like snow. The pretty ice crystal formations are typically associated with a scenic winter wonderland as they create a beautiful white landscape as they form on plants and trees. This type of frost features ‘feathers’, ‘needles’ and ‘spines’, and might also be found on any other object that is exposed to supersaturated air that is below freezing temperature.