Frost and Ice

Ground Frost

Ground frost occurs where ice forms on the ground where the temperatures has fallen below the freezing point of water. Ground frost occurs more frequently than Air frost due to the fact that the ground can lose heat much quicker than the surrounding air. In recent times lazy presentations on the TV have allowed the term grass frost to be used. This is unofficial and inaccurate and the term shouldn’t be used. I can’t see the term Windscreen frost be used when Ice only forms on Car Windscreens. The only reason why frost is more prevalent on Grass at times is because concrete can hold onto the heat better. However the correct terms are Ground Frost and not Grass Frost.

Air Frost

An Air Frost occurs when the temperature falls below the freezing point of water at a height of at least One Meter above ground.

Hoar Frost

Hoar Frost may also form from dew as temperatures drop to around -3C to -5C. This also know as Silver Frost or White frost. Hoar frost can also be derived from sublimation. It’s also easy to confuse Hoar frost with Rime or Glaze.

Rime

Rime is a granular deposit of ice which maybe white or Milky in appearance. This is formed by rapid freezing of super cooled water on cold exposed objects. Rime may also form feathers which point into the wind. Under calm conditions they may form in any direction. This why it’s confused with Hoar Frost. Rime is created via foggy conditions or by cloud droplets. These droplets which are supercooled freeze as soon as they make contact with any object. If the fog persists large amounts of ice can be formed. This may cause structural damage.

Glaze or Black Ice

Glaze forms when supercooled liquid precipitation falls in the form of freezing rain or drizzle onto exposed objects at or just above zero. The liquid doesn’t freeze instantly allowing the liquid to spread out forming a thin layer of smooth ice. Unlike Hoar Frost or Rime the glaze is transparent. Glaze or Black ice tend to occur when a warm air mass displaces a cold air mass. The picture below shows Glaze after freezing rain covering Snow